Power GUI Programming with VisualAge C++/powergui.iso/trialva/ibmcppw/help/cppwsomg.inf

OS/2 Help File | 1996-02-16 | 643.6 KB | 18,974 lines

ΓòÉΓòÉΓòÉ 1. About This Book ΓòÉΓòÉΓòÉ

Before you begin to use this information, it would be helpful to understand how
to navigate through it. You can use the Table of Contents and Index facility to
locate topics and the Search facility to search the text of this document. You
can use hypertext links to acquire related information on the current topic.
Hypertext links appear in a different color. For example, here is a link to
another panel: Communicating Your Comments to IBM. By double-clicking on the
text of the link or by pressing Enter on a highlighted link, you will open a
panel of related information. When you open a panel, the first link has the
focus; to shift the focus to other links, use the Tab key.

You should also understand:

     How to Use the Contents
     How to Obtain Additional Information
     How to Use Menu Bar Choices
     How to Cut and Paste Examples


ΓòÉΓòÉΓòÉ 1.1. Notices ΓòÉΓòÉΓòÉ

(C) Copyright International Business Machines Corporation, 1995, 1996. All
rights reserved.

Note to U.S. Government Users - Documentation related to restricted rights -
Use, duplication, or disclosure is subject to restrictions set forth in GSA ADP
Schedule Contract with IBM Corp.

This edition applies to Version 3.5 of IBM VisualAge for C++ for Windows
(33H4979, 33H4980) and to all subsequent releases and modifications until
otherwise indicated in new editions.  Make sure you are using the correct
edition for the level of the product.

This publication could include technical inaccuracies or typographical errors.
Changes are periodically made to the information herein; any such changes will
be reported in subsequent revisions.

Requests for publications and for technical information about IBM products
should be made to your IBM Authorized Dealer or your IBM Marketing
Representative.

When you send information to IBM, you grant IBM a nonexclusive right to use or
distribute the information in any ways it believes appropriate without
incurring any obligation to you.

Any reference to an IBM licensed program in this publication is not intended to
state or imply that only IBM's licensed program may be used. Any functionally
equivalent product, program, or service that does not infringe any of IBM's
intellectual property rights may be used instead of the IBM product, program,
or service. Evaluation and verification of operation in conjunction with other
products, except  those expressly designated by IBM, is the user's
responsibility.

IBM may have patents or pending patent applications covering subject matter in
this document.  The furnishing of this document does not give you any license
to these patents.  You can send license inquiries, in writing, to the IBM
Director of Licensing, IBM Corporation, 500 Columbus Avenue, Thornwood, NY,
10594, USA.

Licensees of this program who wish to have information about it for the purpose
of enabling: (i) the exchange of information between independent created
programs and other programs (including this one) and (ii) the mutual use of the
information which has been exchanged, should contact IBM Canada Ltd.,
Department 071, 1150 Eglinton Avenue East, North York, Ontario M3C 1H7, Canada.
Such information may be available, subject to appropriate terms and conditions,
including in some cases payment of a fee.

This publication contains examples of data and reports used in daily business
operations. To illustrate them as completely as possible, the examples include
the names of individuals, companies, brands, and products. All of these names
are fictitious and any similarity to the names and addresses used by an actual
business enterprise is entirely coincidental.


ΓòÉΓòÉΓòÉ 1.2. Trademarks and Service Marks ΓòÉΓòÉΓòÉ

The following terms used in this publication are trademarks or service marks of
IBM Corporation in the United States or other countries:

      AIX
      IBM
      IBMLink
      OS/2
      SOMobjects
      System Object Model
      VisualAge

 Windows is a trademark of Microsoft Corporation.

 Other company, product, and service names, which may be denoted by a double
 asterisk(**), may be trademarks or service marks of others.

 IBM's VisualAge products and services are not associated with or sponsored by
 Visual Edge Software, Ltd..


ΓòÉΓòÉΓòÉ 1.3. How to Use the Contents ΓòÉΓòÉΓòÉ

When the Contents window first appears, some topics have a plus (+) sign beside
them. The plus sign indicates that additional topics are available.

To expand the Contents if you are using a mouse, click on the plus sign. If you
are using the keyboard, use the Up or Down Arrow key to highlight the topic,
and press the Plus (+) key. For example, How to Use the Contents has a plus
sign beside it.  To see additional topics for that heading, click on the plus
sign or highlight that topic and press the Plus (+) key.

To view a topic, double-click on the topic (or press the Up or Down Arrow key
to highlight the topic, and then press the Enter key).


ΓòÉΓòÉΓòÉ 1.4. How to Obtain Additional Information ΓòÉΓòÉΓòÉ

After you select a topic, the information for that topic appears in a window.
Highlighted words or phrases indicate that additional information is available.
Certain words and phrases are highlighted in a different color from the
surrounding text. These are called hypertext terms.

If you are using a mouse, double-click on the highlighted word.  If you are
using a keyboard, press the Tab key to move to the highlighted word, and then
press the Enter key.  Additional information then appears in a window.


ΓòÉΓòÉΓòÉ 1.5. How to Use Menu Bar Choices ΓòÉΓòÉΓòÉ

Several choices are available for managing the information presented in this
document. There are three choices on the menu bar: the Services menu, the
Options menu, and the Help menu.

Use the Services menu to operate the active window currently displayed on the
screen. Available actions include the following:

 Placing Bookmarks
   You can set a placeholder so you can retrieve information of interest to
   you.

 Searching for Information
   You can find occurrences of a word or phrase in the current topic, selected
   topics, or all topics.

 Printing Information
   You can print one or more topics. You can also print a set of topics by
   first marking the topics in the Contents list.

 Copying Information to a File
   You can copy a topic that you are viewing to the System Clipboard or to a
   file that you can edit. This method is particularly useful for copying
   syntax definitions and program samples into the application that you are
   developing.

 Select actions from the Options menu, to change the way your Contents list is
 displayed. To expand the Contents and show all levels for all topics, choose
 Expand all from the Options pull-down. You can also press the Ctrl, Shift, and
 * keys together.

 You can select various types of help information from the Help menu.

 For information about any of the menu choices, highlight the choice in the
 menu and press F1.


ΓòÉΓòÉΓòÉ 1.5.1. Placing Bookmarks ΓòÉΓòÉΓòÉ

When you place a bookmark on a topic, it is added to a list of bookmarks you
have previously set.  You can view the list, and you can remove one or all
bookmarks from the list.  If you have not set any bookmarks, the list is empty.

To set a bookmark, do the following:

   1. Select a topic from the Contents.
   2. When that topic appears, select the Bookmark option from the Services
      menu.
   3. If you want to change the name used for the bookmark, type the new name
      in the field.
   4. Click on the Place radio button (or press the Up or Down Arrow key to
      select it).
   5. Click on OK (or select it and press Enter). The bookmark is then added to
      the bookmark list.


ΓòÉΓòÉΓòÉ 1.5.2. Searching for Information ΓòÉΓòÉΓòÉ

You can specify a word or phrase to be searched.  You can also limit the search
to a set of topics by first marking the topics in the Contents list.

To search for a word or phrase in all topics, do the following:

   1. Select the Search option from the Services menu.
   2. Type the word or words to be searched for.
   3. Click on All sections (or press the Up or Down Arrow keys to select it).
   4. Click on Search (or select it and press Enter) to begin the search.
   5. The list of topics where the word or phrase appears is displayed.


ΓòÉΓòÉΓòÉ 1.5.3. Printing Information ΓòÉΓòÉΓòÉ

You can print one or more topics, the index, or the table of contents.  Make
sure that your printer is connected to the serial port, configured correctly,
and ready for input. To print:

   1. Select Print from the Services pull-down menu.
   2. Select what you want to print. Note that the This section and Marked
      sections choices are only available if you are viewing a topic or if you
      have marked topics, respectively.  To mark topics in the table of
      contents, press the Ctrl key and click on the topics, or use the arrow
      keys.
   3. Select Print to print what you've chosen.


ΓòÉΓòÉΓòÉ 1.5.4. Copying Information to a File ΓòÉΓòÉΓòÉ

You can copy a topic that you are viewing in two ways:

     Copy copies the topic that you are viewing into the System Clipboard.

     Copy to file copies the topic that you are viewing into a temporary file
      named text.tmp.  You can later edit that file by using any editor. The
      text.tmp file is placed in the directory where your viewable document
      resides. This copy function does not apply if you are viewing the
      document from the CD-ROM.

 To copy a topic, do the following:

   1. Expand the Contents list and select a topic.
   2. When the topic appears, select Copy to file from the Services menu.
   3. The system puts the text pertaining to that topic into the temporary file
      text.tmp.


ΓòÉΓòÉΓòÉ 1.6. How to Cut and Paste Examples ΓòÉΓòÉΓòÉ

You can copy examples (or information) from this reference/guide/book to
compile, link, and run them, or to paste them into your own code.

To copy an example or information:

   1. Make the topic you want to copy the active window.

   2. From the Services menu, select Copy to file. The text in that topic is
      placed in the temporary file text.tmp, in the same directory as this
      reference. This copy function does not apply if you are viewing the
      document from the CD-ROM.

   3. You can then modify or use text.tmp as you want.

 Note:  Because the system copies the entire contents of the topic to the file,
 you may need to edit it to remove additional text. Most examples in this
 reference are ready to compile, link, and run as they appear, and do not
 require any editing.


ΓòÉΓòÉΓòÉ 1.7. Other Information You Might Find Helpful ΓòÉΓòÉΓòÉ

This product provides a number of online guides and references that we hope
you'll find helpful as you develop applications. This information includes
User's Guides, References, and How Do I help that gives you specific
instructions for performing common tasks. You can get to this online
information from the Information folder inside the main product folder.  You
can also get to it from the Help menu in any of the components of the product.


ΓòÉΓòÉΓòÉ 1.8. Communicating Your Comments to IBM ΓòÉΓòÉΓòÉ

If there is something you like, or dislike, about this book, please let us
know.  You can use one of the methods listed below to send your comments to
IBM.  Please be sure to include the complete title of the publication that you
are commenting on.

The comments you send should only pertain to the information in this document
and its presentation.  To request additional publications or to ask questions
or make comments about the functions of IBM products or systems, you should
talk to your IBM representative or your authorized IBM remarketer.

When you send comments to IBM, you grant IBM a nonexclusive right to use or
distribute your comments in any way it believes appropriate without incurring
any obligation to you.

You can send your comments to IBM in the following ways:

     By mail to the following address:

            IBM Canada Ltd. Laboratory
            Information Development
            2G/345/1150/TOR
            1150 EGLINTON AVENUE EAST
            NORTH YORK, ONTARIO
            CANADA M3C 1H7

     By FAX to the following number:

        -  United States and Canada: [phone]
        -  Other countries ([phone]

     By electronic mail to one of the following IDs.  Be sure to include your
      entire network address if you wish to get a reply.

        -  Internet: [e-mail]
        -  IBMLink: toribm(torrcf)
        -  IBM/PROFS: torolab4(torrcf)
        -  IBMMAIL: ibmmail(caibmwt9)


ΓòÉΓòÉΓòÉ 2. Introduction to the SOMobjects Developer Toolkit ΓòÉΓòÉΓòÉ

This section contains:

     Background
     Introducing SOM and the SOMobjects Toolkit
     What's New in SOMobjects Version 2.1
     Overview of this book


ΓòÉΓòÉΓòÉ 2.1. Background ΓòÉΓòÉΓòÉ

Object-oriented programming(OOP) is an important new programming technology
that offers expanded opportunities for software reuse and extensibility.
Object-oriented programming shifts the emphasis of software development away
from functional decomposition and toward the recognition of units (called
objects) that encapsulate both code and data.  As a result, programs become
easier to maintain and enhance. Object-oriented programs are typically more
impervious to the ripple effects of subsequent design changes than their
non-object-oriented counterparts. This, in turn, leads to improvements in
programmer productivity.

Despite its promise, penetration of object-oriented technology to major
commercial software products has progressed slowly because of certain
obstacles.  This is particularly true of products that offer only a binary
programming interface to their internal object classes (Example: products that
do not allow access to source code).

The first obstacle that developers must confront is the choice of an
object-oriented programming language.

So-called pure object-oriented language (such as Smalltalk) presume a complete
run-time environment (sometimes known as a virtual machine), because their
semantics represent a major departure from traditional, procedure-oriented
system architectures.  So long as the developer works within the supplied
environment, everything works smoothly and consistently.  When the need arises
to interact with foreign environment, however (for example, to make an external
procedure call), the pure-object paradigm ends, and objects must be reduced to
data structures for external manipulation.  Unfortunately, data structures do
not retain the advantage that objects offer with regard to encapsulation and
code reuse.

Hybrid languages such as C++ on the other hand, require less run-time support,
but sometimes result in tight bindings between programs that implement objects
(called "class libraries") and their clients (the programs that use them). That
is, implementation detail is often unavoidably compiled into the client
programs.  Tight binding between class libraries and their clients means that
client programs often must be recompiled whenever simple changes are made in
the library.  Furthermore, no binary standard exists for C++ objects, so the
C++ class libraries produced by one C++ compiler cannot (in general) be used
from C++ programs built with a different C++ compiler.

The second obstacle developers of object-oriented software must confront is
that, because different object-oriented languages and toolkits embrace
incompatible models of what objects are and how they work, software developed
using a particular language or toolkit is naturally limited in scope.  Classes
implemented in one language cannot be readily used from another.  A C++
programmer, for example, cannot easily use classes developed in Smalltalk, nor
can a Smalltalk programmer make effective use of C++ classes. Object-oriented
language and toolkit boundaries become, in effect, barriers to
interoperability.

Ironically, no such barrier exists for ordinary procedure libraries. Software
developers routinely construct procedure libraries that can be shared across a
variety of languages, by adhering to standard linkage conventions.
Object-oriented class libraries are inherently different in that no binary
standards or conventions exist to derive a new class from an existing one,  or
even to invoke a method in a standard way.  Procedure libraries also have the
benefit that their implementations can be freely changed without requiring
client programs to be recompiled, unlike the situation for C++ class libraries.

For developers who need to provide binary class libraries, these are serious
obstacles. In an era of open systems and heterogeneous networking, a
single-language solution is frequently not broad enough.  Certainly, mandating
a specific compiler from a specific vendor in order to use a class library
might be grounds not to include the class library with an operating system or
other general-purpose product.

The System Objects Model (SOM) is IBM's solution to these problems.


ΓòÉΓòÉΓòÉ 2.2. Introducing SOM and the SOMobjects Toolkit ΓòÉΓòÉΓòÉ

The System Object Model (SOM) is a new object-oriented programming technology
for building, packaging, and manipulating binary class libraries.

     With SOM, class implementers describe the interface for a class of
      objects (names of the methods it supports, the return types, parameter
      types, and so forth) in a standard language called the Interface
      Definition Language (IDL).

     They then implement methods in their preferred programming language
      (which may be either an object-oriented programming language or a
      procedural language such as C).

 This means that programmers can begin using SOM quickly, and also extends the
 advantages of OOP to programmers who use non-object-oriented programming
 languages.

 A principal benefit of using SOM is that SOM accommodates changes in
 implementation details and even in certain facets of a class ' interface,
 without breaking the binary interface to a class library and without requiring
 recompilation of client programs.  As a rule of thumb , if changes to a SOM
 class do not require source-code changes in client programs, then those client
 programs will not need to be recompiled. This is not true of many
 object-oriented languages, and it is one of the chief benefits of using SOM.
 For instance, SOM classes can undergo structural changes such as the
 following, yet retain full backward, binary compatibility:

     Adding new methods,

     Changing the size of an object by adding or deleting instance variables,

     Inserting new parent (base) classes above a class in the inheritance
      hierarchy, and

     Relocating methods upward in the class hierarchy.

 In short, implementers can make the typical kinds of changes to an
 implementation and its interfaces that evolving software systems experience
 over time.

 Unlike the object models found in formal object-oriented programming
 languages, SOM is language-neutral.  It preserves the key OOP characteristics
 of encapsulation, inheritance, and polymorphism, without requiring that the
 user of a SOM class and the implementer of a SOM class use the same
 programming language.  SOM is said to be language-neutral for four reasons:

   1. All SOM interactions consist of standard procedure calls. On systems that
      have a standard linkage convention for system calls, SOM interactions
      conform to those conventions. Thus, most programming languages that can
      make external procedure calls can use SOM.

   2. The form of the SOM Application Programming Interface, or API (the way
      that programmers invoke methods, create objects, and so on) can vary
      widely from language to language, as a benefit of the SOM bindings.
      Bindings are a set of macros and procedure calls that make implementing
      and using SOM classes more convenient by tailoring the interface to a
      particular programming language.

   3. SOM supports several mechanisms for method resolution that can be readily
      mapped into the semantics of a wide range of object-oriented programming
      languages. Thus, SOM class libraries can be shared across object-oriented
      languages that have differing object models.  A SOM object can
      potentially be accessed with three different forms of method resolution:

          Offset resolution: roughly equivalent to the C++ virtual function
           concept. Offset resolution implies a static scheme for typing
           objects, with polymorphism based strictly on class derivation.  It
           offers the best performance characteristics for SOM method
           resolution.  Methods accessible through offset resolution are called
           static methods, because they are considered a fixed aspect of an
           object's interface.

          Name-lookup resolution: similar to that employed by Objective-C and
           Smalltalk.  Name resolution supports untyped (sometimes called
           dynamically typed) access to objects, with polymorphism based on the
           actual protocols that objects honor. Name resolution offers the
           opportunity to write code to manipulate objects with little or no
           awareness of the type or shape of the object when the code is
           compiled.

          Dispatch-function resolution: a unique feature of SOM that permits
           method resolution based on arbitrary rules known only in the domain
           of the receiving object. Languages that require special entry or
           exit sequences or local objects that represent distributed object
           domains are good candidates for using dispatch-function resolution.
           This technique offers the highest degree of encapsulation for the
           implementation of an object, with some cost in performance.

   4. SOM conforms fully with the Object Management Group's (OMG) Common Object
      Request Broker Architecture (CORBA) standards. (OMG is an industry
      consortium founded to advance the use of object technology in
      distributed, heterogeneous environments.) In particular,

          Interface to SOM classes are described in CORBA's Interface
           Definition Language, IDL, and the entire SOMobjects Toolkit supports
           all CORBA-defined data types.

          The SOM bindings for the C language are compatible with the C
           bindings prescribed by CORBA.

          All information about the interface to a SOM class is available at
           run time through a CORBA-defined Interface Repository.

 SOM is not intended to replace existing object-oriented languages. Rather, it
 is intended to complement them so that application programs written in
 different programming languages can share common SOM class libraries. For
 example, SOM can be used with C++ to do the following:

     Provide upwardly compatible class libraries, so that when a new version
      of a SOM class is released, client code needn't be recompiled, so long as
      no changes to the client's source code are required.

     Allow other language users (and other C++ compiler users) to use SOM
      classes implemented in C++.

     Allow C++ programs to use SOM classes implemented using other languages.

     Allow other language users to implement SOM classes derived from SOM
      classes implemented in C++.

     Allow C++ programmers to implement SOM classes derived from SOM classes
      implemented using other languages.

     Allow encapsulation (implementation hiding) so that SOM class libraries
      can be shared without exposing private instance variables and methods.

     Allow dynamic (run-time) method resolution in addition to static
      (compile-time) method resolution (on SOM objects).

     Allow information about classes to be obtained and updated at run time.
      (C++ classes are compile-time structures that have no properties at run
      time.)


ΓòÉΓòÉΓòÉ 2.2.1. The SOM Compiler ΓòÉΓòÉΓòÉ

The SOMobjects Toolkit contains a tool called the SOM Compiler that enables
implementers to build classes in which interface and implementation are
decoupled. The SOM Compiler reads the IDL definition of a class interface and
generates:

     an implementation skeleton for the class,

     bindings for implementers, and

     bindings for client programs.

 Bindings are language-specific macros and procedures that make implementing
 and using SOM classes more convenient. These bindings offer a convenient
 interface to SOM that is tailored to a particular programming language.  For
 instance, C programmers can invoke methods in the same way they make ordinary
 procedure calls. The C++ bindings wrap SOM objects as C++ objects, so that C++
 programmers can invoke methods on SOM objects in the same way they invoke
 methods on C++ objects. In addition, SOM objects receive full C++
 typechecking, just as C++ objects do.  Currently, the SOM Compiler can
 generate both C and C++ language bindings for a class. The C and C++ bindings
 will work with a variety of commercial products available from IBM and others.
 Vendors of other programming languages may also offer SOM bindings.  Check
 with your language vendor about possible SOM support.


ΓòÉΓòÉΓòÉ 2.2.2. The SOM Run-Time Library ΓòÉΓòÉΓòÉ

In addition to the SOM Compiler, SOM includes a run-time library. This library
provides, among other things, a set of classes, methods, and procedures used to
create object and invoke methods on them.  The library allows any programming
language to use SOM classes (classes developed using SOM) if that language can:

     Call external procedures,

     Store a pointer to a procedure and subsequently invoke that procedure,
      and

     Map IDL types onto the programming language's native types.

 Thus, the user of a SOM class and the implementer of a SOM class needn't use
 the same programming language, and neither is required to use an
 object-oriented language.  The independence of client language and
 implementation language also extends to subclassing: a SOM class can be
 derived from other SOM classes, and the subclass may or may not be implemented
 in the same language as the parent class(es). Moreover, SOM's run-time
 environment allows applications to access information about classes
 dynamically (at run time).


ΓòÉΓòÉΓòÉ 2.2.3. Frameworks in the SOMobjects Toolkit ΓòÉΓòÉΓòÉ

In addition to SOM itself (the SOM compiler and the SOM run-time library), the
SOMobjects Developer Toolkit also provides a set of frameworks (class
libraries) that can be used in developing object-oriented applications.  These
include Distributed SOM, the Interface Repository Framework, and the Emitter
Framework, and the Metaclass Framework, described below.

Note:  The SOMobjects Base Toolkit, the core version of SOMobject shipped with
       this documentation, contains only Distributed SOM, the Interface
       Repository, and the Metaclass frameworks. The complete SOMobjects
       Developer Toolkit can be ordered from IBM by calling 1-800-342-6672 in
       the U.S. or 1-800-465-7999 in Canada. The part number is 10H9767 for the
       CD-ROM containing SOMobjects for the AIX, OS/2,and Windows 3.x platforms
       (including online documentation). SOMobjects manuals are available
       individually.


ΓòÉΓòÉΓòÉ 2.2.3.1. Distributed SOM ΓòÉΓòÉΓòÉ

Distributed SOM (DSOM) allows application programs to access SOM objects across
address spaces. That is, application programs can access objects in other
processes. The location and implementation of the object are hidden from the
client, and the client accesses the object as if local.  The current release of
DSOM supports distribution of objects among processes within a workstation.


ΓòÉΓòÉΓòÉ 2.2.3.2. Interface Repository Framework ΓòÉΓòÉΓòÉ

The Interface Repository is a database, optionally created and maintained by
the SOM Compiler, that holds all the information contained in the IDL
description of a class of objects. The Interface Repository Framework consists
of the 11 classes defined in the CORBA standard for accessing the Interface
Repository. Thus, the Interface Repository Framework provides run-time access
to all information contained in the IDL description of a class of objects. Type
information is available as TypeCodes-a CORBA-defined way of encoding the
complete description of any data type that can be constructed in IDL.


ΓòÉΓòÉΓòÉ 2.2.3.3. Metaclass Framework ΓòÉΓòÉΓòÉ

Finally, the Metaclass Framework is a collection of SOM metaclasses that
provide functionality that can be useful to SOM class designers for modifying
the default semantics of method invocation and object creation. These
metaclasses are described in The Metaclass Framework.


ΓòÉΓòÉΓòÉ 2.3. What's New in SOMobjects Version 2.1 ΓòÉΓòÉΓòÉ

Version 2.1 of the SOMobjects Developer Toolkit provides enhanced capabilities
and improved performance for both SOM and DSOM. In addition, the Toolkit now
includes support for DirectToSOM (DTS) C++ compilers. New metaclasses in the
Metaclass Framework allow class implementors to define classes that
automatically possess certain convenience facilities.

The enhancements provided with SOMobjects Version 2.1 are described in detail
in the following sections.


ΓòÉΓòÉΓòÉ 2.3.1. General Enhancements ΓòÉΓòÉΓòÉ

C++ programmers can use DirectToSOM (DTS) C++ compilers (available from
independent software vendors) as an alternative to the SOMobjects Toolkit's C++
bindings. (A DTS C++ compiler uses SOM classes to implement C++ objects.) The
support provided by SOMobjects for DTS C++ consists of various enhancements to
the SOM API (useful to SOM programmers in general), and a new emitter that
produces DTS C++ header files corresponding to SOM IDL. DTS C++ programs
include ".hh" header files, which are emitted as described in Generating
binding files.


ΓòÉΓòÉΓòÉ 2.3.2. SOMobjects Enhancements ΓòÉΓòÉΓòÉ

A new default process enables SOMobjects to initialize and destroy objects more
efficiently (using the somDefaultInit and somDestruct methods).

A new kind of method is supported, nonstatic, that is similar to a C++
nonstatic member function. It is normally invoked using offset resolution but
can use any form of SOMobjects method resolution. For more information, see a
description of the nonstatic modifier in Modifier statements.

A new kind of data, staticdata, is supported that is similar to C++ static data
members. A staticdata variable is not stored in an object; rather, the
ClassData structure of the implementing class contains a pointer to the
variable. For a description, see Modifier statements.

Two new modifiers, somallocateand somdeallocate, can be used to indicate that a
user-written procedure should be executed to allocate or deallocate memory for
class instances when the somAllocate or somDeallocate method is invoked. For
descriptions of these modifiers, see Modifier statements.

The capability to cast objects is added. See the methods somCastObj and
somResetObj.

Support is provided for loading and unloading class libraries. This was first
introduced in SOMobjects for Windows 3.x and is now available for Windows NT
and Windows 95. See the SOM_ClassLibrary macro in the SOM Programming Reference


ΓòÉΓòÉΓòÉ 2.3.3. DSOM Enhancements ΓòÉΓòÉΓòÉ

New SOM IDL modifiers are provided for memory management of parameters:
memory_management= corba, caller_owns_parameters, caller_owns_result,
object_owns_parameters, and object_owns_result. The individual modifiers are
described in Modifier statements. For information about memory management, see
Memory management. This memory-management support also includes the new method
somdReleaseResources and the functions  somdExceptionFree and SOMD_NoORBfree,
described in the SOM Programming Reference.

Support is added for the CORBA constant OBJECT_NIL. In addition, the is_nil
method of SOMDObject can now be used for local objects as well as NULL
pointers.

Support is added for passing self-referential structs and unions (those valid
in IDL) in remote method calls.

Support is added for local/remote transparency in DSOM's object-destruction
methods. See "Destroying remote objects."

Users can now define customized base proxy classes. See "Customizing the
default base proxy class."

Users can now specify an upper limit on the number of threads that a server can
spawn, using the SOMDNUMTHREADS environment variable. See this variable under
"Preparing the environment."

Improvements have been made in error handling and performance.

Additional sample programs are available with the SOMobjects Toolkit.


ΓòÉΓòÉΓòÉ 2.3.4. Metaclass Framework ΓòÉΓòÉΓòÉ

A SOMMBeforeAfter metaclass enables the programming of "before/after"
metaclasses, whose instances execute a particular method before and after each
method invocation. See The Metaclass Framework for information about the new
Metaclass Framework. Individual metaclasses, along with related classes and
methods, are documented in the SOM Programming Reference.

A utility metaclass-SOMMTraced-is provided for tracing.


ΓòÉΓòÉΓòÉ 2.4. New Restrictions and Deprecated Methods ΓòÉΓòÉΓòÉ

While implementing the Metaclass Framework, IBM learned that metaclasses must
be programmed so that the capabilities they implement will be preserved when
various metaclasses are combined (using multiple inheritance) into a
SOM-derived metaclass. To assure this result, the Metaclass Framework
metaclasses have been programmed using a Cooperative Metaclass. However, IBM is
not yet ready to include the Cooperative Metaclass among the officially
supported features of SOMobjects.

To prevent user-defined metaclasses from interfering with the operation of the
Cooperative Metaclass and consequently with the Metaclass Framework, SOMobjects
programmers are strongly urged to observe the following restriction when
programming new metaclasses:

      User-defined metaclasses can introduce new class methods and class
      variables, but should not override any of the methods introduced by the
      SOMClass class.

 SOMobjects users whose metaclass programming requirements cannot be met within
 the above restrictions will be given access to the Cooperative Metaclass and
 its documentation. Note, however, that metaclasses developed using the
 Cooperative Metaclass may require reprogramming when an officially supported
 Cooperative Metaclass is later introduced.

 In addition, use of a number of (public) methods introduced by SOMClass is now
 deprecated because they are useful only from overridden SOMClass methods.
 These methods are listed under the heading "Deprecated methods" in the
 documentation for SOMClass within the SOM Programming Reference, until such
 time as SOMobjects is ready to officially provide a framework within which
 their use will not interfere with the internal operation of SOMobjects itself.

 The aforementioned deprecated methods are not available in the Windows NT/95
 Version of SOMObjects.


ΓòÉΓòÉΓòÉ 3. Tutorial for Implementing SOM Classes ΓòÉΓòÉΓòÉ

This tutorial contains five examples showing how SOM classes can be implemented
to achieve various functionality. Obviously, for any person who wishes to
become a class implementor, this tutorial is essential. However, even for those
programmers who expect only to use SOM classes that were implemented by others,
the tutorial is also necessary, as it presents several concepts that will help
clarify the process of using SOM classes.


ΓòÉΓòÉΓòÉ 3.1. Basic Concepts of the System Object Model (SOM) ΓòÉΓòÉΓòÉ

The System Object Model (SOM), provided by the SOMobjects Developer Toolkit, is
a set of libraries, utilities, and conventions used to create binary class
libraries that can be used by application programs written in various
object-oriented programming languages, such as C++ and Smalltalk, or in
traditional procedural languages, such as C and Cobol. The following paragraphs
introduce some of the basic terminology used when creating classes in SOM:

     An object is an OOP entity that has behavior (its methods or operations)
      and state (its data values).  In SOM, an object is a run-time entity with
      a specific set of methods and instance variables.  The methods are used
      by a client programmer to make the object exhibit behavior (that is, to
      do something), and the instance variables are used by the object to store
      its state.  (The state of an object can change over time, which allows
      the object's behavior to change.)  When a method is invoked on an object,
      the object is said to be the receiver or target of the method call.

     An object's implementation is determined by the procedures that execute
      its methods, and by the type and layout of its instance variables.  The
      procedures and instance variables that implement an object are normally
      encapsulated  (hidden from the caller), so a program can use the object's
      methods without knowing anything about how those methods are implemented.
      Instead, a user is given access to the object's methods through its
      interface (a description of the methods in terms of the data elements
      required as input and the type of value each method returns).

     An interface through which an object may be manipulated is represented by
      an object type.  That is, by declaring a type for an object variable, a
      programmer specifies the interface that is intended to be used to access
      that object.  SOM IDL (the SOM Interface Definition Language) is used to
      define object interfaces.  The interface names used in these IDL
      definitions are also the type names used by programmers when typing SOM
      object variables.

     In SOM, as in most approaches to object-oriented programming, a class
      defines the implementation of objects. That is, the implementation of any
      SOM object (as well as its interface) is defined by some specific SOM
      class. A class definition begins with an IDL specification of the
      interface to its objects, and the name of this interface is used as the
      class name as well. Each object of a given class may also be called an
      instance of the class, or an instantiation of the class.

     Inheritance, or class derivation, is a technique for developing new
      classes from existing classes. The original class is called the base
      class, or the parent class, or sometimes the direct ancestor class. The
      derived class is called a child class or a subclass.  The primary
      advantage of inheritance is that a derived class inherits all of its
      parent's methods and instance variables.  Also through inheritance, a new
      class can override (or redefine) methods of its parent, in order to
      provide enhanced functionality as needed.  In addition, a derived class
      can introduce new methods of its own. If a class results from several
      generations of successive class derivation, that class "knows" all of its
      ancestors's methods (whether overridden or not), and an object (or
      instance) of that class can execute any of those methods.

     SOM classes can also take advantage of multiple inheritance, which means
      that a new class is jointly derived from two or more parent classes.  In
      this case, the derived class inherits methods from all of its parents
      (and all of its ancestors), giving it greatly expanded capabilities.  In
      the event that different parents have methods of the same name that
      execute differently, SOM provides ways for avoiding conflicts.

     In the SOM run time, classes are themselves objects.  That is, classes
      have their own methods and interfaces, and are themselves defined by
      other classes. For this reason, a class is often called a class object.
      Likewise, the terms class methods and class variables are used to
      distinguish between the methods/variables of a class object vs. those of
      its instances.  (Note that the type of an object is not the same as the
      type of its class, which as a "class object" has its own type.)

     A class that defines the implementation of class objects is called a
      metaclass.  Just as an instance of a class is an object, so an instance
      of a metaclass is a class object.  Moreover, just as an ordinary class
      defines methods that its objects respond to, so a metaclass defines
      methods that a class object responds to.  For example, such methods might
      involve operations that execute when a class (that is, a class object) is
      creating an instance of itself (an object).  Just as classes are derived
      from parent classes, so metaclasses can be derived from parent
      metaclasses, in order to define new functionality for class objects.

     The SOM system contains three primitive classes that are the basis for
      all subsequent classes:

      SOMObject           the root ancestor class for all SOM classes,

      SOMClass            the root ancestor class for all SOM metaclasses, and

      SOMClassMgr         the class of the SOMClassMgrObject, an object created
                          automatically during SOM initialization, to maintain
                          a registry of existing classes and to assist in
                          dynamic class loading/unloading.

      SOMClass is defined as a subclass (or child) of SOMObject and inherits
      all generic object methods; this is why instances of a metaclass are
      class objects  (rather than simply classes) in the SOM run time.

 SOM classes are designed to be language neutral. That is, SOM classes can be
 implemented in one programming language and used in programs of another
 language. To achieve language neutrality, the interface for a class of objects
 must be defined separately from its implementation. That is, defining
 interface and implementation requires two completely separate steps (plus an
 intervening compile), as follows:

     An interface is the information that a program must know in order to use
      an object of a particular class. This interface is described in an
      interface definition (which is also the class definition), using a formal
      language whose syntax is independent of the programming language used to
      implement the class's methods. For SOM classes, this is the SOM Interface
      Definition Language (SOM IDL). The interface is defined in a file known
      as the IDL source file (or, using its extension, this is often called the
      .idl file).

      An interface definition is specified within the interface declaration (or
      interface statement) of the .idl file, which includes:

      (a)        the interface name (or class name) and the name(s) of the
                 class's parent(s), and

      (b)        the names of the class's attributes and the signatures of its
                 new methods. (Recall that the complete set of available
                 methods also includes all inherited methods.)

      Each method signature includes the method name, and the type and order of
      its arguments, as well as the type of its return value (if any).
      Attributes are instance variables for which "set" and "get" methods will
      automatically be defined, for use by the application program.  (By
      contrast, instance variables that are not attributes are hidden from the
      user.)

     Once the IDL source file is complete, the SOM Compiler is used to analyze
      the .idl file and create the implementation template file, within which
      the class implementation will be defined. Before issuing the SOM Compiler
      command, sc, the class implementor can set an environment variable that
      determines which emitters (output-generating programs) the SOM Compiler
      will call and, consequently, which programming language and operating
      system the resulting binding files will relate to. (Alternatively, this
      emitter information can be placed on the command line for sc.)  In
      addition to the implementation template file itself, the binding files
      include two language-specific header files that will be #included in the
      implementation template file and in application program files. The header
      files define many useful SOM macros, functions, and procedures that can
      be invoked from the files that include the header files.

     The implementation of a class is done by the class implementor in the
      implementation template file (often called just the implementation file
      or the template file).  As produced by the SOM Compiler, the template
      file contains stub procedures  for each method of the class.  These are
      incomplete method procedures that the class implementor uses as a basis
      for implementing the class by writing the corresponding code in the
      programming language of choice.

 In summary, the process of implementing a SOM class includes using the SOM IDL
 syntax to create an IDL source file that specifies the interface to a class of
 objects - that is, the methods and attributes that a program can use to
 manipulate an object of that class.  The SOM Compiler is then run to produce
 an implementation template file and two binding (header) files that are
 specific to the designated programming language and operating system. Finally,
 the class implementor writes language-specific code in the template file to
 implement the method procedures.

 At this point, the next step is to write the application (or client)
 program(s) that use the objects and methods of the newly implemented class.
 (Observe, here, that a programmer could write an application program using a
 class implemented entirely by someone else.)  If not done previously, the SOM
 compiler is run to generate usage bindings for the new class, as appropriate
 for the language used by the client program (which may be different from the
 language in which the class was implemented). After the client program is
 finished, the programmer compiles and links it using a language-specific
 compiler, and then executes the program. (Notice again, the client program can
 invoke methods on objects of the SOM class without knowing how those methods
 are implemented.)


ΓòÉΓòÉΓòÉ 3.1.1. Development of the Tutorial examples ΓòÉΓòÉΓòÉ

     Example 1-Implementing a simple class with one method
      Prints a default message when the "sayHello" method is invoked on an
      object of the "Hello" class.

     Example 2-Adding an attribute to the Hello class
      Defines a "msg" attribute for the "sayHello" method to use. The client
      program "sets" a message; then the "sayHello" method "gets" the message
      and prints it. (There is no defined message when an object of the "Hello"
      class is first created.)

     Example 3- Overriding an inherited method
      Overrides the SOMobjects method somPrintSelf so that invoking this method
      on an object of the "Hello" class will not only display the class name
      and the object's location, but will also include the object's message
      attribute.

     Example 4-Initializing a SOM object.
      Overrides the default initialization method,  somDefaultInit, to
      illustrate how an object's instance variables can be initialized when the
      object is created.

     Example 5-Using multiple inheritance
      Extends the "Hello" class to provide it with multiple inheritance (from
      the "Disk;" and "Printer" classes.) The "Hello" interface defines an enum
      and an "output" attribute that takes its value from the enum (either
      "screen," "printer," or "disk"). The client program "sets" the form of
      "output" before invoking the "sayHello" method to send a "msg"(defined as
      in Example 4).


ΓòÉΓòÉΓòÉ 3.2. Basic Steps for Implementing SOM Classes ΓòÉΓòÉΓòÉ

Implementing and using SOM classes in C or C++ involves the following steps,
which are explicitly illustrated in the examples of this tutorial:

   1. Define the interface to objects of the new class (that is, the interface
      declaration), by creating a .idl file.

   2. Run the SOM Compiler on the .idl file by issuing the sc command to
      produce the following binding files:

          Template implementation file


           a .c file for C programmers, or


           a .cpp file for C++ programmers;

          Header file to be included in the implementation file


           a .ih file for C programmers, or


           a .xih file for C++ programmers; and

          Header file to be included in client programs that use the class


           a .h file for C clients, or


           a .xh file for C++ clients.

      To specify whether the SOM Compiler should produce C or C++ bindings, set
      the value of the SMEMIT environment variable or use the "-s" option of
      the sc or somc command, as described in Section 4.3, "The SOM Compiler."
      By default, the SOM Compiler produces C bindings.

   3. Customize the implementation, by adding code to the template
      implementation file.

   4. Create a client program that uses the class.

   5. Compile and link the client code with the class implementation, using a C
      or C++ compiler.

   6. Execute the client program.

 The following examples illustrate appropriate syntax for defining interface
 declarations in a .idl file, including designating the methods that the
 class's instances will perform.  In addition, example template implementation
 files contain typical code that the SOM Compiler produces. Explanations
 accompanying each example discuss topics that are significant to the
 particular example; full explanations of the SOM IDL syntax are contained in
 Chapter 4, "SOM IDL and the SOM Compiler." Customization of each
 implementation file (step 3) is illustrated in C and C++.

 Note:

   1. The Tutorial assumes you will work through the examples in order. If you
      do not do so, the code that the SOM Compiler generates from your revised
      .idl file may vary slightly from what you see in the Tutorial.

   2. When the SOMobjects Toolkit is installed, a choice is made between
      "somcorba" and "somstars" for the style of C bindings the SOM Compiler
      will generate.  The Tutorial examples use the "somcorba" style, where an
      interface name used as a type indicates a pointer to an object, as
      required by strict CORBA bindings.  Consequently, as the examples show, a
      "*" does not explicitly appear for types that are pointers to objects.
      If your system was installed for "somstars" C bindings, you can set the
      environment variable SMADDSTAR=1 or use the SOM Compiler option
      "-maddstar" to request bindings that use explicit pointer stars. For more
      information, see "Declaring object variables" in Chapter 3, "Using SOM
      Classes in Client Programs" and "Object types" in Chapter 4, "SOM IDL and
      the SOM Compiler."


ΓòÉΓòÉΓòÉ 3.2.1. Example 1-Implementing a Simple Class with One Method ΓòÉΓòÉΓòÉ

Example 1 defines a class "Hello" which introduces one new method, "sayHello".
When invoked from a client program, the "sayHello" method will print the fixed
string "Hello, World!"  The example follows the six steps described in the
preceding topic, "Basic Steps for Implementing SOM Classes."

   1. Define the interface to class "Hello", which inherits methods from the
      root class SOMObject and introduces one new method, "sayHello". Define
      these IDL specifications in the file "hello.idl".

      The "interface" statement introduces the name of a new class and any
      parents (base classes) it may have (here, the root class SOMObject).  The
      body of the interface declaration introduces the method "sayHello."
      Observe that method declarations in IDL have syntax similar to C and C++
      function prototypes:

            #include <somobj.idl>  //# Get the parent class definition.

            interface Hello : SOMObject
            /*  This is a simple class that demonstrates how to define the
             *  interface to a new class of objects in SOM IDL.
             */
            {
                void sayHello();
                // This method outputs the string "Hello, World!".
                   /* On Windows, use: string sayHello();
                    * This method returns the string "Hello, World!". */
            };

      Note that the method "sayHello" has no (explicit) arguments and returns
      no value (except on Windows, which returns a string). The characters "//"
      start a line comment, which finishes at the end of the line.  The
      characters "/*" start a block comment which finishes with the "*/". Block
      comments do not nest. The two comment styles can be used interchangeably.
      Throw-away comments are also permitted in a .idl file; they are ignored
      by the SOM Compiler.  Throw-away comments start with the characters "//#"
      and terminate at the end of the line.

      Note:  For simplicity, this IDL fragment does not include a releaseorder
             modifier; consequently, the SOM Compiler will issue a warning for
             the method "sayHello". For directions on using the releaseorder
             modifier to remove this warning, see the topic "Modifier
             statements" in Chapter 4, "Implementing Classes in SOM."

   2. Run the SOM Compiler to produce binding files and an implementation
      template (that is, issue the sc command):

            > sc -s"c;h;ih" hello.idl      (for C bindings)
            > sc -s"xc;xh;xih" hello.idl   (for C++ bindings)       or OS/2)

      When set to generate C binding files, the SOM Compiler generates the
      following implementation template file, named "hello.c". The template
      implementation file contains stub procedures for each new method.  These
      are procedures whose bodies are largely vacuous, to be filled in by the
      implementor. (Unimportant details have been removed for this tutorial.)

            #include <hello.ih>
            /*
             *  This method outputs the string "Hello, World!".
             */

            SOM_Scope  void   SOMLINK  sayHello(Hello somSelf, Environment *ev)
            {
                /* HelloData *somThis = HelloGetData(somSelf); */
                HelloMethodDebug("Hello", "sayHello");
            }

      The terms SOM_Scope and SOMLINK appear in the prototype for all stub
      procedures, but they are defined by SOM and are not of interest to the
      developer. In the method procedure for the "sayHello" method, "somSelf"
      is a pointer to the target object (here, an instance of the class
      "Hello") that will respond to the method. A "somSelf" parameter appears
      in the procedure prototype for every method, since SOM requires every
      method to act on some object.

      The target object is always the first parameter of a method's procedure,
      although it should not be included in the method's IDL specification. The
      second parameter (which also is not included in the method's IDL
      specification) is the parameter (Environment *ev).  This parameter can be
      used by the method to return exception information if the method
      encounters an error.  (Contrast the prototype for the "sayHello" method
      in steps 1 and 2 above.)

      The remaining lines of the template above are not pertinent at this
      point.  (For those interested, they are discussed in Chapter 5,
      "Implementing SOM Classes.")  The file is now ready for customization
      with the C code needed to implement method "sayHello".

      When set to generate C++ binding files, the SOM Compiler generates an
      implementation template file, "hello.C" (on AIX) or "hello.cpp (on OS/2),
      similar to the one above.  (Chapter 5 discusses the implementation
      template in more detail.)

      Recall that, in addition to generating a template implementation file,
      the SOM Compiler also generates implementation bindings (in a header file
      to be included in the implementation file) and usage bindings (in a
      header file to be included in client programs).  These files are named
      "hello.ih" and "hello.h" for C bindings, and are "hello.xih" and
      "hello.xh" for C++ bindings. Notice that the "hello.c" file shown above
      includes the "hello.ih" implementation binding file.

   3. Customize the implementation, by adding code to the template
      implementation file.

      Modify the body of the "sayHello" method procedure in the "hello.c" (or,
      for C++), implementation file so that the "sayHello"            o" method
      prints "Hello, World!":

            SOM_Scope void   SOMLINK sayHello(Hello somSelf, Environment *ev)
            {
                /* HelloData *somThis = HelloGetData(somSelf); */
                HelloMethodDebug("Hello","sayHello");

                printf("Hello, World!\n");
            }

   4. Create a client program that uses the class.

      Write a program "main" that creates an instance (object) of the "Hello"
      class and invokes the method "sayHello" on that object.

      A C programmer would write the following program in "main.c", using the
      bindings defined in the "hello.h" header file:

            #include <hello.h>

            int main(int argc, char *argv[])
            {
                /* Declare a variable to point to an instance of Hello */
                Hello obj;

                /* Create an instance of the Hello class */
                obj = HelloNew();

                /* Execute the "sayHello" method */
                _sayHello(obj, somGetGlobalEnvironment());

                /* Free the instance: */
                _somFree(obj);
                return (0);
            }

      Notice the statement obj = HelloNew();  The "hello.h" header file
      automatically contains the SOM-defined macro <className>New(), which is
      used to create an instance of the <className> class (here, the "Hello"
      class).

      Also notice that, in C, a method is invoked on an object by using the
      form:

            _<methodName>(<objectName>, <environment_argument>, <other_method_arguments>)

      as used above in the statement _sayHello(obj, somGetGlobalEnvironment()).
      As shown in this example, the SOM-provided somGetGlobalEnvironment
      function can be used to supply the (Environment *) argument of the
      method.

      Finally, the code uses the method somFree, which SOM also provides, to
      free the object created by HelloNew().  Notice that  somFree does not
      require an (Environment *) argument. This is because the method
      procedures for some of the classes in the SOMobjects Toolkit (including
      SOMObject, SOMClass, and SOMClassMgr) do not have an Environment
      parameter, to ensure compatibility with the previous release of SOM.  The
      documentation for each SOM-kernel method in the SOMobjects Developer
      Toolkit: Programmers Reference Manual indicates whether an Environment
      parameter is used.

      A C++ programmer would write the following program in "main.cpp", using
      the bindings defined in the "hello.xh" header file:

            #include <hello.xh>

            int main(int argc, char *argvэ∙)
            {
                /* Declare a variable to point to an instance of Hello */
                Hello *obj;

                /* Create an instance of the Hello class */
                obj = new Hello;

                /* Execute the "sayHello" method */
                obj->sayHello(somGetGlobalEnvironment());

                obj->somFree();
                return (0);
            }

      Notice that the only argument passed to the "sayHello" method by a C++
      client program is the Environment pointer.  (Contrast this with the
      invocation of "sayHello" in the C client program, above.

   5. Compile and link the client code with the class implementation.

      Note: The environment variable SOMBASE represents the directory in which
      SOM has been installed.

      For C programmers:

            > set LIB=%SOMBASE%\lib;%LIB%
            > icc -I. -I%SOMBASE%\include -Fe hello main.c hello.c  somtk.lib

      For C++ programmers:

            > set LIB=%SOMBASE%\lib;%LIB%
            > icc -I. -I\%SOMBASE%\include -Fe hello main.cpp hello.cpp  somtk.lib

   6. Execute the client program.

            > hello
            Hello, World!

      Example 2 will extend the "Hello" class to introduce an "attribute."

      File Extensions for SOM Files

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            Γöé IDL source file:           Γöé .idl   Γöé for all users        Γöé
            Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
            Γöé Implementation template file:    Γöé .c    Γöé for C, all systems      Γöé
            Γöé                   Γöé      Γöé               Γöé
            Γöé                   Γöé      Γöé               Γöé
            Γöé                   Γöé      Γöé               Γöé
            Γöé                   Γöé .cpp   Γöé for C++           Γöé
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ΓòÉΓòÉΓòÉ 3.2.2. Example 2-Adding an Attribute to the Hello class ΓòÉΓòÉΓòÉ

Example 1 introduced a class "Hello" which has a method "sayHello" that prints
the fixed string "Hello, World!"  Example 2 extends the "Hello" class so that
clients can customize the output from the method "sayHello".

   1. Modify the interface declaration for the class definition in "hello.idl."

      Class "Hello" is extended by adding an attribute that we call "msg".
      Declaring an attribute is equivalent to defining "get" and "set" methods.
      For example, specifying:

               attribute string msg;

      is equivalent to defining the two methods:

               string _get_msg();
               void  _set_msg(in string msg);

      Thus, for convenience, an attribute can be used (rather than an instance
      variable) in order to use the automatically defined "get" and "set"
      methods without having to write their method procedures. The new
      interface specification for "Hello" that results from adding attribute
      "msg" to the "Hello" class is as follows (with some comment lines
      omitted):

               #include <somobj.idl>

               interface Hello : SOMObject
               {
                 void  sayHello();

                 attribute string msg;
                      //# This is equivalent to defining the methods:
                      //#     string _get_msg();
                      //#      void  _set_msg(string msg);
               };

   2. Re-run the SOM Compiler on the updated idl file, as in example 1.  This
      produces new header files and updates the existing implementation file,
      if needed, to reflect changes made to the .idl file.  In this example,
      the implementation file is not modified by the SOM Compiler.

   3. Customize the implementation.

      Customize the implementation file by modifying the print statement in the
      "sayHello" method procedure. This example prints the contents of the
      "msg" attribute (which must be initialized in the client program) by
      invoking the "_get_msg" method. Notice that, because the "_get_msg"
      method name begins with an underscore, the method is invoked with two
      leading underscores (for C only).

               SOM_Scope void   SOMLINK sayHello(Hello somSelf, Environment  *ev)
               {
                   /* HelloData *somThis = HelloGetData(somSelf); */
                   HelloMethodDebug("Hello", "sayHello");

                   printf("%s\n", __get_msg(somSelf, ev));

                   /* for  C++, use   somSelf->_get_msg(ev);  */
               }

      This implementation assumes that "_set_msg" has been invoked to
      initialize the "msg" attribute before the "_get_msg" method is invoked by
      the "sayHello" method.  This initialization can be done within the client
      program.

   4. Update the client program.

      Modify the client program so that the "_set_msg" method is invoked to
      initialize the "msg" attribute before the "sayHello" method is invoked.
      Notice that, because the "_set_msg" method name begins with an
      underscore, the C client program invokes the method with two  leading
      underscores.

      For C programmers:

               #include <hello.h>

               int main(int argc, char *argv[])
               {
                   Hello obj;

                   obj = HelloNew();
                   /* Set the msg text */
                   __set_msg(obj, somGetGlobalEnvironment(), "Hello World Again");

                   /* Execute the "sayHello" method */
                   _sayHello(obj, somGetGlobalEnvironment());

                   _somFree(obj);
                   return (0);
               }

      For C++ programmers:

               #include <hello.xh>

               int main(int argc, char *argv[])
               {
                   Hello *obj;
                   obj = new Hello;

                   /* Set the msg text */
                   obj->_set_msg(somGetGlobalEnvironment(), "Hello World Again");

                   /* Execute the "sayHello" method */
                   obj->sayHello(somGetGlobalEnvironment());

                   obj->somFree();
                   return (0);
               }

   5. Compile and link the client program, as before.

   6. Execute the client program:

               > hello
               Hello World Again

      The next example extends the "Hello" class to override (redefine) one of
      the methods it inherits from its parent class, SOMObject.


ΓòÉΓòÉΓòÉ 3.2.3. Attributes vs instance variables ΓòÉΓòÉΓòÉ

As an alternative to defining "numberObjs" as an attribute, it could be defined
as an instance variable, with a "get_numberObjs" method also defined for
retrieving its value.  Instance variables are declared in an implementation
statement, as shown below:

   interface Hello
   {
   string get_msg() ;
   void set_msg(in string msg);

   #ifdef __SOMIDL__
   implementation
   {
       string message;
   };
   #endif
   };

As demonstrated in this example, one disadvantage to using an instance variable
is that the "get_msg" and "set_msg" methods must be defined in the
implementation file by the class implementor. For attributes, by contrast,
default implementations of the "get" and "set" methods are generated
automatically by the SOM Compiler in the .ih and .xih header files.

Note: For some attributes (particularly those involving structures, strings,
and pointers) the default implementation generated by the SOM Compiler for the
"set" method may not be suitable. This happens because the SOM Compiler only
performs a "shallow copy," which typically is not useful for distributed
objects with these types of attributes.  In such cases, it is possible to write
your own implementations, as you do for any other method, by specifying the
"noset/noget" modifiers for the attribute. (See the subtopic "Modifier
statements" in Chapter 4 "SOM IDL and the SOM Compiler.")

Regardless of whether you let the SOM Compiler generate your implementations or
not, if access to instance data is required, either from a subclass or a client
program, then this access should be facilitated by using an attribute.
Otherwise, instance data can be defined in the "implementation" statement as
above (using the same syntax as used to declare variables in C or C++), with
appropriate methods defined to access it. For more information about
"implementation" statements, see the topic "Implementation statements" in
Chapter 4.

As an example where instance variables would be used (rather than attributes),
consider a class "Date" that provides a method for returning the current date.
Suppose the date is represented by three instance variables-"mm", "dd", and
"yy". Rather than making "mm", "dd", and "yy" attributes (and allowing clients
to access them directly), "Date" defines "mm", "dd", and "yy" as instance
variables in the "implementation" statement, and defines a method "get_date"
that converts "mm", "dd", and "yy" into a string of the form "mm/dd/yy":

   interface Date
   {
           string get_date() ;

   #ifdef __SOMIDL__
   implementation
   {
       long mm,dd,yy;
   };
   #endif
   };

To access instance variables that a class introduces from within the class
implementation file, two forms of notation are available:

somThis->variableName

or

_variableName

For example, the implementation for "get_date" would

      access the "mm" instance variable as somThis->mm or _mm,
      access "dd"  as somThis->dd or _dd, and
      access "yy" as somThis->yy or _yy.

 In C++ programs, the _variableName  form is available only if the programmer
 first defines the macro VARIABLE_MACROS (that is, enter #define
 VARIABLE_MACROS) in the implementation file prior to including the .xih file
 for the class.


ΓòÉΓòÉΓòÉ 3.2.4. Example 3-Overriding an Inherited Method ΓòÉΓòÉΓòÉ

An important aspect of OOP programming is the ability of a subclass to replace
an inherited method implementation with a new implementation especially
appropriate to its instances.  This is called overriding a method. Sometimes, a
class may introduce methods that every descendant class is expected to
override.  For example, SOMobjects introduces the somPrintSelf method, and a
good SOM programmer will generally override this method when implementing a new
class.

The purpose of somPrintSelf is to print a brief description of an object.  The
method can be useful when debugging an application that deals with a number of
objects of the same class-assuming the class designer has overridden
somPrintSelf with a message that is useful in distinguishing different objects
of the class.  For example, the implementation of somPrintSelf provided by
SOMobjects simply prints the class of the object and its address in memory.
SOMclass overrides this method so that, when somPrintSelf is invoked on a class
object, the name of the class will print.

This example illustrates how somPrintSelf might be overridden for the "Hello"
class.  An important identifying characteristic of "Hello" objects is the
message they hold; thus, the following steps illustrate how somPrintSelf could
be overridden in "Hello" to provide this information.

   1. Modify the interface declaration in "hello.idl."

      To override the somPrintSelf method in "Hello", additionalinformation
      must be provided in "hello.idl" in the form of an implementation
      statement, which gives extra information about the class, its methods and
      attributes, and any instance variables.  (The previous examples omitted
      the optional "implementation" statement, because it was not needed.)

      In the current example, the "implementation" statement introduces the
      modifiers  for the "Hello" class, as follows.

               #include <somobj.idl>

               interface Hello : SOMObject
               {
                 void sayHello();

                 attribute string msg;

                 #ifdef __SOMIDL__
                 implementation
                 {
                         //# Method Modifiers:
                         somPrintSelf: override;
                         // Override the inherited implementation of somPrintSelf.
                 };
                 #endif

                 };

      Here, "somPrintSelf:" introduces a list of modifiers  for the (inherited)
      somPrintSelf method in the class "Hello".  Modifiers are like C/C ++
      #pragma commands and give specific implementation details to the
      compiler. This example uses only one modifier, "override." Because of the
      "override" modifier, when somPrintSelf is invoked on an instance of class
      "Hello", Hello's implementation of somPrintSelf (in the implementation
      file) will be called, instead of the implementation inherited from the
      parent class, SOMObject.

      The "#ifdef__SOMIDL__" and "#endif" are standard C and C++ preprocessor
      commands that cause the "implementation" statement to be read only when
      using the SOM IDL compiler (and not some other IDL compiler).

   2. Re-run the SOM Compiler on the updated .idl file, as before.  The SOM
      Compiler extends the existing implementation file from Example 2 to
      include new stub procedures as needed (in this case, for somPrintSelf).
      Below is a shortened version of the C language implementation file as
      updated by the SOM Compiler; C++ implementation files are similarly
      revised.  Notice that the code previously added to the "sayHello" method
      is not disturbed when the SOM Compiler updates the implementation file.

               #include <hello.ih>

               SOM_Scope void   SOMLINK sayHello(Hello somSelf, Environment  *ev)
               {
                   /* HelloData *somThis = HelloGetData(somSelf); */
                   HelloMethodDebug("Hello","sayHello");

                   printf("%s\n", __get_msg(somSelf, ev));
               }

               SOM_Scope void   SOMLINK  somPrintSelf(Hello somSelf)
               {
                   HelloData *somThis = HelloGetData(somSelf);
                   HelloMethodDebug("hello","somPrintSelf");

                     Hello_parent_SOMObject_somPrintSelf(somSelf);
               }

      Note that the SOM Compiler added code allowing the "Hello" class to
      redefine somPrintSelf.  The SOM Compiler provides a default
      implementation for overriding the somPrintSelf method.  This default
      implementation simply calls the "parent method" [the procedure that the
      parent class of "Hello" (SOMObject) uses to implement the somPrintSelf
      method]. This parent method call is accomplished by the macro
      Hello_parent_SOMObject_somPrintSelf defined in "hello.ih."

      Notice that the stub procedure for overriding the somPrintSelf method
      does not include an Environment parameter. This is because somPrintSelf
      is introduced by SOMObject, which does not include the Environment
      parameter in any of its methods (to ensure backward compatibility). The
      signature for a method cannot change after it has been introduced.

   3. Customize the implementation.

      Within the new somPrintSelf method procedure, display a brief description
      of the object, appropriate to "Hello" objects.  Note that the parent
      method call is not needed, so it has been deleted.  Also, direct access
      to instance data introduced by the "Hello" class is not required, so the
      assignment to "somThis" has been commented out (see the first line of the
      procedure).

               SOM_Scope void   SOMLINK somPrintSelf(Hello somSelf)
               {
                   HelloData *somThis = HelloGetData(somSelf);
                   HelloMethodDebug("Hello","somPrintSelf");

                 somPrintf("--a %s object at location %x with msg:s\n",
                               _somGetClassName(somSelf),
                               somSelf,
                               __get_msg(somSelf,0));
               }

   4. Update the client program to illustrate the change (also notice the new
      message text):

      For C programmers:

               #include <hello.h>

               int main(int argc, char *argvэ∙)
               {
                   Hello obj;
                   Environment *ev = somGetGlobalEnvironment();

                   obj = HelloNew();

                   /* Set the msg text */
                   __set_msg(obj, ev, "Hi There");

                   /* Execute the "somPrintSelf" method */
                   _somPrintSelf(obj);

                        _somFree(obj);
                        return (0);
               }

      For C++ programmers:

               #include <hello.xh>

               int main(int argc, char *argv[])
               {
                   Hello *obj;
                   Environment *ev = somGetGlobalEnvironment();

                   obj = new Hello;

                   /* Set the msg text */
                   __setmsg(obj, ev, "Hi There");

                  /* Execute the "somPrintSelf" method */
                     obj->somPrintSelf();

                   obj->somFree();
                   return (0);
               }

   5. Compile and link the client program, as before.

   6. Execute the client program, which now outputs the message:

               > hello
                -- a Hello object at location 20062838 with msg: Hi There


ΓòÉΓòÉΓòÉ 3.2.5. Example 4 - Initializing a SOM Object ΓòÉΓòÉΓòÉ

The previous example showed how to override the method somPrintSelf, introduced
by SOMObject. As mentioned in that example, somPrintSelf should generally be
overridden when implementing a new class. Another method introduced by
SOMObject  that should generally be overridden is somDefaultInit. The purpose
of somDefaultInit is to provide a "default" initializer  for the instance
variables introduced by a class.

This example shows how to override somDefaultInit to give each "Hello" object's
message an initial value when the object is first created. To learn more about
initializers than shown in this example (including how to introduce new
initializers that take arbitrary arguments, and how to explicitly invoke
initializers) read "Initializing and Uninitializing Objects," in Chapter 5,
"Implementing Classes in SOM."

The overall process of overriding somDefaultInit is similar to that of the
previous example. First, the IDL for "Hello" is modified. In addition to an
override  modifier, an init modifier is used to indicate that a stub procedure
for an initialization method is desired (the stub procedures for initializers
are different from those of normal methods).

   1. Modify the interface declaration in "hello.idl."


            #include <somobj.idl>

            interface Hello : SOMObject
            {
              void sayHello();

              attribute string msg;

            #ifdef __SOMIDL__
            implementation
            {
                //# Method Modifiers:
                somPrintSelf: override;
                somDefaultInit: override, init;
            };
            #endif

            };

   2. Re-run the SOM Compiler on the updated hello.idl file, as before. SOM
      Compiler extends the existing implementation file. Below is the
      initializer stub procedure that the SOM Compiler adds to the C language
      implementation file; C++ implementation files would be similarly revised:


            SOM_Scope void SOMLINK
                      somDefaultInit(Hello somSelf, somInitCtrl *ctrl)
            {
              HelloData *somThis; /* set by BeginInitializer */
              somInitCtrl globalCtrl;
              somBooleanVector myMask;
              HelloMethodDebug("Hello", "somDefaultInit");
              Hello_BeginInitializer_somDefaultInit;

              Hello_Init_SOMObject_somDefaultInit(somSelf, ctrl);
              /*
               * local Hello initialization code added by programmer
               */
            }

   3. Customize the implementation.

      Here, the "msg" instance variable is set in the implementation template
      (rather than in the client program, as before). Thus, the "msg" is
      defined as part of the "Hello" object's initialization.

            SOM_Scope void SOMLINK
                           somDefaultInit(Hello somSelf, somInitCtrl *ctrl)
            {
              HelloData *somThis; /* set by BeginInitializer */
              somInitCtrl globalCtrl;
              somBooleanVector myMask;
              HelloMethodDebug("Hello", "somDefaultInit");
              Hello_BeginInitializer_somDefaultInit;

              Hello_Init_SOMObject_somDefaultInit(somSelf, ctrl);
              /*
               * local Hello initialization code added by programmer
               */
              __set_msg(somSelf, "Initial Message");
            }

   4. Update the client program to illustrate default initialization.


            #include <hello.h>
            main()
            {
                Hello h = HelloNew();

                /* Execute the "somPrintSelf" method */
                _somPrintSelf(h);
            }

   5. Compile and link the client program, as before.

   6. Execute the client program.

  > hello
  -- a Hello object at 200633A8 with msg: Initial Message


ΓòÉΓòÉΓòÉ 3.2.6. Example 5-Using Multiple Inheritance ΓòÉΓòÉΓòÉ

The "Hello" class is useful for writing messages to the screen.  So that
clients can also write messages to printers and disk files, this example
references two additional classes: "Printer" and "Disk."  The "Printer" class
will manage messages to a printer, and the "Disk" class will manage messages
sent to files. These classes can be defined as follows:

   #include <somobj.idl>

   interface Printer : SOMObject
   {
       void stringToPrinter(in string s) ;
        // This method writes a string to a printer.
   };

   #include <somobj.idl>

   interface Disk : SOMObject
   {
        void stringToDisk(in string s) ;
        // This method writes a string to disk.
   };

This example assumes the "Printer" and "Disk" classes are defined separately
(in "print.idl" and "disk.idl," for example), are implemented in separate
files, and are linked with the other example code.  Given the implementations
of the "Printer" and "Disk" interfaces, the "Hello" class can use them by
inheriting from them, as illustrated next.

   1. Modify the interface declaration in "hello.idl".

               #include <disk.idl>
               #include <printer.idl>

               interface Hello : Disk, Printer
               {
                   void sayHello();

                   attribute string msg;

                   enum outputTypes {screen, printer, disk};
                   // Declare an enumeration for the different forms of output

                   attribute outputTypes output;
                   // The current form of output

               #ifdef __SOMIDL__

                 implementation {
                 somDefaultInit: override, init;
                 };
               #endif //#  __SOMIDL__
               };

      Notice that SOMObject is not listed as a parent of "Hello" above, because
      SOMObject is a parent of "Disk" (and of "Printer").

      The IDL specification above declares an enumeration "outputTypes" for the
      different forms of output, and an attribute "output" whose value will
      depend on where the client wants the output of the "sayHello" method to
      go.

      Note: SOM IDL allows the use of structures, unions (though with a syntax
      different from C or C++), enumerations, constants, and typedefs, both
      inside and outside the body of an interface statement. Declarations that
      appear inside an interface body will be emitted in the header file (that
      is, in "hello.h" or "hello.xh").  Declarations that appear outside of an
      interface body do not appear in the header file (unless required by a
      special #pragma directive; see the SOM Compiler options in Chapter 4).

      SOM IDL also supports all of the C and C++ preprocessor directives,
      including conditional compilation, macro processing, and file inclusion.

   2. Re-run the SOM Compiler on the updated idl file..

      Unfortunately, when this is done, the implementation for somDefaultInit
      is not correctly updated to reflect the addition of two new parents to
      "Hello."  This is because the implementation file emitter never changes
      the bodies of existing method procedures.  As a result, method procedures
      for initializer methods are not given new parent calls when the parents
      of a class are changed.  One way to deal with this (when the parents of a
      class are changed) is to temporarily rename the method procedure for
      initializer methods, and then run the implementation emitter. Once this
      is done, the code in the renamed methods can be merged into the new
      templates, which will include all the appropriate parent method calls.
      When this is done here, the new implementation for somDefaultInit would
      appear as:

                  SOM_Scope void SOMLINK
                                 somDefaultInit(Hello somSelf, somInitCtrl *ctrl)
             {
               HelloData *somThis; /* set by BeginInitializer */
               somInitCtrl globalCtrl;
               somBooleanVector myMask;
               HelloMethodDebug("Hello", "somDefaultInit");
               Hello_BeginInitializer_somDefaultINit;

                Hello_Init_Disk_somDefaultInit(somSelf, ctrl);
                Hello_Init_Printer_somDefaultInit(somSelf, ctrl);
               /*
                * local Hello initialization code added by programmer
               /*
               __set_msg(somSelf, "Initial Message");
            }

   3. Continue to customize the implementation file, hello.c, as follows.
      Notice that the "sayHello" method (last discussed in Example 2) is now
      modified to allow alternate ways of outputing a "msg".

            SOM_Scope void  SOMLINK sayHello(Hello somSelf, Environment *ev)
            {
                /* HelloData *somThis = HelloGetData(somSelf) ; */
                HelloMethodDebug("Hello","sayHello") ;
                switch (__get_output(somSelf, ev) ) {
                        /*  for C++, use:  somSelf->_get_output(ev) */
                case Hello_screen:
                    printf("%s\n", __get_msg(somSelf, ev) );
                        /*  for C++, use:  somSelf->_get_msg(ev) */
                    break;
                case Hello_printer:
                    _stringToPrinter(somSelf, ev, __get_msg(somSelf, ev) );
                        /* for C++, use:
                         * somSelf->stringToPrinter(ev, somSelf->_get_msg(ev) );
                         */
                    break;
                case Hello_disk:
                    _stringToDisk(somSelf, ev, __get_msg(somSelf, ev) );
                        /*  for C++, use:
                         *   somSelf->stringToDisk(ev, somSelf->_get_msg(ev) );
                         */
                    break;
                }
            }

      The "switch" statement invokes the appropriate method depending on the
      value of the "output" attribute.  Notice how the "case" statements
      utilize the enumeration values of "outputTypes" declared in "hello.idl"
      by prefacing the enumeration names with the class name (Hello_screen,
      Hello_printer, and Hello_disk).

   4. Update the client program, as illustrated next:

               #include <hello.h>
                       /* for C++, use "hello.xh" and <stdio.h> */

               int main(int argc, char *argv[])
               {
                   Hello a = HelloNew();
                   Environment *ev = somGetGlobalEnvironment();

                   /*Invoke "sayHello" on an object and use each output */
                   _sayHello(a, ev) ; /* for c++, use:a->sayHello(ev);*/
                    __set_output(a, ev, Hello_printer);
                                             /* C++: a->_set_output(ev,Hello_printer);*/
                   _sayHello(a, ev);
                   __set_output(a, ev, Hello_disk);
                              /* C++:a->_set_output(ev,Hello-disk);       */
                   _sayHello(a, ev) ;

                   _somFree(a0 ;  /* for C++, use: a->somFree(); */
                    return (0);
               }

   5. Compile and link the client program as before, except also include the
      implementation files for the "Printer" and "Disk" classes in the
      compilation.

   6. Execute the client program.  Observe that the message that prints is the
      "msg" defined in Example 4 as part of the somDefaultInit initialization
      of the "Hello" object.

       Initial Message

       Initial Message               - goes to a Printer

       Initial Message               - goes to Disk

 This tutorial has described features of SOM IDL that will be useful to C and
 C++ programmers.  SOM IDL also provides features such as full type checking,
 constructs for declaring private methods, and constructs for defining methods
 that receive and return pointers to structures.  Chapter 4, "SOM IDL and the
 SOM Compiler" gives complete description of the SOM IDL syntax and also
 describes how to use the SOM Compiler.  In addition, Chapter 5, "Implementing
 Classes in SOM," provides helpful information for completing the
 implementation template, for using initializer (somDefaultInit or user-defined
 initialization methods), for defining SOM class libraries, and for customizing
 various aspects of SOMobjects execution.


ΓòÉΓòÉΓòÉ 4. Using SOM Classes in Client Programs ΓòÉΓòÉΓòÉ

This chapter discusses how to use SOM classes that have already been fully
implemented. That is, these topics describe the steps that a programmer uses to
instantiate an object and invoke some method(s) on it from within an
application program.

Who should read this chapter?

     Programmers who wish to use SOM classes that were originally developed by
      someone else will need to know the information in this chapter. These
      programmers often may not need the information from any subsequent
      chapters.

     By contrast, class implementers who are creating their own SOM classes
      should continue with Chapter 4, "SOM IDL and the SOM Compiler," and
      Chapter 5, "Implementing Classes in SOM "for complete information on the
      SOM Interface Definition Language (SOM IDL) syntax and other details of
      class implementation.

 Programs that use a class are referred to as client programs. A client program
 can be written in  C, in C++, or in another language. As noted, this chapter
 describes how client programs can use SOM classes (classes defined using SOM,
 as described in Chapter 2, "Tutorial for Implementing SOM Classes" and in
 Chapter 4, "SOM IDL and the SOM Compiler"and Chapter 5 "Implementing Classes
 in SOM"). Using a SOM class involves creating instances of a class, invoking
 methods on objects, and so forth. All of the methods, functions, and macros
 described here can also be used by class implementers within the
 implementation file for a class.

 Note: "Using a SOM class," as described in this chapter, does not include
 subclassing the class in a client program. In particular, the C++ compatible
 SOM classes made available in the .xh binding file can not be subclassed in
 C++ to create new C++ or SOM classes.

 Some of the macros and functions described here are supplied as part of SOM's
 C and C++ usage bindings. These bindings are functions and macros defined in
 header files to be included in client programs. The usage bindings make it
 more convenient for C and C++ programmers to create and use instances of SOM
 classes. SOM classes can be also used without the C or C++ bindings, however.
 For example, users of other programming languages can use SOM classes, and C
 and C++ programmers can use a SOM class without using its language bindings.
 The language bindings simply offer a more convenient programmer's interface to
 SOM. Vendors of other languages may also offer SOM bindings; check with your
 language vendor for possible SOM support.

 To use the C or C++ bindings for a class, a client program must include a
 header file for the class (using the #include preprocessor directive). For a C
 language client program, the file <classFileStem>.h must be included. For a
 C++ language client program, the file <classFileStem>.xh must be included. The
 SOM Compiler generates these header files from an IDL interface definition.
 The header files contain definitions of the macros and functions that make up
 the C or C++ bindings for the class. Whether the header files include bindings
 for the class's private methods and attributes (in addition to the public
 methods and attributes) depends on the IDL interface definition available to
 the user, and on how the SOM Compiler was invoked when generating bindings.

 Usage binding headers automatically include any other bindings upon which they
 may rely. Client programs not using the C or C++ bindings for any particular
 class of SOM object (for example, a client program that does not know at
 compile time what classes it will be using) should simply include the
 SOM-supplied bindings for  SOMObject, provided in the header file "somobj.h"
 (for C programs) or"somobj.xh" (for C++ programs).

 For each task that a user of a SOM class might want to perform, this chapter
 shows how the task would be accomplished by:

     a C programmer using the C bindings,
     a C++ programmer using the C++ bindings, or
     a programmer not using SOM's C or C++ language bindings.

 If neither of the first two approaches is applicable, the third approach can
 always be used.


ΓòÉΓòÉΓòÉ 4.1. An Example Client Program ΓòÉΓòÉΓòÉ

Following is a C program that uses the class "Hello" (as defined in the
Tutorial in Chapter 2). The "Hello" class provides one attribute, "msg", of
type string, and one method, "sayHello". The "sayHello" method simply displays
the value of the "msg" attribute of the object on which the method is invoked.

   #include <hello.h>  /* include the header file for Hello */

   int main(int argc, char *argv[])
   {
       /* declare a variable (obj) that is a
        * pointer to an instance of the Hello class: */
       Hello obj;

       /* create an instance of the Hello class
        * and store a pointer to it in obj: */
       obj = HelloNew();

       /* invoke method _set_msg on obj with the argument
        * "Hello World Again". This method sets the value of
        * obj's 'msg' attribute to the specified string.
        */
        __set_msg(obj, somGetGlobalEnvironment(), "Hello World Again");

        /* invoke method sayHello on obj. This method prints
         * the value of obj's 'msg' attribute.  */
        _sayHello(obj, somGetGlobalEnvironment());

        _somFree(obj);
        return(0);
   }

The C++ version of the foregoing client program is shown below:

   #include <hello.xh>  /* include the header file for Hello */

   int main(int argc, char *argv[])
   {
       /* declare a variable (obj) that is a
        * pointer to an instance of the Hello class: */
       Hello *obj;

       /* create an instance of the Hello class
        * and store a pointer to it in obj: */
       obj = new Hello;

      /* invoke method _set_msg on obj with the argument
       * "Hello World Again". This method sets the value of
       * obj's 'msg' attribute to the specified string.  */
      obj->_set_msg(somGetGlobalEnvironment(), "Hello World Again");

      /* invoke method sayHello on obj. This method prints
       * the value of obj's 'msg' attribute.  */
      obj->sayHello(somGetGlobalEnvironment());

      obj->somFree();
      return(0);
   }

These client programs both produce the output:

Hello World Again


ΓòÉΓòÉΓòÉ 4.2. Using SOM Classes: the Basics ΓòÉΓòÉΓòÉ

This section covers the following subjects:

     Declaring object variables
     Creating instances of a class
     Invoking methods on objects
     Using class objects
     Compiling and linking


ΓòÉΓòÉΓòÉ 4.2.1. Declaring object variables ΓòÉΓòÉΓòÉ

When declaring an object variable, an object interface name defined in IDL is
used as the type of the variable. The exact syntax is slightly different for C
vs. C++ programmers. Specifically,

 <interfaceName>  obj ;   in C programs or
 <interfaceName> *obj ;   in C++ programs

 declares "obj" to be a pointer to an object that has type <interfaceName>. In
 SOM, objects of this type are instances of the SOM class named
 <interfaceName>, or of any SOM class derived from this class. Thus, for
 example,

 Animal obj;              in C programs or
 Animal *obj;             in C++ programs

 declares "obj" as pointer to an object of type "Animal" that can be used to
 reference an instance of the SOM class "Animal" or any SOM class derived from
 "Animal". Note that the type of an object need not be the same as its class;
 an object of type "Animal" might not be an instance of the "Animal" class
 (rather, it might be an instance of some subclass of "Animal" - the "Cat"
 class, perhaps).

 All SOM objects are of type SOMObject, even though they may not be instances
 of the SOMObject class. Thus, if it is not known at compile time what type of
 object the variable will point to, the following declaration can be used:

 SOMObject obj;           in C programs or
 SOMObject *obj;          in C++ programs.

 Because the sizes of SOM objects are not known at compile time, instances of
 SOM classes must always be dynamically allocated. Thus, a variable declaration
 must always define a pointer to an object.

 Note: In the C usage bindings, as within an IDL specification, an interface
 name used as a type implicitly indicates a pointer to an object that has that
 interface (this is required by the CORBA specification). The C usage bindings
 for SOM classes therefore hide the pointer with a C typedef for
 <interfaceName>. But this is not appropriate in the C++  usage bindings, which
 define a C++ class for <interfaceName>. Thus, it is not correct in C++ to use
 a declaration of the form:

 <interfaceName>  obj ;   not valid in C++ programs

 Note: If a C programmer also prefers to use explicit pointers to
 <interfaceName> types, then the SOM Compiler option -maddstar can be used when
 the C binding files are generated, and the explicit " *" will then be required
 in declarations of object variables. (This option is required for
 compatibility with existing SOM OIDL code. For information on using the
 -maddstar option, see "Running the SOM Compiler" in Chapter 4, "SOM IDL and
 the SOM Compiler.")

 Users of other programming languages must also define object variables to be
 pointers to the data structure used to represent SOM objects. The way this is
 done is programming-language dependent. The header file "somtypes.h" defines
 the structure of SOM objects for the C language.


ΓòÉΓòÉΓòÉ 4.2.2. Creating instances of a class ΓòÉΓòÉΓòÉ

For C programmers with usage bindings, SOM provides the <className>New and the
<className>Renew macros for creating instances of a class.

These macros are illustrated with the following two examples, each of which
creates a single instance of class "Hello":

   obj = HelloNew();
   obj = HelloRenew(buffer);


ΓòÉΓòÉΓòÉ 4.2.2.1. Using <className>New ΓòÉΓòÉΓòÉ

After verifying that the <className> class object exists, the <className>New
macro invokes the somNew method on the class object.  This allocates enough
space for a new instance of <className>, creates a new instance of the class,
initializes this new object by invoking somDefaultInit on it, and then returns
a pointer to it.  The <className>New macro automatically creates the the class
object for <className>, as well as its ancestor classes and metaclass, if these
objects have not already been created.

After a client program has finished using an object created using the
<className >New macro, the object should be freed by invoking the method
somFree on it:

   _somFree(obj);

After uninitializing the object by invoking somDestruct on it, somFree calls
the class object for storage deallocation. This is important because storage
for an object created using the <className>New macro is allocated by the class
of the object. Thus, only the class of the object can know how to reclaim the
object's storage. object for storage deallocation.


ΓòÉΓòÉΓòÉ 4.2.2.2. Using <className>Renew ΓòÉΓòÉΓòÉ

After verifying that the <className> class object exists, the <className>Renew
macro invokes the somRenew method on the class object.  <className>Renew is
only used when the space for the object has been allocated previously. (Perhaps
the space holds an old uninitialized object that is not needed anymore.) This
macro converts the given space into a new, initialized instance of <className>
and returns a pointer to it.  The programmer is responsible for ensuring that
the argument of <className>Renew  points to a block of storage large enough to
hold an instance of class <className>. The SOM method somGetInstanceSize can be
invoked on the class to determine the amount of memory required. Like
<className>New, the <className>Renew macro automatically creates any required
class objects that have not already been created.

Hint: When creating a large number of class instances, it may be more efficient
to allocate at once enough memory to hold all the instances, and then invoke
<className>Renew once for each object to be created, rather than performing
separate memory allocations.


ΓòÉΓòÉΓòÉ 4.2.2.3. Using <className>NewClass ΓòÉΓòÉΓòÉ

The C and C++ usage bindings for a SOM class also provide static linkage to a
<className>NewClass function that can be used to create the class object.  This
can be useful if the class object is needed before its instances are created.

For example, the following C code uses the function HelloNewClass to create the
"Hello" class object. The arguments to this function are defined by the usage
bindings, and indicate the version of the class implementation that is assumed
by the bindings. (For more detail on creation of classes, see the later
section, "Creating a class object.") Once the class object has been created,
the example invokes the method somGetInstanceSize  on this class to determine
the size of a "Hello" object, uses SOMMalloc to allocate storage, and then uses
the HelloRenew macro to create ten instances of the "Hello" class:

   #include <hello.h>
   main()
   {
   SOMClass helloCls; /*  A pointer for the Hello class object */
   Hello objA[10];    /*  an array of Hello instances */
   unsigned char *buffer;
   int i;
   int size;

   /* create the Hello class object:  */
   helloCls =  HelloNewClass(Hello_MajorVersion, Hello_MinorVersion);

   /* get the amount of space needed for a Hello instance:
    * (somGetInstanceSize is a method provided by SOM.) */
   size =  _somGetInstanceSize(helloCls);
   size = ((size+3)/4)*4;  /* round up to doubleword multiple */

   /* allocate the total space needed for ten instances: */
   buffer =  SOMMalloc(10*size);

   /* convert the space into ten separate Hello instances: */
   for (i=0; i<10; i++)
       objA[i] = HelloRenew(buffer+i*size);
   ...
   ...
   /* Uninitialize the objects and free them */
   for (i=0; i<10; i++)
     _somDestruct(objA[i],0,0);
   SOMFree(buffer);
   }

When an object created with the <className>Renew macro is no longer needed, its
storage must be freed using the dual to whatever method was originally used to
allocate the storage. Two method pairs are typical:

     For example, if an object was originally initialized using the
      <className>New macro, then, as discussed previously, the client should
      use the somFree  method on it.

     On the other hand, if the program uses the SOMMalloc function to allocate
      memory, as illustrated in the example above, then the SOMFree function
      must be called to free the objects' storage (because SOMFree is the dual
      to SOMMalloc). Before this is done, the objects in the region to be freed
      should be deinitialized by invoking the somDestruct method on them. This
      allows each object to free any memory that may have been allocated
      without the programmer's knowledge. (The somFree method also calls the
      somDestruct method.)

 Note:  In the somDestruct method call above, the first zero indicates that
        memory should not be freed by the class of the object (that is, the
        programmer will do it explicitly).  The second zero indicates that the
        class of the object is responsible for overall control of object
        uninitialization.  For further discussion, see "Initializing and
        Uninitializing Objects," in Chapter 5, "Implementing Classes in SOM."

 For C++ programmers with usage bindings, instances of a class <className> can
 be created with a new operator provided by the usage bindings of each SOM
 class.  The new operator automatically creates the class object for
 <className>, as well as its ancestor classes and metaclass,  if they do not
 yet exist.  After verifying the existence of the desired class object, the new
 operator then invokes the somNewNoInit  method on the class.  This allocates
 memory and creates a new instance of the class,  but it does not initialize
 the new object. Initialization of the new object is then performed using one
 of the C++ constructors defined by the usage bindings. (For further
 discussion, see "Initializing and Uninitializing Objects," in Chapter 5,
 "Implementing Classes in SOM.") Two variations of the new operator require no
 arguments. When either is used, the C++ usage bindings provide a default
 constructor that invokes the somDefaultInit method on the new object.  Thus, a
 new object initialized by somDefaultInit would be created using either of the
 forms:

 new <className>


 new <classname>()

 For example:

     obj = new Hello;
     obj1 = new Hello();

 For convenience, pointers to SOM objects created using the new operator can be
 freed using the delete operator, just as for normal C++ objects (or, the
 somFree method could be used):

    delete obj;
    obj1->somFree;

 When previously allocated space will be used to hold a new object, C++
 programmers should use the somRenew method, described below. C++ bindings do
 not provide a macro for this purpose.

 somNew and somRenew: C and C++ programmers, as well programmers using other
 languages, can create instances of a class using the SOM methods somNew and
 somRenew, invoked on the class object. As discussed and illustrated above for
 the C bindings, the class object must first be created using the
 <className>NewClass procedure (or, perhaps, using the somFindClass method-see
 the section "Using class objects," to follow later in this chapter).

 The somNew method invoked on the class object creates a new instance of the
 class, initializes the object using somDefaultInit, and then returns a pointer
 to the new object. For instance, the following C example creates a new object
 of the "Hello" class.

     #include <hello.h>
     main()
     {
       SOMClass helloCls;   /* a pointer to the Hello class */
       Hello obj;           /* a pointer to an Hello instance */
       /* create the Hello class  */
       helloCls = HelloNewClass(Hello_MajorVersion, Hello_MinorVersion);
       obj = _somNew(helloCls); /* create the Hello instance */
     }

 An object created using the somNew method should be freed by invoking the
 somFree method on it after the client program is finished using the object.

 The somRenew method invoked on the class object creates a new instance of a
 class using the given space, rather than allocating new space for the object.
 The method converts the given space into an instance of the class, initializes
 the new object using somDefaultInit, and then returns a pointer to it. The
 argument to somRenew must point to a block of storage large enough to hold the
 new instance. The method somGetInstanceSize can be used to determine the
 amount of memory required. For example, the following C++ code creates ten
 instances of the "Hello" class:

     #include <hello.xh>
     #include <somcls.xh>
     main()
     {
       SOMClass *helloCls; // a pointer to the Hello class
       Hello *objA[10]  //  an array of Hello instance pointers
       unsigned char *buffer;
       int i;
       int size;

      // create the Hello class object
      helloCls =  HelloNewClass(Hello_MajorVersion, Hello_MinorVersion);

      // get the amount of space needed for a Hello instance:
      size = helloCls-> somGetInstanceSize();
      size = ((size+3)/4)*4;  // round up to doubleword multiple

      // allocate the total space needed for ten instances
      buffer =  SOMMalloc(10*size);

      // convert the space into ten separate Hello objects
      for (i=0; i<10; i++)
         objA[i] = helloCls-> somRenew(buffer+i*size);

      // Uninitialize the objects and free them
      for (i=0; i<10; i++)
         objA[i]-> somDestruct(0,0);
      SOMFree(buffer);
     }

 The somNew  and somRenew methods are useful for creating instances of a class
 when the header file for the class is not included in the client program at
 compile time. (The name of the class might be specified by user input, for
 example.) However, the <className>New macro (for C) and the new operator (for
 C++) can only be used for classes whose header file is included in the client
 program at compile time.

 Objects created using the somRenew method should be freed by the client
 program that allocated it, using the dual to whatever allocation approach was
 initially used. If the somFree method is not appropriate (because the method
 somNew was not initially used), then, before memory is freed, the object
 should be explicitly deinitialized by invoking the somDestruct method on it.
 (The somFree method calls the somDestruct method.  Refer to the previous C
 example for Renew for an explanation of the arguments to somDestruct.)


ΓòÉΓòÉΓòÉ 4.2.3. Invoking methods on objects ΓòÉΓòÉΓòÉ

This topic describes the general way to invoke methods in C/C+ + and in other
languages, and then presents subtopics for more specialized situations.


ΓòÉΓòÉΓòÉ 4.2.3.1. Making typical method calls ΓòÉΓòÉΓòÉ

For C programmers with usage bindings: To invoke a method in C, use the macro:

 _<methodName> (receiver, args)

(that is, an underscore followed by the method name). Arguments to the macro
are the receiver of the method followed by all of the arguments to the method.
For example:

   _foo(obj, somGetGlobalEnvironment(), x, y);

This invokes method "foo" on "obj" (the remaining arguments are arguments to
the method "foo"). This expression can be used anywhere that a standard
function call can be used in C.

Required arguments

In C, calls to methods defined using IDL require at least two arguments- a
pointer to the receiving object (the object responding to the method) and a
value of type (Environment *). The Environment data structure is specified by
CORBA, and is used to pass environmental information between a caller and a
called method. For example, it is used to return exceptions. (For more
information on how to supply and use the Environment structure, see the later
section entitled "Exceptions and error handling.")

In the IDL definition of a method, by contrast, the receiver and the
Environment pointer are not listed as parameters to the method. (Unlike the
receiver, the Environment pointer is considered a method parameter, even though
it is never explicitly specified in IDL. For this reason, it is called an
implicit method parameter.) For example, if a method is defined in a .idl file
with two parameters, as in:

   int foo (in char c, in float f);

then, with the C usage bindings, the method would be invoked with four
arguments, as in:

   intvar = _foo(obj, somGetGlobalEnvironment(), x, y);

where "obj" is the object responding to the method and "x" and "y" are the
arguments corresponding to "c" and "f", above.

If the IDL specification of the method includes a context specification, then
the method has an additional (implicit) context parameter. Thus, when invoking
the method, this argument must follow immediately after the Environment pointer
argument.  (None of the SOM-supplied methods require context arguments.)  The
Environment  and context method parameters are prescribed by the CORBA
standard.

If the IDL specification of the class that introduces the method includes the
callstyle=oidl modifier, then the (Environment*) and context arguments should
not be supplied when invoking the method. That is, the receiver of the method
call is followed immediately by the arguments to the method (if any). Some of
the classes supplied in the SOMobjects Toolkit (including SOMObject, SOMClass,
and SOMClassMgr) are defined in this way, to ensure compatibility with the
previous release of SOM. The SOM Programming Reference specifies for each
method whether these arguments are used.

If you use a C expression to compute the first argument to a method call (the
receiver), you must use an expression without side effects, because the first
argument is evaluated twice by the _<methodName> macro expansion. In
particular, a somNew method call or a macro call of <className>New can not be
used as the first argument to a C method call, because doing so would create
two new class instances rather than one.

Following the initial, required arguments to a method (the receiving object,
the Environment, if any, and the context, if any), you enter any additional
arguments required by that method, as specified in IDL.  For a discussion of
how IDL in/out/inout argument types may to C/C++ data types, see the topic
"Parameter list" in Chapter 4, "SOM IDL and the SOM Compiler."

Short form vs long form

If a client program uses the bindings for two different classes that introduce
or inherit two different methods of the same name, then the _<methodName> macro
described above (called the short form) will not be provided by the bindings,
because the macro would be ambiguous in that circumstance. The following long
form macro, however, is always provided by the usage bindings for each class
that supports the method:

  <className>_<methodName>(receiver, args)

For example, method "foo" supported by class "Bar" can be invoked as:

  Bar_foo(obj, somGetGlobalEnvironment(), x, y)   (in C)

where "obj"  has type "Bar" and "x" and "y" are the arguments to method "foo".

In most cases (where there is no ambiguity, and where the method is not a
va_list method, as described in the subsequent subtopic "Using 'va_list'
methods"), a C programmer may use either the short or the long form of a method
invocation macro interchangeably. However, only the long form complies with the
CORBA standard for C usage bindings.  If you wish to write code that can be
easily ported to other vendor platforms that support the CORBA standard, use
the long form exclusively. The long form is always available for every method
that a class supports. The short form is provided both as a programming
convenience and for source code compatibility with release 1 of SOM.

In order to use the long form, a programmer will usually know what type an
object is expected to have.  If this is not known, but the different methods
have the same signature, the method can be invoked using name-lookup
resolution, as described in a following subtopic of this section.

For C++  programmers with usage bindings:  To invoke a method, use the standard
C++ form shown below:

  obj-><methodName> (args)

where args are the arguments to the method. For instance, the following example
invokes method "foo" on "obj":

   obj->foo(somGetGlobalEnvironment(), x, y)
Required arguments

All methods introduced by classes declared using IDL (except those having the
SOM IDL callstyle=oidl modifier) have at least one parameter-a value of type
(Environment *). The Environment data structure is used to pass environmental
information between a caller and a called method. For example, it is used to
return exceptions. For more information on how to supply and use the
Environment structure, see the later section entitled "Exceptions and error
handling."

The Environment pointer is an implicit parameter; in the IDL definition of a
method, the Environment pointer is not explicitly listed as a parameter to the
method. For example, if a method is defined in IDL with two explicit
parameters, as in:

   int foo (in char c, in float f);

then the method would be invoked from C++ bindings with three arguments, as in:

   intvar = obj->foo(somGetGlobalEnvironment(), x, y);

where "obj" is the object responding to the method and "x" and "y" are the
arguments corresponding to "c" and "f", above.

If the IDL specification of the method includes a context specification, then
the method has a second implicit parameter, of type context, and the method
must be invoked with an additional context argument.  This argument must follow
immediately after the Environment pointer argument.  (No SOM-supplied methods
require context arguments.)  The Environment and context method parameters are
prescribed by the CORBA standard.

If the IDL specification of the class that introduces the method includes the
callstyle=oidl modifier, then the (Environment  *) and context arguments should
not be supplied when the method is invoked. Some of the classes supplied in the
SOMobjects Toolkit (including SOMObject, SOMClass, and SOMClassMgr) are defined
in this way, to ensure compatibility with the previous release of SOM. The SOM
Programming Reference specifies for each method whether these arguments are
used.

Following the initial, required arguments to a method (the receiving object,
the Environment, if any, and the context, if any), you enter any additional
arguments required by that method, as specified in IDL.  For a discussion of
how IDL in/out/inout argument types map to C/C++ data types, see the topic
"Parameter list" in Chapter 4, "SOM IDL and the SOM Compiler."

For non-C/C++ programmers:  To invoke a static method  (that is, a method
declared when defining an OIDL or IDL object interface) without using the C or
C++ usage bindings, a programmer can use the somResolve procedure. The
somResolve  procedure takes as arguments a pointer to the object on which the
method is to be invoked and a method token for the desired method. It returns a
pointer to the method's procedure (or raises a fatal error if the object does
not support the method). Depending on the language and system, it may be
necessary to cast this procedure pointer to the appropriate type; the way this
is done is language-specific.

The method is then invoked by calling the procedure returned by somResolve (the
means for calling a procedure, given a pointer to it, is language-specific),
passing the method's receiver, the Environment pointer (if necessary), the
context argument (if necessary) and the remainder of the method's arguments, if
any. (See the section above for C programmers; the arguments to a method
procedure are the same as the arguments passed using the long form of the
C-language method-invocation macro for that method.)

Using somResolve requires the programmer to know where to find the method token
for the desired method. Method tokens are available from class objects that
support the method (via the method somGetMethodToken), or from a global data
structure, called the ClassData structure,  corresponding to the class that
introduces the method. In C and C++ programs with access to the definitions for
ClassData structures provided by usage bindings, the method token for method
methodName introduced by class className may be accessed by the following
expression:

  <className>ClassData.<methodName>

For example, the method token for method "sayHello"introduced by class "Hello"
is stored at location HelloClassData.sayHello, for C and C++ programmers.  The
way method tokens are accessed in other languages is language-specific.

As an  example of using offset resolution to invoke methods from a programming
language other than C/C++, one would do the following to create an instance of
a SOM Class X in Smalltalk:

   1. Initialize the SOM run-time environment, if it has not previously been
      initialized, using the somEnvironmentNew function.

   2. If the class object for class X has not yet been created, use somResolve
      with arguments SOMClassMgrObject (returned by somEnvironmentNew in step
      1) and the method token for the somFindClass method,  to obtain a method
      procedure pointer for the somFindClass method. Use the method procedure
      for somFindClass to create the class object for class X:  Call the
      procedure with arguments SOMClassMgrObject, the result of calling the
      somIdFromString function with argument "X", and the major and minor
      version numbers for class X (or zero). The procedure returns the class
      object for class X.

   3. Use somResolve with arguments representing the class object for X
      (returned by somFindClass in step 2) and the method token for the somNew
      method, to obtain a method procedure pointer for method somNew. (The
      somNew method is used to create instances of class X.)

   4. Call the method procedure for somNew (using the method procedure pointer
      obtained in step 3) with the class object for X (returned by somFindClass
      in step 3) as the argument. The procedure returns a new instance of class
      X.

 In addition to somResolve, SOM also supplies the somClassResolve procedure.
 Instead of an object, the somClassResolve procedure takes a class as its first
 argument, and then selects a method procedure from the instance method table
 of the passed class. (The somResolve procedure, by contrast, selects a method
 procedure from the instance method table of the class of which the passed
 object is an instance.) The somClassResolve procedure therefore supports
 casted  method resolution. See the SOM Programming Reference for more
 information on somResolve  and somClassResolve.

 If the programmer does not know at compile time which class introduces the
 method to be invoked, or if the programmer cannot directly access method
 tokens, then the procedure somResolveByName can be used to obtain a method
 procedure using name-lookup resolution, as described in the next section.

 If the signature of the method to be invoked is not known at compile time, but
 can be discovered at run time, use somResolve or somResolveByName  to get a
 pointer to the somDispatch method procedure, then use it to invoke the
 specific method, as described below under "Method name or signature not known
 at compile time."


ΓòÉΓòÉΓòÉ 4.2.3.2. Accessing Attributes ΓòÉΓòÉΓòÉ

In addition to methods, SOM objects can also have attributes. An attribute is
an IDL shorthand for declaring methods, and does not necessarily indicate the
presence of any particular instance data in an object of that type. Attribute
methods are called "get" and "set" methods. For example, if a class "Hello"
declares an attribute called "msg", then object variables of type "Hello" will
support the methods _get_msg and _set_msg to access or set the value of the
"msg" attribute. (Attributes that are declared as "readonly" have no "set"
method, however.)

The "get" and "set" methods are invoked in the same way as other methods. For
example, given class "Hello" with attribute "msg" of type string, the following
code segments set and get the value of the "msg" attribute:

For C:

   #include <hello.h>
   Hello obj;
   Environment *ev = somGetGlobalEnvironment();

   obj = HelloNew();
   __set_msg(obj, ev, "Good Morning");/*note: two leading underscores */
   printf("%s\n", __get_msg(obj, ev));

For C++:

   #include <hello.xh>
   #include <stdio.h>
   Hello *obj;
   Environment *ev = somGetGlobalEnvironment();

   obj = new Hello;
   obj->_set_msg(ev, "Good Morning");
   printf("%s\n", obj->_get_msg(ev));

Attributes available with each class, if any, are described in the
documentation of the class itself in the SOM Programming Reference.


ΓòÉΓòÉΓòÉ 4.2.3.3. Using name-lookup method resolution ΓòÉΓòÉΓòÉ

For C/C++  programmers:  Offset resolution is the most efficient way to select
the method procedure appropriate to a given method call. Client programs can,
however, invoke a method using "name-lookup" resolution instead of offset
resolution. The C and C++ bindings for method invocation use offset resolution
by default, but methods defined with the namelookup SOM IDL modifier result in
C bindings in which the short form invocation macro uses name-lookup resolution
instead.  Also, for both C and C++ bindings, a special lookup_<methodName>
macro is defined.

Name-lookup resolution is appropriate in the case where a programmer knows at
compile time which arguments will be expected by a method (that is, its
signature), but does not know the type of the object on which the method will
be invoked. For example, name-lookup resolution can be used when two different
classes introduce different methods of the same name and signature, and it is
not known which method should be invoked (because the type of the object is not
known at compile time).

Name-lookup resolution is also used to invoke dynamic methods (that is, methods
that have been added to a class's interface at run time rather than being
specified in the class's IDL specification). For more information on
name-lookup method resolution, see the topic "Method Resolution" in Chapter 4,
"SOM IDL and the SOM Compiler."

For C: To invoke a method using name-lookup resolution, when using the C
bindings for a method that has been implemented with the namelookup modifier,
use either of the following macros:

      _<methodName> (receiver, args)

      lookup_<methodName> (receiver, args)

 Thus, the short-form method invocation macro results in name-lookup resolution
 (rather than offset resolution), when the method has been defined as a
 namelookup method. (The long form of the macro for offset resolution is still
 available in the C usage bindings.) If the method takes a variable number of
 arguments, then the first form shown above is used when supplying a variable
 number of arguments, and the second form is used when supplying a va_list
 argument in place of the variable number of arguments.

 For C++: To invoke a method using name-lookup resolution, when using the C++
 bindings for a method that has been defined with the namelookup modifier, use
 either of the following macros:

      lookup_<methodName> (receiver, args)

      <className>_lookup_<methodName> (receiver, args)

 If the method takes a variable number of arguments, then the first form shown
 above is used when supplying a variable number of arguments, and the second
 form is used when supplying a va_list argument in place of the variable number
 of arguments. Note that the offset-resolution forms for invoking methods using
 the C++ bindings are also still available, even if the method has been defined
 as a namelookup method.

 For C/C++  To invoke a method using name-lookup resolution, when the method
 has not been defined as a namelookup method:

     Use the somResolveByName procedure (described in the following section),
      or any of the methods somLookupMethod, somFindMethod or somFindMethodOk
      to obtain a pointer to the procedure that implements the desired method.

     Then, invoke the desired method by calling that procedure, passing the
      method's intended receiver, the Environment pointer (if needed), the
      context argument (if needed), and the remainder of the method's
      arguments, if any.

 The somLookupMethod, somFindMethod and somFindMethodOK methods are invoked on
 a class object  (the class of the method receiver should be used), and take as
 an argument the somId for the desired method (which can be obtained from the
 method's name using the somIdFromString function). For more information on
 these methods, see the SOM Programming Reference.

 Important Note: SOM provides many ways for a SOM user to acquire a pointer to
 a method procedure. Once this is done, it becomes the user's responsibility to
 make appropriate use of this procedure.

     First, the procedure should only be used on objects for which this is
      appropriate-otherwise, run-time errors are likely to result.

     Second, when the procedure is used, it is essential that the compiler be
      given correct information concerning the signature of the method and the
      linkage required by the method.  (On many systems, there are different
      ways to pass method arguments, and linkage information tells a compiler
      how to pass the arguments indicated by a method's signature).

 SOM method procedures on Windows NT and Windows 95 must be called with "_ _
 stdcall" linkage. In Contrast, SOM method procedures on OS/2 must be called
 with "system" linkage.  On AIX, there is only one linkage convention for
 procedure calls. While C and C++ provide standard ways to indicate a method
 signature, the way to indicate linkage information depends on the specific
 compiler and system.  For each method declared using OIDL or IDL,  the C and
 C++ usage bindings therefore use conditional macros and a typedef to name a
 type that has the correct linkage convention. This type name can then be used
 by programmers with access to the usage bindings for the class that introduces
 the method whose procedure pointer is used. The type is named
 somTD_<className>_<methodName>. This is illustrated in the following example,
 and further details are provided in the section below, titled "Obtaining a
 method's procedure pointer."


ΓòÉΓòÉΓòÉ 4.2.3.4. A name-lookup example ΓòÉΓòÉΓòÉ

The following example shows the use of name-lookup by a SOM client programmer.
Name-lookup resolution is appropriate when a programmer knows that an object
will respond to a method of some given name, but does not know enough about the
type of the object to use offset method resolution. How can this happen? It
normally happens when a programmer wants to write generic code, using methods
of the same name and signature that are applicable to different classes of
objects, and yet these classes have no common ancestor that introduces the
method. This can easily occur in single-inheritance systems (such as Smalltalk
and SOM release 1) and can also happen in multiple-inheritance systems such as
SOM release 2-when class hierarchies designed by different people are brought
together for clients' use.

If multiple inheritance is available, it is always possible to create a common
class ancestor into which methods of this kind can be migrated. A refactoring
of this kind often implements a semantically pleasing generalization that
unifies common features of two previously unrelated class hierarchies. This
step is most practical, however, when it does not require the redefinition or
recompilation of current applications that use offset resolution. SOM is unique
in that it allows this.

However, such refactoring must redefine the classes that originally introduced
the common methods (so the methods can be inherited from the new "unifying"
class instead). A client programmer who simply wants to create an application
may not control the implementations of the classes. Thus, the use of
name-lookup method resolution seems the best alternative for programmers who do
not want to define new classes, but simply to make use of available ones.

For example, assume the existence of two different SOM classes, "classX" and
"classY", whose only common ancestor is SOMObject, and who both introduce a
method named "reduce" that accepts a string as an argument and returns a long.
We assume that the classes were not designed in conjunction with each other. As
a result, it is unlikely that the "reduce" method was defined with a namelookup
modifier.

Following is a C++ generic procedure that uses name-lookup method resolution to
invoke the "reduce" method on its argument, which may be either of type
"classX" or "classY". Note that there is no reason to include classY's usage
bindings, since the typedef provided for the "reduce" method procedure in
"classX" is sufficient for invoking the method procedure, independently of
whether the target object is of type "classX" or "classY".

   #include <classX.xh> // use classX's method proc typedef

   // this procedure can be invoked on a target of type
   // classX or classY.

   long generic_reduce1(SOMObject *target, string arg)
   {
      somTD_classX_reduce reduceProc = (somTD_classX_reduce)
      somResolveByName(target, "reduce");
      return reduceProc(target, arg);
   }

On the other hand, If the classes were designed in conjunction with each other,
and the class designer felt that programmers might want to write generic code
appropriate to either class of object, the namelookup modifier might have been
used. This is a possibility in SOM release 2, even with multiple inheritance,
but it is much more likely that the class designer would use multiple
inheritance to introduce the reduce method in a separate class, and then use
this other class as a parent for both classX and classY (thereby allowing the
use of offset resolution).

In any case, if the "reduce" method in "classX" were defined as a namelookup
method, the following code would be appropriate.  Note that the name-lookup
support provided by "classX" usage bindings is still appropriate for use on
targets that do not have type "classX".  As a result, the "reduce" method
introduced by "classY" need not have been defined as a namelookup method.

   #include <classX.xh> // use classX's name-lookup support

   // this procedure can be invoked on a target of type
   // classX or classY.

   long generic_reduce2(SOMObject *target, string arg)
   {
       return lookup_reduce(target, arg);
   }

For non-C/C++  programmers:  Name-lookup  resolution is useful for non-C/C++
programmers when the type of an object on which a method must be invoked is not
known at compile time or when method tokens cannot be directly accessed by the
programmer. To invoke a method using name-lookup resolution when not using the
C or C++ usage bindings, use the somResolveByName procedure to acquire a
procedure pointer. How the programmer indicates the method arguments and the
linkage convention in this case is compiler specific.

The somResolveByName  procedure takes as arguments a pointer to the object on
which the method is to be invoked and the name of the method, as a string. It
returns a pointer to the method's procedure (or NULL if the method is not
supported by the object). The method can then be invoked by calling the method
procedure, passing the method's receiver, the Environment pointer (if
necessary), the context argument (if necessary), and the rest of the method's
arguments, if any. (See the section above for C programmers; the arguments to a
method procedure are the same as the arguments passed to the long-form
C-language method-invocation macro for that method.)

As an example of invoking methods using name-lookup resolution using the
procedure somResolveByName, the following steps are used to create an instance
of a SOM Class X in Smalltalk:

   1. Initialize the SOM run-time environment (if it is not already
      initialized) using the somEnvironmentNew function.

   2. If the class object for class X has not yet been created, use
      somResolveByName with the arguments SOMClassMgrObject (returned by
      somEnvironmentNew in step 1) and the string "somFindClass", to obtain a
      method procedure pointer for the somFindClass method. Use the method
      procedure for somFindClass to create the class object for class X: Call
      the method procedure with these four arguments: SOMClassMgrObject; the
      variable holding class X 's somId (the result of calling the
      somIdFromString function with argument "X"); and the major and minor
      version numbers for class X (or zero). The result is the class object for
      class X.

   3. Use somResolveByName with arguments the class object for X (returned by
      somFindClass in step 2) and the string "somNew", to obtain a method
      procedure pointer for method somNew. (This somNew method is used to
      create instances of a class.)

   4. Call the method procedure for somNew (using the method procedure pointer
      obtained in step 3) with the class object for X (returned by somFindClass
      in step 3) as the argument. The result is a new instance of class X. How
      the programmer indicates the method arguments and the linkage convention
      is compiler-specific.


ΓòÉΓòÉΓòÉ 4.2.3.5. Obtaining a method's procedure pointer ΓòÉΓòÉΓòÉ

Method resolution is the process of obtaining a pointer to the procedure that
implements a particular method for a particular object at run time.  The method
is then invoked subsequently by calling that procedure, passing the method's
intended receiver, the Environment pointer (if needed), the context argument
(if needed), and the method's other arguments, if any. C and C++ programmers
may wish to obtain a pointer to a method's procedure for efficient repeated
invocations.

Obtaining a pointer to a method's procedure is achieved in one of two ways,
depending on whether the method is to be resolved using offset resolution or
name-lookup resolution. Obtaining a method's procedure pointer via offset
resolution is faster, but it requires that the name of the class that
introduces the method and the name of the method be known at compile time.  It
also requires that the method be defined as part of that class's interface in
the IDL specification of the class. (See the topic "Method Resolution" in
Chapter 4, "SOM IDL and the SOM Compiler" for more information on offset and
name-lookup method resolution.)


ΓòÉΓòÉΓòÉ 4.2.3.5.1. Offset resolution ΓòÉΓòÉΓòÉ

To obtain a pointer to a procedure using offset resolution, the C/C++ usage
bindings provide the SOM_Resolve and SOM_ResolveNoCheck macros. The usage
bindings themselves use the first of these, SOM_Resolve, for offset-resolution
method calls. The difference in the two macros is that the SOM_Resolve macro
performs consistency checking on its arguments, but the macro
SOM_ResolveNoCheck, which is faster, does not. Both macros require the same
arguments:

      SOM_Resolve(<receiver>, <className>, <methodName>)

      SOM_ResolveNoCheck(<receiver>, <className>, <methodName>)

 where the arguments are as follows:

 receiver          The object to which the method will apply. It should be
                   specified as an expression without side effects.

 className         The name of the class that introduces the method.

 methodName        The name of the desired method.

 These two names (className and methodName) must be given as tokens, rather
 than strings or expressions. (For example, as Animal rather than "Animal".) If
 the symbol SOM_TestOn is defined and the symbol SOM_NoTest is not defined in
 the current compilation unit, then SOM_Resolve verifies that receiver is an
 instance of className or some class derived from className. If this test
 fails, an error message is output and execution is terminated.

 The SOM_Resolve and SOM_ResolveNoCheck  macros use the procedure somResolve to
 obtain the entry-point address of the desired method procedure (or raise a
 fatal error if methodName is not introduced by className). This result can be
 directly applied to the method arguments, or stored in a variable of generic
 procedure type (for example, somMethodPtr) and retained for later method use.
 This second possibility would result in a loss of information, however, for
 the reasons now given.

 The SOM_Resolve or SOM_ResolveNoCheck macros are especially useful because
 they cast the method procedure they obtain to the right type to allow the C or
 C++ compiler to call  this procedure with system linkage and with the
 appropriate arguments. This is why the result of SOM_Resolve is immediately
 useful for calling the method procedure, and why storing the result of
 SOM_Resolve in a variable of some "generic" procedure type results in a loss
 of information. The correct type information can be regained, however, because
 the type used by SOM_Resolve for casting the result of somResolve  is
 available from C/C++ usage bindings using the typedef name
 somTD_<className>_<methodName>. This type name describes a pointer to a method
 procedure for methodName introduced by class className. If the final argument
 of the method is a va_list, then the method procedure returned by SOM_Resolve
 or SOM_ResolveNoCheck must be called with a va_list  argument, and not a
 variable number of arguments.

 Below is a C example of using SOM_Resolve to obtain a method procedure pointer
 for method "sayHello", introduced by class "Hello", and using it to invoke the
 method on "obj." (Assume that the only argument required by the "sayHello"
 method is the Environment pointer.)

     somMethodProc *p;
     SOMObject obj = HelloNew();
     p = SOM_Resolve(obj, Hello, sayHello);
     ((somTD_Hello_sayHello)p) (obj, somGetGlobalEnvironment());

 SOM_Resolve and SOM_ResolveNoCheck can only be used to obtain method
 procedures for static methods  (methods that have been declared in an IDL
 specification for a class) and not methods that are added to a class at run
 time. See the SOM Programming Reference for more information and examples on
 SOM_Resolve and SOM_ResolveNoCheck.


ΓòÉΓòÉΓòÉ 4.2.3.5.2. Name-lookup method resolution ΓòÉΓòÉΓòÉ

To obtain a pointer to a method's procedure using name-lookup  resolution, use
the somResolveByName procedure (described in the following section), or any of
the somLookupMethod, somFindMethod and somFindMethodOK methods. These methods
are invoked on a class object that supports the desired method, and they take
an argument specifying the a somId for the desired method (which can be
obtained from the method's name using the somIdFromString function). For more
information on these methods and for examples of their use, see the SOM
Programming Reference.


ΓòÉΓòÉΓòÉ 4.2.3.6. Method name or signature not known at compile time ΓòÉΓòÉΓòÉ

If the programmer does not know a method's name at compile time (for example,
it might be specified by user input), then the method can be invoked in one of
two ways, depending upon whether its signature is known:

     Suppose the signature of the method is known at compile time (even though
      the method name is not). In that case, when the name of the method
      becomes available at run time, the somLookupMethod, somFindMethod or
      somFindMethodOk methods or the somResolveByName procedure can be used to
      obtain a pointer to the method's procedure using name-lookup method
      resolution, as described in the preceding topics. That method procedure
      can then be invoked, passing the method's intended receiver, the
      Environment pointer (if needed), the context argument (if needed), and
      the remainder of the method's arguments.

     If the method's signature is unknown until run time, then
      dispatch-function resolution is indicated, as described in the next
      topic.


ΓòÉΓòÉΓòÉ 4.2.3.6.1. Dispatch-function method resolution ΓòÉΓòÉΓòÉ

If the signature of the method is not known at compile time (and hence the
method's argument list cannot be constructed until run time), then the method
can be invoked at run time by (a) placing the arguments in a variable of type
va_list at run time and (b) either using the somGetMethodData method followed
by use of the somApply function, or by invoking the somDispatch or
somClassDispatch method. Using somApply is more efficient, since this is what
the somDispatch method does, but it requires two steps instead of one. In
either case, the result invokes a "stub" procedure called an apply stub, whose
purpose is to remove the method arguments from the va_list, and then pass them
to the appropriate method procedure in the way expected by that procedure. For
more information on these methods and for examples of their use, see the
somApply function, and the somGetMethodData, somDispatch, and somClassDispatch
methods in the SOM Programming Reference.


ΓòÉΓòÉΓòÉ 4.2.4. Using class objects ΓòÉΓòÉΓòÉ

Using a class object encompasses three aspects: getting the class of an object,
creating a new class object, or simply referring to a class object through the
use of a pointer.


ΓòÉΓòÉΓòÉ 4.2.4.1. Getting the class of an object ΓòÉΓòÉΓòÉ

To get the class that an object is an instance of, SOM provides a method called
somGetClass. The somGetClass method takes an object as its only argument and
returns a pointer to the class object of which it is an instance. For example,
the following statements store in "myClass" the class object of which "obj" is
an instance.

myClass = _somGetClass(obj);  (for C)
myClass = obj->somGetClass();  (for C++)

Getting the class of an object is useful for obtaining information about the
object; in some cases, such information cannot be obtained directly from the
object, but only from its class. The section below entitled "Getting
information about a class" describes the methods that can be invoked on a class
object after it is obtained using somGetClass.

The somGetClass method can be overridden by a class to provide enhanced or
alternative semantics for its objects. Because it is usually important to
respect the intended semantics of a class of objects, the somGetClass method
should normally be used to access the class of an object.

In a few special cases, it is not possible to make a method call on an object
in order to determine its class. For such situations, SOM provides the
SOM_GetClass  macro. In general, the somGetClass method and the SOM_GetClass
macro may have different behavior (if somGetClass has been overridden). This
difference may be limited to side effects, but it is possible for their results
to differ as well. The SOM_GetClass macro should only be used when absolutely
necessary.


ΓòÉΓòÉΓòÉ 4.2.4.2. Creating a class object ΓòÉΓòÉΓòÉ

A class object is created automatically the first time the <className>New
macro (for C) or the new operator (C++) is invoked to create an instance of
that class. In other situations, however, it may be necessary to create a class
object explicitly, as this section describes.


ΓòÉΓòÉΓòÉ 4.2.4.2.1. Using <className>Renew or somRenew ΓòÉΓòÉΓòÉ

It is sometimes necessary to create a class object before creating any
instances of the class. For example, creating instances using the <className>
Renew macro or the somRenew method requires knowing how large the created
instance will be, so that memory can be allocated for it. Getting this
information requires creating the class object (see the example under "Creating
instances of a class" early in this chapter). As another example, a class
object must be explicitly created when a program does not use the SOM bindings
for a class. Without SOM bindings for a class, its instances must be created
using somNew or somRenew, and these methods require that the class object be
created in advance.

Use the <className>NewClass procedure to create a class object:

     When using the C/C++ language bindings for the class, and

     When the name of the class is known at compile time.


ΓòÉΓòÉΓòÉ 4.2.4.2.2. Using <className>NewClass ΓòÉΓòÉΓòÉ

The <className>NewClass procedure initializes the SOM run-time environment, if
necessary, creates the class object (unless it already exists), creates class
objects for the ancestor classes and metaclass of the class, if necessary, and
returns a pointer to the newly created class object. After its creation, the
class object can be referenced in client code using the macro

 _<className>                            (for C and C++ programs)

 or the expression

 <className>ClassData.classObject        (for C and C++ programs).

 The <className>NewClass procedure takes two arguments, the major version
 number and minor version number of the class. These numbers are checked
 against the version numbers built into the class library to determine if the
 class is compatible with the client's expectations. The class is compatible if
 it has the same major version number and the same or a higher minor version
 number. If the class is not compatible, an error is raised. Major version
 numbers usually only change when a significant enhancement or incompatible
 change is made to a class. Minor version numbers change when minor
 enhancements or fixes are made. Downward compatibility is usually maintained
 across changes in the minor version number. Zero can be used in place of
 version numbers to bypass version number checking.

 When using SOM bindings for a class, these bindings define constants
 representing the major and minor version numbers of the class at the time the
 bindings were generated. These constants are named <className>_MajorVersion
 and <className>_MinorVersion. For example, the following procedure call:

     AnimalNewClass(Animal_MajorVersion, Animal_MinorVersion);

 creates the class object for class "Animal". Thereafter, _Animal can be used
 to reference the "Animal" class object.

 The preceding technique for checking version numbers is not failsafe. For
 performance reasons, the version numbers for a class are only checked when the
 class object is created, and not when the class object or its instances are
 used. Thus, run-time errors may result when usage bindings for a particular
 version of a class are used to invoke methods on objects created by an earlier
 version of the class.


ΓòÉΓòÉΓòÉ 4.2.4.2.3. Using somFindClass or somFindClsInFile ΓòÉΓòÉΓòÉ

To create a class object when  not using the C/C++  language bindings for the
class, or when the class name is not known at compile time:

     First, initialize the SOM run-time environment by calling the
      somEnvironmentNew function (unless it is known that the SOM run-time
      environment has already been initialized).

     Then, use the somFindClass or somFindClsInFile method to create the class
      object. (The class must already be defined in a dynamically linked
      library, or DLL.)

 The somEnvironmentNew function initializes the SOM run-time environment. That
 is, it creates the four primitive SOM objects (SOMClass, SOMObject,
 SOMClassMgr, and the SOMClassMgrObject), and it initializes SOM global
 variables. The function takes no arguments and returns a pointer to the
 SOMClassMgrObject.

 Note:  Although somEnvironmentNew must be called before using other SOM
        functions and methods, explicitly calling somEnvironmentNew is usually
        not necessary when using the C/C++ bindings,  because the macros for
        <className>NewClass,  <className>New , and <className>Renew call it
        automatically, as does the new operator for C++. Calling
        somEnvironmentNew repeatedly does no harm.

 After the SOM run-time environment has been initialized, the methods
 somFindClass and somFindClsInFile can be used to create a class object. These
 methods must be invoked on the class manager, which is pointed to by the
 global variable SOMClassMgrObject.  (It is also returned as the result of
 somEnvironmentNew.)

 The somFindClass method takes the following arguments:

 classId                  A somId identifying the name of the class to be
                          created. The somIdFromString function returns a
                          classId  given the name of the class.

 major version number     The expected major version number of the class.

 minor version number     The expected minor version number of the class.

 The version numbers are checked against the version numbers built into the
 class library to determine if the class is compatible with the client's
 expectations.

 The somFindClass  method dynamically loads the DLL containing the class's
 implementation, if needed, creates the class object (unless it already exists)
 by invoking its <className>NewClass procedure, and returns a pointer to it. If
 the class could not be created, somFindClass returns NULL. For example, the
 following C code fragment creates the class "Hello" and stores a pointer to it
 in "myClass":

     SOMClassMgr cm = somEnvironmentNew();
     somId classId = somIdFromString("Hello");
     SOMClass myClass = _somFindClass(SOMClassMgrObject, classId
                            Hello_MajorVersion, Hello_MinorVersion);
     ...
     SOMFree(classId);

 The somFindClass method uses somLocateClassFile to get the name of the library
 file containing the class. If the class was defined with a "dllname" class
 modifier, then somLocateClassFile returns that file name; otherwise, it
 assumes that the class name is the name of the library file. The
 somFindClsInFile method is similar to somFindClass, except that it takes an
 additional (final) argument-the name of the library file containing the class.
 The somFindClsInFile method is useful when a class is packaged in a DLL along
 with other classes and the "dllname" class modifier has not been given in the
 class's IDL specification. Invoking methods without corresponding class usage
 bindings

 This topic builds on the preceding discussion, and illustrates how a client
 program can apply dynamic SOM mechanisms to utilize classes and objects for
 which specific usage bindings are not available. This process can be applied
 when a class implementor did not ship the C/C++ language bindings.
 Furthermore, the process allows more programming flexibility, because it is
 not necessary to know the class and method names at compile time in order to
 access them at run time. (At run time, however, you must be able to provide
 the method arguments, either explicitly or via a va_list, and provide a
 generalized way to handle return values.) As an example application, a
 programmer might create an online class viewer that can access many classes
 without requiring usage bindings for all those classes, and the person using
 the viewer can select class names at run time.

 As another aspect of flexibility, a code sequence similar to the following C++
 example could be re-used to access any class or method. After getting the
 somId for a class name, the example uses the somFindClass method to create the
 class object. The somNew method is then invoked to create an instance of the
 specified class, and the somDispatch method is used to invoke a method on the
 object.

  #include <stdio.h>
  #include <somcls.xh>

  int main()
  {
          SOMClass *classobj;
          somId tempId;
          somId methId;
          SOMObject *s2;
          Environment * main_ev = somGetGlobalEnvironment();

          tempId = SOM_IdFromString("myClassName");
          classobj = SOMClassMgrObject->somFindClass(tempId,0,0);
          SOMFree(tempId);

          if (NULL==classobj)
          {
                  printf("somFindClass could not find the selected class\n");
          }
          else
          {
                  s2 = (SOMObject *) (classobj->somNew());
                  methId = somIdFromString("sayHello");
                  if (s2->somDispatch((somToken *) 0, methId, s2, ev))
                          printf("Method successfully called.\n");
          }

          return 0;
  }


ΓòÉΓòÉΓòÉ 4.2.4.3. Referring to class objects ΓòÉΓòÉΓòÉ

Saving a pointer as the class object is created: The <className>NewClass macro
and the somFindClass method, used to create class objects, both return a
pointer to the newly created class object. Hence, one way to obtain a pointer
to a class object is to save the value returned by <className>NewClass or
somFindClass  when the class object is created.

Getting a pointer after the class object is created: After a class object has
been created, client programs can also get a pointer to the class object from
the class name. When the class name is known at compile time and the client
program is using the C or C++ language bindings, the macro

      _<className>

 can be used to refer to the class object for <className>. Also, when the class
 name is known at compile time and the client program is using the C or C++
 language bindings, the expression

      <className>ClassData.classObject

 refers to the class object for <className>. For example, _Hello refers to the
 class object for class "Hello" in C or C++ programs, and
 HelloClassData.classObject refers to the class object for class "Hello." in C
 or C++ programs.

 Getting a pointer to the class object from an instance:  If any instances of
 the class are known to exist, a pointer to the class object can also be
 obtained by invoking the somGetClass method on such an instance. (See "Getting
 the class of an object," above.)

 Getting a pointer in other situations: If the class name is not known until
 run time, or if the client program is not using the C or C++ language
 bindings, and no instances of the class are known to exist, then the
 somClassFromId method can be used to obtain a pointer to a class object after
 the class object has been created. The somClassFromId method should be invoked
 on the class manager, which is pointed to by the global variable
 SOMClassMgrObject. The only argument to the method is a somId for the class
 name, which can be obtained using the somIdFromString function. The method
 somClassFromId returns a pointer to the class object of the specified class.
 For example, the following C code stores in "myClass" a pointer to the class
 object for class "Hello" (or NULL, if the class cannot be located):

     SOMClassMgr cm = somEnvironmentNew();
     somId classId = somIdFromString("Hello");
     SOMClass myClass =_somClassFromId(SOMClassMgrObject,classId
                            Hello_MajorVersion, Hello_MinorVersion);
     SOMFree(classId);


ΓòÉΓòÉΓòÉ 4.2.5. Compiling and linking ΓòÉΓòÉΓòÉ

This section describes how to compile and link C and C++ client programs.
Compiling and linking a client program with a SOM class is done in one of two
ways, depending on how the class is packaged.

Note: If you are building an application that uses a combination of C and C++
compiled object modules, then the C++ linker must be used to link them.

If the class is not packaged as a library (that is, the client program has the
implementation source code for the class, as in the examples given in the SOM
IDL tutorial), then the client program can be compiled together with the class
implementation file as follows. (This assumes that the client program and the
class are both implemented in the same language, C or C++. If this is not the
case, then each module must be compiled separately to produce an object file
and the resulting object files linked together to form an executable.)

In the following examples, the environment variable SOMBASE refers to the
directory in which SOM has been installed. The examples also assume that the
header files and the import library for the "Hello" class reside in the
"include" and "lib" directories where SOM has been installed.  If this is not
the case, additional path information should be supplied for these files. For
client program "main" and class "Hello": For C programmers:

   > set LIB=%SOMBASE%\lib;%LIB%
   > icc -I. -I%SOMBASE%\include main.c hello.c somtk.lib

For C++ programmers:

   > set LIB=%SOMBASE%\lib;%LIB%
   > icc -I. -I%SOMBASE%\include main.cpp hello.cpp somtk.lib

If the class is packaged as a class library, then the client program, "main",
is compiled as above, except that the class implementation file is not part of
the compilation. Instead, the "import library" provided with the class library
is used to resolve the symbolic references that appear in "main". For example,
to compile the C client program "main.c" that uses class "Hello":

Under *OS/2:

   > set LIB=%SOMBASE%\lib;%LIB%
   > icc -I. -I%SOMBASE%\include main.c somtk.lib hello.lib


ΓòÉΓòÉΓòÉ 4.3. Language-neutral Methods and Functions ΓòÉΓòÉΓòÉ

This section describes methods, functions, and macros that client programs can
use regardless of the programming language in which they are written. In other
words, these functions and methods are not part of the C or C++ bindings.


ΓòÉΓòÉΓòÉ 4.3.1. Generating output ΓòÉΓòÉΓòÉ

The following functions and methods are used to generate output, including
descriptions of SOM objects. They all produce their output using the
character-output procedure held by the global variable SOMOutCharRoutine. The
default procedure for character output simply writes the character to stdout,
but it can be replaced to change the output destination of the methods and
functions below. (See Chapter 5 for more information on customizing SOM.)

 somDumpSelf              (method) writes a detailed description of an object,
                          including its class, its location, and its instance
                          data. The receiver of the method is the object to be
                          dumped. An additional argument is the "nesting level"
                          for the description. [All lines in the description
                          will be indented by (2 * level) spaces.]

 somPrintSelf             (method) Writes a brief description of an object,
                          including its class and location in memory. The
                          receiver of the method is the object to be printed.

 somPrintf                (function) SOM's version of the C "printf" function.
                          It generates character stream output via
                          SOMOutCharRoutine. It has the same interface as the C
                          "printf" function.

 somVprintf               (function) Represents the "vprint" form of somPrintf.
                          Its arguments are a formatting string and a va_list
                          holding the remaining arguments.

 somPrefixLevel           (function) Generates (via somPrintf) spaces to prefix
                          a line at the indicated level. The return type is
                          void. The argument is an integer specifying the
                          level. The number of spaces generated is (2 * level).

 somLPrintf               (function) Combines somPrefixLevel and somPrintf. The
                          first argument is the level of the description (as
                          for somPrefixLevel) and the remaining arguments are
                          as for somPrintf (or for the C "printf" function).

 See the SOM Programming Reference for more information on a specific function
 or method.


ΓòÉΓòÉΓòÉ 4.3.2. Getting information about a class ΓòÉΓòÉΓòÉ

The following methods are used to obtain information about a class or to locate
a particular class object:

 somCheckVersion          Checks a class for compatibility with the specified
                          major and minor version numbers. The receiver of the
                          method is the SOM class about which information is
                          needed. Additional arguments are values of the major
                          and minor version numbers. The method returns TRUE if
                          the class is compatible, or FALSE otherwise.

 somClassFromId           Finds the class object of an existing class when
                          given its somId, but without loading the class. The
                          receiver of the method is the class manager (pointed
                          to by the global variable SOMClassMgrObject). The
                          additional argument is the class's somId. The method
                          returns a pointer to the class (or NULL if the class
                          does not exist).

 somDescendedFrom         Tests whether one class is derived from another. The
                          receiver of the method is the class to be tested, and
                          the potential ancestor class is the argument. The
                          method returns TRUE if the relationships exists, or
                          FALSE otherwise.

 somFindClass             Finds or creates the class object for a class, given
                          the class's somId and its major and minor version
                          numbers. The receiver of the method is the class
                          manager (pointed to by the global variable
                          SOMClassMgrObject). Additional arguments are the
                          class's somId and the major and minor version
                          numbers. The method returns a pointer to the class
                          object, or NULL if the class could not be created.

 somFindClsInFile         Finds or creates the class object for a class. This
                          method is similar to somFindClass, except the user
                          also provides the name of a file to be used for
                          dynamic loading, if needed. The receiver of the
                          method is the class manager (pointed to by the global
                          variable SOMClassMgrObject). Additional arguments are
                          the class's somId, the major and minor version
                          numbers, and the file name. The method returns a
                          pointer to the class object, or NULL if the class
                          could not be created.

 somGetInstancePartSize   Obtains the size of the instance variables introduced
                          by a class. The receiver of the method is the class
                          object. The method returns the amount of space, in
                          bytes, needed for the instance variables.

 somGetInstanceSize       Obtains the total size requirements for an instance
                          of a class. The receiver of the method is the class
                          object. The method returns the amount of space, in
                          bytes, required for the instance variables introduced
                          by the class itself and by all of its ancestor
                          classes.

 somGetName               Obtains the name of a class. The receiver of the
                          method is the class object. The method returns the
                          class name.

 somGetNumMethods         Obtains the number of methods available for a class.
                          The receiver of the method is the class object. The
                          method returns the total number of currently
                          available methods (static or otherwise, including
                          inherited methods).

 somGetNumStaticMethods   Obtains the number of static methods available for a
                          class. (A static method is one declared in the
                          class's interface specification [.idl] file.) The
                          receiver of the method is the class object. The
                          method returns the total number of available static
                          methods, including inherited ones.

 somGetParents            Obtains a sequence of the parent (base) classes of a
                          specified class. The receiver of the method is the
                          class object. The method returns a pointer to a
                          linked list of the parent (base) classes (unless the
                          receiver is SOMObject, for which it returns NULL).

 somGetVersionNumbers     Obtains the major and minor version numbers of a
                          class. The return type is void, and the two arguments
                          are pointers to locations in memory where the method
                          can store the major and minor version numbers (of
                          type long).

 somSupportsMethod        Indicates whether instances of a given class support
                          a given method. The receiver of the somSupportsMethod
                          method is the class object. The argument is the somId
                          for the method in question. The somSupportsMethod
                          method returns TRUE if the method is supported, or
                          FALSE otherwise.

 See the SOM Programming Reference for more information on a specific method.


ΓòÉΓòÉΓòÉ 4.3.3. Getting information about an object ΓòÉΓòÉΓòÉ

The following methods and functions are used to obtain information about an
object (instance) or to determine whether a variable holds a valid SOM object.


ΓòÉΓòÉΓòÉ 4.3.3.1. Methods ΓòÉΓòÉΓòÉ

 somGetClass              Gets the class object of a specified object. The
                          receiver of the method is the object whose class is
                          desired. The method returns a pointer to the object's
                          corresponding class object.

 somGetClassName          Obtains the class name of an object. The receiver of
                          the method is the object whose class name is desired.
                          The method returns a pointer to the name of the class
                          of which the specified object is an instance.

 somGetSize               Obtains the size of an object. The receiver of the
                          method is the object. The method returns the amount
                          of contiguous space, in bytes, that is needed to hold
                          the object itself (not including any additional space
                          that the object may be using or managing outside of
                          this area).

 somIsA                   Determines whether an object is an instance of a
                          given class or of one of its descendant classes. The
                          receiver of the method is the object to be tested. An
                          additional argument is the name of the class to which
                          the object will be compared. This method returns TRUE
                          if the object is an instance of the specified class
                          or if (unlike somIsInstanceOf) it is an instance of
                          any descendant class of the given class; otherwise,
                          the method returns FALSE.

 somIsInstanceOf          Determines whether an object is an instance of a
                          specific class (but not of any descendant class).The
                          receiver of the method is the object. The argument is
                          the name of the class to which the object will be
                          compared. The method returns TRUE if the object is an
                          instance of the specified class, or FALSE otherwise.

 somRespondsTo            Determines whether an object supports a given method.
                          The receiver of the method is the object. The
                          argument is the somId for the method in question. (A
                          somId can be obtained from a string by using the
                          somIdFromString function.) The somRespondsTo method
                          returns TRUE if the object supports the method, or
                          FALSE otherwise.


ΓòÉΓòÉΓòÉ 4.3.3.2. Functions ΓòÉΓòÉΓòÉ

 somIsObj                 Takes as its only argument an address (which may not
                          be valid). The function returns TRUE (1) if the
                          address contains a valid SOM object, or FALSE (0)
                          otherwise. This function is designed to be failsafe.

 See the SOM Programming Reference for more information on a specific method or
 function.


ΓòÉΓòÉΓòÉ 4.3.4. Debugging ΓòÉΓòÉΓòÉ

The following macros are used to conditionally generate output for debugging.
All output generated by these macros is written using the replaceable
character-output procedure pointed to by the global variable SOMOutCharRoutine.
The default procedure simply writes the character to stdout, but it can be
replaced to change the output destination of the methods and functions below.
(See Chapter 5 for more information on customizing SOM.)

Debugging output is produced or suppressed based on the settings of three
global variables, SOM_TraceLevel, SOM_WarnLevel, and SOM_AssertLevel:

     SOM_TraceLevel controls the behavior of the <className>MethodDebug macro;

     SOM_WarnLevel controls the behavior of the macros SOM_WarnMsg, SOM_TestC,
      and SOM_Expect; and

     SOM_AssertLevel controls the behavior of the SOM_Assert macro.

 Available macros for generating debugging output are as follows:

 <className>MethodDebug   (macro for C and C++ programmers using the SOM
                          language bindings for <className>)
                          The arguments to this macro are a class name and a
                          method name. If the SOM_TraceLevel global variable
                          has a nonzero value, the <className>MethodDebug macro
                          produces a message each time the specified method (as
                          defined by the specified class) is executed. This
                          macro is typically used within the procedure that
                          implements the specified method. (The SOM Compiler
                          automatically generates calls to the
                          <className>MethodDebug macro within the
                          implementation template files it produces.) u To
                          suppress method tracing for all methods of a class,
                          put the following statement in the implementation
                          file after including the header file for the class:

                                                    #define <className>MethodDebug(c,m) \
                                                                       SOM_NoTrace(c,m)

                          This can yield a slight performance improvement. The
                          SOMMTraced metaclass, discussed below, provides a
                          more extensive tracing facility that includes method
                          parameters and returned values.

 SOM_TestC                The SOM_TestC macro takes as an argument a boolean
                          expression. If the boolean expression is TRUE
                          (nonzero) and SOM_AssertLevel is greater than zero,
                          then an informational message is output. If the
                          expression is FALSE (zero) and SOM_WarnLevel is
                          greater than zero, a warning message is produced.

 SOM_WarnMsg              The SOM_WarnMsg macro takes as an argument a
                          character string. If the value of SOM_WarnLevel is
                          greater than zero, the specified message is output.

 SOM_Assert               The SOM_Assert macro takes as arguments a boolean
                          expression and an error code (an integer). If the
                          boolean expression is TRUE (nonzero) and
                          SOM_AssertLevel  is greater than zero, then an
                          informational message is output. If the expression is
                          FALSE (zero), and the error code indicates a
                          warning-level error and SOM_WarnLevel is greater than
                          zero, then a warning message is output. If the
                          expression is FALSE and the error code indicates a
                          fatal error, then an error message is produced and
                          the process is terminated.

 SOM_Expect               The SOM_Expect macro takes as an argument a boolean
                          expression. If the boolean expression is FALSE (zero)
                          and SOM_WarnLevel is set to be greater than zero,
                          then a warning message is output. If condition is
                          TRUE and SOM_AssertLevel is set to be greater than
                          zero, then an informational message is output.

 See the SOM Programming Reference for more information on a specific macro.

 The somDumpSelf and somPrintSelf methods can be useful in testing and
 debugging. The somPrintSelf method produces a brief description of an object,
 and the somDumpSelf method produces a more detailed description. See the SOM
 Programming Reference for more information.


ΓòÉΓòÉΓòÉ 4.3.5. Checking the validity of method calls ΓòÉΓòÉΓòÉ

The C and C++ language bindings include code to check the validity of method
calls at run time. If a validity check fails, the SOM_Error macro ends the
process. (SOM_Error is described below.) To enable method-call validity
checking, place the following directive in the client program prior to any
#include directives for SOM header files:

#define SOM_TestOn

Alternatively, the -DSOM_TestOn option can be used when compiling the client
program to enable method-call validity checking.


ΓòÉΓòÉΓòÉ 4.3.6. Exceptions and error handling ΓòÉΓòÉΓòÉ

In the classes provided in the SOM run-time library (that is, SOMClass,
SOMObject, and SOMClassMgr), error handling is performed by a user-replaceable
procedure, pointed to by the global variable SOMError, that produces an error
message and an error code and, if appropriate ate, ends the process where the
error occurred. (Chapter 5 describes how to customize the error handling
procedure.)

Each error is assigned a unique integer error code. Errors are grouped into
three categories, based on the last digit of the error code:

 SOM_Ignore               This category of error represents an informational
                          event. The event is considered normal and can be
                          ignored or logged at the user's discretion. Error
                          codes having a last digit of 2 belong to this
                          category.

 SOM_Warn                 This category of error represents an unusual
                          condition that is not a normal event, but is not
                          severe enough to require program termination. Error
                          codes having a last digit of 1 belong to this
                          category.

 SOM_Fatal                This category of error represents a condition that
                          should not occur or that would result in loss of
                          system integrity if processing were allowed to
                          continue. In the default error handling procedure,
                          these errors cause the termination of the process in
                          which they occur. Error codes having a last digit of
                          9 belong to this category.

 The various codes for all errors detected by SOM are listed in Appendix A,
 "Customer Support and Error Codes."

 When errors are encountered in client programs or user defined-classes, the
 following two macros can be used to invoke the error-handling procedure:

 SOM_Error                The SOM_Error macro takes an error code as its only
                          argument and invokes the SOM error handling procedure
                          (pointed to by the global variable SOMError) to
                          handle the error. The default error handling
                          procedure prints a message that includes the error
                          code, the name of the source file, and the line
                          number where the macro was invoked. If the last digit
                          of the error code indicates a serious error (of
                          category SOM_Fatal), the process causing the error is
                          terminated. (Chapter 5 describes how to customize the
                          error handling procedure.)

 SOM_Test                 The SOM_Test macro takes a boolean expression as an
                          argument. If the expression is TRUE (nonzero) and the
                          SOM_AssertLevel is greater than zero, then an
                          informational message is output. If the expression is
                          FALSE (zero), an error message is produced and the
                          program is terminated.

 See the SOM Programming Reference for more information on a specific macro.

 Other classes provided by the SOMobjects Toolkit (including those in the DSOM,
 and Interface Repository frameworks, and the utility classes and metaclasses)
 handle errors differently. Rather than invoking SOMError with an error code,
 their methods return exceptions via the (Environment *) inout parameter
 required by these methods. The following sections describe the exception
 declarations, the standard exceptions, and how to set and get exception
 information in an Environment structure.


ΓòÉΓòÉΓòÉ 4.3.6.1. Exception declarations ΓòÉΓòÉΓòÉ

As discussed in Chapter 4 in the section entitled "SOM Interface Definition
Language," a method may be declared to return zero or more exceptions. IDL
exceptions are implemented by simply passing back error information after a
method call, as opposed to the "catch/throw" model where an exception is
implemented by a long jump or signal. Associated with each type of exception is
a name, and optionally, a struct-like data structure for holding error
information. A method declares the types of exceptions it may return in a
raises expression.

Below is an example IDL declaration of a "BAD_FLAG" exception, which may be
"raised" by a"checkFlag" method, as part of a "MyObject" interface:

   interface MyObject {
      exception BAD_FLAG { long ErrCode; char Reason[80];}

      void checkFlag(in unsigned long flag) raises(BAD_FLAG);
   };

An exception structure contains whatever information is necessary to help the
caller understand the nature of the error. The exception declaration can be
treated like a struct definition:  i.e., whatever you can access in an IDL
struct, you can access in an exception declaration. Alternatively, the
structure can be empty, whereby the exception is just identified by its name.

The SOM Compiler will map the exception declaration in the above example to the
following C language constructs:

   typedef struct BAD_FLAG {
      long ErrCode;
      char Reason[80];
   } BAD_FLAG;

   #define ex_BAD_FLAG "MyObject::BAD_FLAG"

When an exception is detected, the "checkFlag" method must call SOMMalloc to
allocate a "BAD_FLAG" structure, initialize it with the appropriate error
information, and make a call to somSetException (see "Setting an exception
value," below) to record the exception value in the Environment structure
passed in the method call. The caller, after invoking "checkFlag", can check
the Environment structure that was passed to the method to see if there was an
exception, and if so, extract the exception value from the Environment (see
"Getting an exception value," below.)


ΓòÉΓòÉΓòÉ 4.3.6.2. Standard exceptions ΓòÉΓòÉΓòÉ

In addition to user-defined exceptions (those defined explicitly in an IDL
file), there are several predefined exceptions for system run-time errors. A
system exception  can be returned on any method call. (That is, they are
implicitly declared for every method whose class uses IDL call style, and they
do not appear in any raises expressions.) The standard exceptions are listed in
Table 2 of Section 4.2, "SOM Interface Definition Language". Most of the
predefined system exceptions pertain to Object Request Broker errors.
Consequently, these types of exceptions are most likely to occur in DSOM
applications (Chapter 6).

Each of the standard exceptions has the same structure: an error code (to
designate the subcategory of the exception) and a completion status code. For
example, the NO_MEMORY standard exception has the following definition:

   enum completion_status {YES, NO, MAYBE};
   exception NO_MEMORY { unsigned long minor;
                         completion_status completed; };

The completion status value indicates whether the method was never initiated
(NO), completed execution prior to the exception (YES), or the completion
status is indeterminate (MAYBE).

Since all the standard exceptions have the same structure, file "somcorba.h"
(included by "som.h") defines a generic StExcep typedef which can be used
instead of the specific typedefs:

   typedef struct StExcep {
        unsigned long minor;
        completion_status completed;
   } StExcep;

The standard exceptions are defined in an IDL module called StExcep, in the
file named "stexcep.idl", and the C definitions can be found in "stexcep.h".


ΓòÉΓòÉΓòÉ 4.3.6.3. The Environment ΓòÉΓòÉΓòÉ

The Environment is a data structure that contains environmental information
that can be passed between a caller and a called object when a method is
executed. For example, it is used to pass information about the user id of a
client, to return exception data to the client following a method call, and so
on.

A pointer to an Environment variable is passed as an argument to method calls
(unless the method's class has the callstyle=oidl SOM IDL modifier). The
Environment typedef is defined in "som.h", and an instance of the structure is
allocated by the caller in any reasonable way: on  the stack (by declaring a
local variable and initializing it using the macro SOM_InitEnvironment),
dynamically (using the SOM_CreateLocalEnvironment macro), or by calling the
somGetGlobalEnvironment function to allocate an Environment structure to be
shared by objects running in the same thread.

For class libraries that use callstyle=oidl, there is no explicit Environment
parameter. For these libraries, exception information may be passed using the
per-thread Environment structure returned by the somGetGlobalEnvironment
procedure.


ΓòÉΓòÉΓòÉ 4.3.6.4. Setting an exception value ΓòÉΓòÉΓòÉ

To set an exception value in the caller's Environment structure, a method
implementation makes a call to the somSetException procedure:

   void  somSetException ( Environment *ev,
                               exception_type major,
                               string exception_name,
                               void *params);

where "ev" is a pointer to the Environment structure passed to the method,
"major" is an exception_type, "exception_name" is the string name of the
exception (usually the constant defined by the IDL compiler, for example,
ex_BAD_FLAG), and "params"  is a pointer to an (initialized) exception
structure which must be allocated by SOMMalloc:

   typedef enum exception_type {
      NO_EXCEPTION, USER_EXCEPTION, SYSTEM_EXCEPTION
   } exception_type;

It is important to reiterate that somSetException expects the params argument
to be a pointer to a structure that was allocated using SOMMalloc. When
somSetException is called, the client passes ownership of the exception
structure to the SOM run-time environment. The SOM run-time  environment will
free the structure when the exception is reset (that is, upon the next call to
somSetException), or when the somExceptionFree procedure is called.

Note that somSetException simply sets the exception value; it performs no exit
processing. If there are multiple calls to somSetException before the method
returns, the caller will only see the last exception value.


ΓòÉΓòÉΓòÉ 4.3.6.5. Getting an exception value ΓòÉΓòÉΓòÉ

After a method returns, the calling client program can look at the Environment
structure to see if there was an exception. The Environment struct is mostly
opaque, except for an exception type field named _major:

   typedef struct Environment {
      exception_type   _major;
      ...
   } Environment;

If ev._major != NO_EXCEPTION, there was an exception returned by the call. The
caller can retrieve the exception name and value (passed as parameters in the
somSetException call) from an Environment struct via the following functions:

      string somExceptionId (Environment *ev);

      somToken somExceptionValue (Environment *ev);

 The somExceptionId function returns the exception name, if any, as a string.
 The function somExceptionValue returns a pointer to the value of the
 exception, if any, contained in the exception structure. If NULL is passed as
 the Environment pointer in either of the above calls, an implicit call is made
 to somGetGlobalEnvironment.

 The somExceptionFree procedure will free any memory in the Environment
 associated with the last exception:

      void  somExceptionFree (Environment *ev);

 If preferred, developers can alternatively use the CORBA "exception_free" API
 to free the memory in an Environment structure.

 Note: File "somcorba.h" (included by "som.h") provides the following aliases
 for strict compliance with CORBA programming interfaces:

     #ifdef CORBA_FUNCTION_NAMES
     #define exception_id    somExceptionId
     #define exception_value somExceptionValue
     #define exception_free  somExceptionFree
     #endif /* CORBA_FUNCTION_NAMES */


ΓòÉΓòÉΓòÉ 4.3.6.6. Example ΓòÉΓòÉΓòÉ

Let us define an IDL interface for a "MyObject" object, which declares a
"BAD_FLAG" exception, which can be raised by the "checkFlag"method, in a file
called "myobject.idl":

   interface MyObject {
     exception BAD_FLAG { long ErrCode;   char Reason[80]; }

     void checkFlag(in unsigned long flag) raises(BAD_FLAG);
   };

The SOM IDL compiler will map the exception to the following C language
constructs, in myobject.h:

   typedef struct BAD_FLAG {
      long ErrCode;
      char Reason[80];
   } BAD_FLAG;

   #define ex_BAD_FLAG "MyObject::BAD_FLAG"

A client program that invokes the "checkFlag" method might contain the
following error handling code.  (Note: The error checking code below lies in
the user-written procedure, "ErrorCheck," so the code need not be replicated
through the program.)

size='auto'.
   #include "som.h"
   #include "myobject.h"
   boolean ErrorCheck(Environment *ev);   /* prototype */

   main()
   {
       unsigned long flag;
       Environment ev;
       MyObject myobj;
       char     *exId;
       BAD_FLAG *badFlag;
       StExcep  *stExValue;

       myobj = MyObjectNew();
       flag  = 0x01L;
       SOM_InitEnvironment(&ev);

        /* invoke the checkFlag method, passing the Environment param */
       _checkFlag(myobj, &ev, flag);

       /* check for exception */
       if (ErrorCheck(&ev))
       {
          /* ... */
          somExceptionFree(&ev);   /* free the exception memory */
       }

       /* ... */
   }

   /* error checking procedure */
   boolean ErrorCheck(Environment *ev)
   {
     switch (ev._major)
     {
     case SYSTEM_EXCEPTION:
       /* get system exception id and value */
       exId      =  somExceptionId(ev);
       stExValue =  somExceptionValue(ev);
       /* ... */
       return( TRUE);

     case USER_EXCEPTION:
       /* get user-defined exception id and value */
       exId =  somExceptionId(ev);
       if (strcmp(exId, ex_BAD_FLAG) == 0)
       {
          badFlag = (BAD_FLAG *)  somExceptionValue(ev);
          /* ... */
       }
       /* ... */
       return( TRUE);

     case NO_EXCEPTION:
       return( FALSE);
     }
   }

The implementation of the "checkFlag" method might contain the following
error-handling code:

   #include "som.h"
   #include "myobject.h"

   void  checkFlag(MyObject somSelf, Environment *ev,
                   unsigned long flag)
   {
      BAD_FLAG *badFlag;
      /* ... */

      if ( /* flag is invalid */ )
      {
         badFlag = (BAD_FLAG *) SOMMalloc(sizeof(BAD_FLAG));
         badFlag->ErrCode = /* bad flag code */;
         strcpy(badFlag->Reason, "bad flag was passed");
         somSetException(ev, USER_EXCEPTION,
                         ex_BAD_FLAG, (void *)badFlag);
         return;
      }
      /* ... */
   }


ΓòÉΓòÉΓòÉ 4.3.7. Memory management ΓòÉΓòÉΓòÉ

The memory management functions used by SOM are a subset of those supplied in
the ANSI C standard library. They have the same calling interface and the same
return types as their ANSI C equivalents, but include supplemental error
checking. Errors detected by these functions are passed to SOMError (described
in the previous section). The correspondence between SOM memory management
functions and their ANSI C standard library equivalents is shown below:

ΓöîΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÉ
Γöé SOM FUNCTION             Γöé EQUIVALENT ANSI C LIBRARY ROUTINE  Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMMALLOC              Γöé MALLOC                Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMCALLOC              Γöé CALLOC                Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMREALLOC              Γöé REALLOC               Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMFREE               Γöé FREE                 Γöé
ΓööΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÿ

SOMMalloc, SOMCalloc, SOMRealloc, and SOMFree are actually global variables
that point to the SOM memory management functions (rather than being the names
of the functions themselves), so that users can replace them with their own
memory management functions if desired. (See chapter 5 for a discussion of
replacing the SOM memory management functions.)

Clearing memory for objects

The memory associated with objects initialized by a client program must also be
freed by the client.  The SOM-provided method somFree  is used to release the
storage containing the receiver object:

 #include "origcls.h"

main ()
{
   OrigCls myObject;
   myObject = OrigClsNew ();

    /*  Code to use myObject  */
    _somFree (myObject);
{

Clearing memory for the Environment

Any memory associated with an exception in an Environment structure is
typically freed using the somExceptionFree function. (Or, the CORBA
"exception_free" API can be used.) The somExceptionFree function takes the
following form (also see "Example" in the previous topic for an application
example):

  void somExceptionFree(Environment *ev);

Note: For information on managing the memory, objects, and exceptions used by
DSOM applications, see "Memory management" in Chapter 6, "Distributed SOM
(DSOM)."


ΓòÉΓòÉΓòÉ 4.3.8. SOM manipulations using somId's ΓòÉΓòÉΓòÉ

A somId is similar to a number that represents a zero-terminated string. A
somId is used in SOM to identify method names, class names, and so forth. For
example, many of the SOM methods that take a method or class name as a
parameter require a value of type somId rather than string. All SOM
manipulations using somIds are case insensitive, although the original case of
the string is preserved.

During its first use with any of the following functions, a somId is
automatically converted to an internal representation (registered). Because the
representation of a somId changes, a special SOM type (somId) is provided for
this purpose. Names and the corresponding somId can be declared at compile
time, as follows:

   string example = "exampleMethodName";
   somId exampleId = &example;

or a somId can be generated at run time, as follows:

   somId myMethodId;
   myMethodId = somIdFromString("exampleMethodName");

SOM provides the following functions that generate or use a somId:

 somIdFromString          Finds the somId that corresponds to a string. The
                          method takes a string as its argument, and returns a
                          value of type somId that represents the string. The
                          returned somId must later be freed using SOMFree.

 somStringFromId          Obtains the string that corresponds to a somId. The
                          function takes a somId as its argument and returns
                          the string that the somId represents.

 somCompareIds            Determines whether two somId values are the same
                          (that is, represent the same string). This function
                          takes two somId values as arguments. It returns TRUE
                          (1) if the somIds represent the same string, or FALSE
                          (0) otherwise.

 somCheckId               Determines whether SOM already knows a somId. The
                          function takes a somId as its argument. It verifies
                          whether the somId is registered and in normal form,
                          registers it if necessary, and returns the input
                          somId.

 somRegisterId            The same as somCheckId, except it returns TRUE (1) if
                          this is the first time the somId has been registered,
                          or FALSE (0) otherwise.

 somUniqueKey             Finds the unique key for a somId. The function takes
                          a somId identifier as its argument, and returns the
                          unique key for the somId-a number that uniquely
                          represents the string that the somId represents. This
                          key is the same as the key for another somId if and
                          only if the other somId refers to the same string as
                          the input somId.

 somTotalRegIds           Finds the total number of somIds that have been
                          registered, as an unsigned long. This function is
                          used to determine an appropriate argument to
                          somSetExpectedIds, below, in later executions of the
                          program. The function takes no input arguments.

 somSetExpectedIds        Indicates how many unique somIds SOM can expect to
                          use during program execution, which, if accurate, can
                          improve the space and time utilization of the program
                          slightly. This routine must be called before the SOM
                          run-time environment is initialized (that is, before
                          the function somEnvironmentNew is invoked and before
                          any objects are created). This is the only SOM
                          function that can be invoked before the SOM run-time
                          environment is initialized. The input argument is an
                          unsigned long. The function has no return value.

 See the SOM Programming Reference for more information on a specific function.


ΓòÉΓòÉΓòÉ 5. SOM IDL and the SOM Compiler ΓòÉΓòÉΓòÉ

This chapter first discusses how to define SOM classes and then describes the
SOM Compiler. To allow a class of objects to be implemented in one programming
language and used in another (that is, to allow a SOM class to be language
neutral), the interface to objects of this class must be specified separately
from the objects' implementation.

To summarize:  As a first step, a file known as the .idl file is used to
declare classes and their methods, using SOM's language#neutral Interface
Definition Language (IDL). Next, the SOM Compiler is run on the .idl file to
produce a template implementation file that contains stub method procedures for
the new and overridden methods; this preliminary code corresponds to the
computer language that will implement the class. Then, the class implementer
fills in the stub procedures with code that implements the methods (or
redefines overridden methods) and sets instance data. (This implementation
process is the subject of Chapter 5, "Implementing Classes in SOM.") At this
point, the implementation file can be compiled and linked with a client program
that uses it (as described in Chapter 3, "Using SOM Classes in Client
Programs").

Syntax for SOM IDL and the SOM Compiler are presented in this chapter, along
with helpful information for using them correctly.


ΓòÉΓòÉΓòÉ 5.1. Interface vs Implementation ΓòÉΓòÉΓòÉ

The interface to a class of objects contains the information that a client must
know to use an object-namely, the names of its attributes and the signatures of
its methods.  The interface is described in a formal language independent of
the programming language used to implement the object's methods. In SOM, the
formal language used to define object interfaces is the Interface Definition
Language (IDL), standardized by CORBA.

The implementation of a class of objects (that is, the procedures that
implement methods) is written in the implementer's preferred programming
language. This language can be object-oriented (for instance, C++) or
procedural (for instance, C).

A completely implemented class definition, then, consists of two main files:

     An IDL specification of the interface to instances of the class (the
      interface definition file (or .idl file) and

     Method procedures written in the implementer's language of choice (the
      implementation file).

 The interface definition file has an .idl extension, as noted. The
 implementation file, however, has an extension specific to the language in
 which it is written. For example, implementations written in C have a .c
 extension, and implementations written in C++ have a .cpp extension.

 To assist users in implementing SOM classes, the SOMobjects Toolkit provides a
 SOM Compiler. The SOM Compiler takes as input an object interface definition
 file  (the .idl file) and produces a set of binding files that make it
 convenient to implement and use a SOM class whose instances are objects that
 support the defined interface. The binding files and their purposes are as
 follows:

     An implementation template that serves as a guide for how the
      implementation file for the class should look. The class implementer
      fills in this template file with language-specific code to implement the
      methods that are available on the class' instances.

     Header files to be included (a) in the class's implementation file and
      (b) in client programs that use the class.

 These binding files produced by the SOM Compiler bridge the gap between  SOM
 and the object model used in object-oriented languages (such as C++), and they
 allow SOM to be used with non-object-oriented languages (such as C). The SOM
 Compiler currently produces binding files for the C and C++ programming
 languages. SOM can also be used with other programming languages; the bindings
 simply offer a more convenient programmer's interface to SOM. Vendors of other
 languages may also offer SOM bindings; check with your language vendor for
 possible SOM support.

 The subsequent sections of this chapter provide full syntax for SOM IDL and
 the SOM Compiler.


ΓòÉΓòÉΓòÉ 5.2. SOM Interface Definition Language ΓòÉΓòÉΓòÉ

This section describes the syntax of SOM's Interface Definition Language (SOM
IDL). SOM IDL complies with CORBA's standard for IDL; it also adds constructs
specific to SOM. (For more information on the CORBA standard for IDL, see The
Common Object Request Broker: Architecture and Specification, published by
Object Management Group and x/Open.) The full grammar for SOM IDL is given in
Appendix C. Instructions for converting existing OIDL-syntax files to IDL are
given in Appendix B. The current section describes the syntax and semantics of
SOM IDL using the following conventions:

      Constants (words to be used literally, such as keywords) appear in bold.
      User-supplied elements appear in italics.
      { }       Groups related items together as a single item.

      [ ]       Encloses an optional item.

      *         Indicates zero or more repetitions of the preceding item.

      +         Indicates one or more repetitions of the preceding item.

      |         Separates alternatives.

      _         Within a set of alternatives, an underscore indicates the
                default, if defined.

 IDL is a formal language used to describe object interfaces. Because, in SOM,
 objects are implemented as instances of classes, an IDL object interface
 definition specifies for a class of objects what methods (operations) are
 available, their return types, and their parameter types. For this reason, we
 often speak of an IDL specification for a class (as opposed to simply an
 object interface). Constructs specific to SOM discussed below further
 strengthen this connection between SOM classes and the IDL language.

 IDL generally follows the same lexical rules as C and C++, with some
 exceptions. In particular:

     IDL uses the ISO Latin-1 (8859.1) character set (as per the CORBA
      standard).

     White space is ignored except as token delimiters.

     C and C++ comment styles are supported.

     IDL supports standard C/C++ preprocessing, including macro substitution,
      conditional compilation, and source file inclusion.

     Identifiers (user-defined names for methods, attributes, instance
      variables, and so on) are composed of alphanumeric and underscore
      characters, (with the first character alphabetic) and can be of arbitrary
      length, up to an operating-system limit of about 250 characters.

     Identifiers must be spelled consistently with respect to case throughout
      a specification.

     Identifiers that differ only in case yield a compilation error.

     There is a single name space for identifiers (thus, using the same
      identifier for a constant and a class name within the same naming scope,
      for example, yields a compilation error).

     Integer, floating point, character, and string literals are defined just
      as in C and C++.

 The terms listed in the Keywords table are reserved keywords and may not be
 used otherwise. Keywords must be spelled using upper- and lower-case
 characters exactly as shown in the table. For example, "void" is correct, but
 "Void" yields a compilation error.

 A typical IDL specification for a single class, residing in a single .idl
 file, has the following form. (Also see the later section, "Module
 declarations to define multiple interfaces in an .idl file.") The order is
 unimportant, except that names must be declared (or forward referenced) before
 they are referenced. The subsequent topics of this section describe the
 requirements for these specifications:

 Include directives       (optional)

 Type declarations        (optional)

 Constant declarations    (optional)

 Exception declarations   (optional)

 Interface declaration    (optional)

 Module declaration       (optional)

                          Keywords for SOM IDL

                                                    ΓöîΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÉ
                                                    Γöé any           Γöé FALSE           Γöé readonly         Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé attribute        Γöé float           Γöé sequence         Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé boolean         Γöé implementation      Γöé short          Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé case           Γöé in            Γöé string          Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé char           Γöé inout           Γöé struct          Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé class          Γöé interface         Γöé switch          Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé const          Γöé long           Γöé TRUE           Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé context         Γöé module          Γöé TypeCode         Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé default         Γöé octet           Γöé typedef         Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé double          Γöé oneway          Γöé unsigned         Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé enum           Γöé out            Γöé union          Γöé
                                                    Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
                                                    Γöé exception        Γöé raises          Γöé void           Γöé
                                                    ΓööΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÿ


ΓòÉΓòÉΓòÉ 5.2.1. Include directives ΓòÉΓòÉΓòÉ

The IDL specification for a class normally contains #include statements that
tell the SOM Compiler where to find the interface definitions (the .idl files)
for:

     Each of the class's parent (direct base) classes, and
     The class's metaclass (if specified).

 The #include statements must appear in the above order. For example, if class
 "C" has parents "foo" and "bar" and metaclass "meta", then file "C.idl" must
 begin with the following #include statements:

     #include <foo.idl>
     #include <bar.idl>
     #include <meta.idl>

 As in C and C++, if a filename is enclosed in angle brackets (< >), the search
 for the file will begin in system-specific locations. If the filename appears
 in double quotation marks (" "), the search for the file will begin in the
 current working directory, then move to the system-specific locations.


ΓòÉΓòÉΓòÉ 5.2.2. Type and constant declarations ΓòÉΓòÉΓòÉ

IDL specifications may include type declarations and constant declarations as
in C and C++, with the restrictions/extensions described below.  [Note: For any
reader not familiar with C, a recommended reference is The C Programming
Language (2nd edition, 1988, Prentice Hall) by Brian W. Kernighan and Dennis M.
Ritchie. See pages 36-40 for a discussion of type and constant declarations.]

IDL supports the following basic types (these basic types are also defined for
C and C++ client and implementation programs, using the SOM bindings):


ΓòÉΓòÉΓòÉ 5.2.2.1. Integral types ΓòÉΓòÉΓòÉ

IDL supports only the integral types short,  long, unsigned short, and unsigned
long, which represent the following value ranges:

 short                    -2[15] .. 2[15]-1
 long                     -2[31] .. 2[31]-1
 unsigned short           0 .. 2[16]-1
 unsigned long            0 .. 2[32]-1


ΓòÉΓòÉΓòÉ 5.2.2.2. Floating point types ΓòÉΓòÉΓòÉ

IDL supports the float and double floating-point types. The float type
represents the IEEE single-precision floating-point numbers; double represents
the IEEE double-precision floating-point numbers.


ΓòÉΓòÉΓòÉ 5.2.2.3. Character type ΓòÉΓòÉΓòÉ

IDL supports a char type, which represents an 8-bit quantity. The ISO Latin-1
(8859.1) character set defines the meaning and representation of graphic
characters. The meaning and representation of null and formatting characters is
the numerical value of the character as defined in the ASCII (ISO 646)
standard. Unlike C/C++, type char cannot be qualified as signed or unsigned.
(The octet type, below, can be used in place of unsigned char.)


ΓòÉΓòÉΓòÉ 5.2.2.4. Boolean type ΓòÉΓòÉΓòÉ

IDL supports a boolean type for data items that can take only the values TRUE
and FALSE.


ΓòÉΓòÉΓòÉ 5.2.2.5. Octet type ΓòÉΓòÉΓòÉ

IDL supports an octet type, an 8-bit quantity guaranteed not to undergo
conversion when transmitted by the communication system. The octet type can be
used in place of the unsigned char type.


ΓòÉΓòÉΓòÉ 5.2.2.6. Any type ΓòÉΓòÉΓòÉ

IDL supports an any type, which permits the specification of values of any IDL
type. In the SOM C and C++ bindings, the any type is mapped onto the following
struct:

   typedef struct any {
       TypeCode _type;
       void *_value;
   } any;

The "_value" member for an any type is a pointer to the actual value. The
"_type" member is a pointer to an instance of a TypeCode that represents the
type of the value. The TypeCode provides functions for obtaining information
about an IDL type. Chapter 7, "The Interface Repository Framework," describes
TypeCodes and their associated functions.


ΓòÉΓòÉΓòÉ 5.2.2.7. Constructed types ΓòÉΓòÉΓòÉ

In addition to the above basic types, IDL also supports three constructed
types: struct, union, and enum. The structure and enumeration types are
specified in IDL the same as they are in C and C++ [Kernighan-Ritchie
references: struct, p. 128; union, p. 147; enum, p. 39], with the following
restrictions:

      Unlike C/C++, recursive type specifications are allowed only through the
      use of the sequence template type (see below).

      Unlike C/C++, structures, discriminated unions, and enumerations in IDL
      must be tagged. For example, "struct { int a; ... }" is an invalid type
      specification. The tag introduces a new type name.

      In IDL, constructed type definitions need not be part of a typedef
      statement; furthermore, if they are part of a typedef statement, the tag
      of the struct must differ from the type name being defined by the
      typedef. For example, the following are valid IDL struct and enum
      definitions:

               struct myStruct {
                  long x;
                  double y;
               };                               /* defines type name myStruct*/

               enum colors { red, white, blue };  /* defines type name colors */

      By contrast, the following definitions are not valid:

               typedef struct myStruct {        /*  NOT VALID  */
                  long x;
                  double y;
               } myStruct;                      /* myStruct has been redefined */

               typedef enum colors { red, white, blue } colors;  /* NOT VALID */

 The valid IDL struct and enum definitions shown above are translated by the
 SOM Compiler into the following definitions in the C and C++ bindings,
 assuming they were declared within the scope of interface "Hello":

     typedef struct Hello_myStruct {  /* C/C++ bindings for IDL struct */
        long x;
        double y;
     } Hello_myStruct;

     typedef unsigned long Hello_colors; /* C/C++ bindings for IDL enum */
     #define Hello_red 1UL
     #define Hello_white 2UL
     #define Hello_blue 3UL

 When an enumeration is defined within an interface statement for a class, then
 within C/C++ programs, the enumeration names must be referenced by prefixing
 the class name. For example, if the colors enum, above, were defined within
 the interface statement for class Hello, then the enumeration names would be
 referenced as Hello_red, Hello_white, and Hello_blue. Notice the first
 identifier in an enumeration is assigned the value 1.

 All types and constants generated by the SOM Compiler are fully qualified.
 That is, prepended to them is the fully qualified name of the interface or
 module in which they appear.  For example, consider the following fragment of
 IDL:

     module M {
         typedef long long_t;
         module  N {
             typedef long long_t;
             interface I {
                 typedef long long_t;
             };
         };
     };

 That specification would generate the following three types:

     typedef long  M_long_t;
     typedef long  M_N_long_t;
     typedef long  M_N_I_long_t;

 For programmer convenience, the SOM Compiler also generates shorter bindings,
 without the interface qualification. Consider the next IDL fragment:

     module M {
         typedef long long_t;
         module  N {
             typedef short short_t;
             interface I {
                 typedef char char_t;
             };
         };
     };

 In the C/C++ bindings of the preceding fragment, you can refer to "M_long_t"
 as "long_t", to "M_N_short_t" as "short_t", and to "M_N_I_char_t" as "char_t".

 However, these shorter forms are available only when their interpretation is
 not ambiguous. Thus, in the first example the shorthand for "M_N_I_long_t"
 would not be allowed, since it clashes with "M_long_t" and "M_N_long_t". If
 these shorter forms are not required, they can be ignored by setting #define
 SOM_DONT_USE_SHORT_NAMES before including the public header files, or by using
 the SOM Compiler option -mnouseshort so that they are not generated in the
 header files.

 In the SOM documentation and samples, both long and short forms are
 illustrated, for both type names and method calls. It is the responsibility of
 each user to adopt a style according to personal preference. It should be
 noted, however, that CORBA specifies that only the long forms must be present.


ΓòÉΓòÉΓòÉ 5.2.2.7.1. Union type ΓòÉΓòÉΓòÉ

IDL also supports a union type, which is a cross between the C union and switch
statements. The syntax of a union type declaration is as follows:

union identifier switch ( switch-type )
  { case+ }

The "identifier" following the union keyword defines a new legal type. (Union
types may also be named using a typedef declaration.) The "switch-type"
specifies an integral, character, boolean, or enumeration type, or the name of
a previously defined integral, boolean, character, or enumeration type. Each
"case" of the union is specified with the following syntax:

case-label+  type-spec  declarator ;

where "type-spec" is any valid type specification; "declarator" is an
identifier, an array declarator (such as, foo[3][5]), or a pointer declarator
(such as, *foo); and each "case-label" has one of the following forms:

case const-expr:
default:

The "const-expr" is a constant expression that must match or be automatically
castable to the "switch-type". A default case can appear no more than once.

Unions are mapped onto C/C++ structs. For example, the following IDL
declaration:

   union Foo switch (long) {
     case 1: long x;
     case 2: float y;
     default: char z;
   };

is mapped onto the following C struct:

   typedef Hello_struct {
     long _d;
     union {
       long x;
       float y;
       char z;
     } _u;
   } Hello_foo;

The discriminator is referred to as "_d", and the union in the struct is
referred to as "_u". Hence, elements of the union are referenced just as in C:

   Foo v;

   /* get a pointer to Foo in v: */
   switch(v->_d) {
     case 1: printf("x = %ld\n", v->_u.x); break;
     case 2: printf("y = %f\n", v->_u.y); break;
     default: printf("z = %c\n", v->_u.z); break;
   }

Note: This example is from The Common Object Request Broker: Architecture and
Specification, revision 1.1, page 90.


ΓòÉΓòÉΓòÉ 5.2.2.8. Template types (sequences and strings) ΓòÉΓòÉΓòÉ

IDL defines two template types not found in C and C++: sequences and strings. A
sequence is a one-dimensional array with two characteristics:  a maximum size
(specified at compile time) and a length (determined at run time). Sequences
permit passing unbounded arrays between objects. Sequences are specified as
follows:

sequence < simple-type [, positive-integer-const] >

where "simple-type" specifies any valid IDL type, and the optional
"positive-integer-const" is a constant expression that specifies the maximum
size of the sequence (as a positive integer).

Note: The "simple-type" cannot have a `*' directly in the sequence statement.
Instead, a typedef for the pointer type must be used. For example, instead of:

   typedef sequence<long *> seq_longptr; // Error: '*' not allowed.

use:

   typedef long * longptr;
   typedef sequence<longptr> seq_longptr;  // Ok.

In SOM's C and C++ bindings, sequences  are mapped onto structs with the
following members:

      unsigned long _maximum;
      unsigned long _length;
      simple-type *_buffer;

 where "simple-type" is the specified type of the sequence.  For example, the
 IDL declaration

     typedef sequence<long, 10> vec10;

 results in the following C struct:

     #ifndef _IDL_SEQUENCE_long_defined
     #define _IDL_SEQUENCE_long_defined
     typedef struct {
         unsigned long _maximum;
         unsigned long _length;
         long *_buffer;
     } _IDL_SEQUENCE_long;
     #endif /* _IDL_SEQUENCE_long_defined */
     typedef _IDL_SEQUENCE_long vec10;

 and an instance of this type is declared as follows:

     vec10 v = {10L, 0L, (long *)NULL};

 The "_maximum" member designates the actual size of storage allocated for the
 sequence, and the "_length" member designates the number of values contained
 in the "_buffer" member. For bounded sequences, it is an error to set the
 "_length" or "_maximum" member to a value larger than the specified bound of
 the sequence.

 Before a sequence is passed as the value of an "in" or "inout" method
 parameter, the "_buffer" member must point to an array of elements of the
 appropriate type, and the "_length" member must contain the number of elements
 to be passed. (If the parameter is "inout" and the sequence is unbounded, the
 "_maximum" member must also be set to the actual size of the array. Upon
 return, "_length" will contain the number of values copied into "_buffer",
 which must be less than "_maximum".) When a sequence is passed as an "out"
 method parameter or received as the return value, the method procedure
 allocates storage for "_buffer" as needed, the "_length" member contains the
 number of elements returned, and the "_maximum" member contains the number of
 elements allocated. (The client is responsible for subsequently freeing the
 memory pointed to by  "_buffer".)

 C and C++ programs using SOM's language bindings can refer to sequence types
 as:

 _IDL_SEQUENCE_type

 where "type" is the effective type of the sequence members. For example, the
 IDL type sequence<long,10> is referred to in a C/C++ program by the type name
 _IDL_SEQUENCE_long. If longint is defined via a typedef to be type long, then
 the IDL type sequence<longint,10> is also referred to by the type name
 _IDL_SEQUENCE_long.

 If the typedef is for a pointer type, then the effective type is the name of
 the pointer type. For example, the following statements define a C/C++ type
 _IDL_SEQUENCE_longptr and not _IDL_SEQUENCE_long:

     typedef long * longptr;
     typedef sequence<longptr> seq_longptr;

 A string is similar to a sequence of type char. It can contain all possible
 8-bit quantities except NULL. Strings are specified as follows:

      string  [ < positive-integer-const > ]

 where the optional "positive-integer-const" is a constant expression that
 specifies the maximum size of the string (as a positive integer, which does
 not include the extra byte to hold a NULL as required in C/C++). In SOM's C
 and C++ bindings, strings are mapped onto zero-byte terminated character
 arrays. The length of the string is encoded by the position of the zero-byte.
 For example, the following IDL declaration:

     typedef string<10> foo;

 is converted to the following C/C++ typedef:

     typedef char *foo;

 A variable of this type is then declared as follows:

     foo s = (char *) NULL;

 C and C++ programs using SOM's language bindings can refer to string types by
 the type name string.

 When an unbounded string is passed as the value of an "inout" method
 parameter, the returned value is constrained to be no longer than the input
 value. Hence, using unbounded strings as "inout" parameters is not advised.


ΓòÉΓòÉΓòÉ 5.2.2.9. Arrays ΓòÉΓòÉΓòÉ

Multidimensional, fixed-size arrays can be declared in IDL as follows:

identifier {[ positive-integer-const ] }+

where the "positive-integer-const" is a constant expression that specifies the
array size, in each dimension, as a positive integer. The array size is fixed
at compile time.


ΓòÉΓòÉΓòÉ 5.2.2.10. Pointers ΓòÉΓòÉΓòÉ

Although the CORBA standard for IDL does not include them, SOM IDL offers
pointer types. Declarators of a pointer type are specified as in C and C++:

type *declarator

where "type" is a valid IDL type specification and "declarator" is an
identifier or an array declarator.


ΓòÉΓòÉΓòÉ 5.2.2.11. Object types ΓòÉΓòÉΓòÉ

The name of the interface to a class of objects can be used as a type. For
example, if an IDL specification includes an interface declaration (described
below) for a class (of objects)  "C1", then "C1" can be used as a type name
within that IDL specification. When used as a type, an interface name indicates
a pointer to an object that supports that interface. An interface name can be
used as the type of a method argument, as a method return type, or as the type
of a member of a constructed type (a struct, union, or enum). In all cases, the
use of an interface name implicitly indicates a pointer to an object that
supports that interface.

As explained in Using SOM Classes in Client Programs, SOM's C usage bindings
for SOM classes also follow this convention.  However, within SOM's C++
bindings, the pointer is made explicit, and the use of an interface name as a
type refers to a class instance itself, rather than a pointer to a class
instance. For example, to declare a variable "myobj" that is a pointer to an
instance of class "Foo" in an IDL specification and in a C program, the
following declaration is required:

   Foo myobj;

However, in a C++ program, the following declaration is required:

   Foo *myobj;

If a C programmer uses the SOM Compiler option -maddstar, then the bindings
generated for C will also require an explicit `*' in declarations. Thus,

 Foo myobj;               in IDL requires

 Foo *myobj;              in both C and C++ programs

 This style of bindings for C is permitted for two reasons:

     It more closely resembles the bindings for C++, thus making it easier to
      change to the C++ bindings at a later date; and

     It is required for compatibility with existing SOM OIDL code.

 Note: The same C and C++ binding emitters should not be run in the same SOM
 Compiler command. For example,

     sc "-sh;xh" cls.idl    // Not valid.

 If you wish to generate both C and C++ bindings, you should issue the commands
 separately:

     sc -sh cls.idl
     sc -sxh cls.idl


ΓòÉΓòÉΓòÉ 5.2.3. Exception declarations ΓòÉΓòÉΓòÉ

IDL specifications may include exception declarations, which define data
structures to be returned when an exception occurs during the execution of a
method. (IDL exceptions are implemented by simply passing back error
information after a method call, as opposed to the "catch/throw" model where an
exception is implemented by a long jump or signal.)  Associated with each type
of exception is a name and, optionally, a struct-like data structure for
holding error information. Exceptions are declared as follows:

      exception  identifier { member* };

 The "identifier" is the name of the exception, and each "member" has the
 following form:

      type-spec declarators ;

 where "type-spec" is a valid IDL type specification and "declarators" is a
 list of identifiers, array declarators, or pointer declarators, delimited by
 commas. The members of an exception structure should contain information to
 help the caller understand the nature of the error. The exception declaration
 can be treated like a struct  definition; that is, whatever you can access in
 an IDL struct, you can access in an exception declaration.  Alternatively, the
 structure can be empty, whereby the exception is just identified by its name.

 If an exception is returned as the outcome of a method, the exception
 "identifier" indicates which exception occurred. The values of the members of
 the exception provide additional information specific to the exception. The
 topic "Method declarations" below describes how to indicate that a particular
 method may raise a particular exception, and Using SOM Classes in Client
 Programs, describes how exceptions are handled, in the section entitled
 "Exceptions and error handling."

 Following is an example declaration of a "BAD_FLAG" exception:

     exception BAD_FLAG { long ErrCode; char Reason[80]; };

 The SOM Compiler will map the above exception declaration to the following C
 language constructs:

     #define ex_BAD_FLAG "::BAD_FLAG"
     typedef struct BAD_FLAG {
         long  ErrCode;
         char  Reason[80];
     } BAD_FLAG;

 Thus, the ex_BAD_FLAG symbol can be used as a shorthand for naming the
 exception.

 An exception declaration within an interface "Hello", such as this:

     interface Hello {
         exception LOCAL_EXCEPTION { long ErrCode; };
     };

 would map onto:

     #define ex_Hello_LOCAL_EXCEPTION "::Hello::LOCAL_EXCEPTION"
     typedef struct Hello_LOCAL_EXCEPTION {
         long  ErrCode;
     } Hello_LOCAL_EXCEPTION;
     #define ex_LOCAL_EXCEPTION ex_Hello_LOCAL_EXCEPTION

 In addition to user-defined exceptions, there are several predefined
 exceptions for system run-time errors. The standard exceptions as prescribed
 by CORBA are shown in the table "Standard Exceptions Defined by CORBA". These
 exceptions correspond to standard run-time errors that may occur during the
 execution of any method (regardless of the list of exceptions listed in its
 IDL specification).

 Each of the standard exceptions has the same structure: an error code (to
 designate the subcategory of the exception) and a completion status code. For
 example, the NO_MEMORY standard exception has the following definition:

     enum completion_status {YES, NO, MAYBE};
     exception NO_MEMORY { unsigned long minor;
                           completion_status completed; };

 The "completion_status" value indicates whether the method was never initiated
 (NO), completed its execution prior to the exception (YES), or the completion
 status is indeterminate (MAYBE).

 Since all the standard exceptions have the same structure,  somcorba.h
 (included by som.h) defines a generic StExcep typedef which can be used
 instead of the specific typedefs:

     typedef struct StExcep {
          unsigned long minor;
          completion_status completed;
     } StExcep;

 The standard exceptions shown in the table "Standard Exceptions Defined by
 CORBA". are defined in an IDL module called StExcep, in the file called
 stexcep.idl, and the C definitions can be found in stexcep.h.

 Standard Exceptions Defined by CORBA

  module StExcep {
    #define ex_body { unsigned long minor; completion_status completed; }

    enum completion_status { YES, NO, MAYBE };

    enum exception_type {NO_EXCEPTION, USER_EXCEPTION, SYSTEM_EXCEPTION};

    exception UNKNOWN              ex_body;   // the unknown exception
    exception BAD_PARAM            ex_body;   // an invalid parameter was passed
    exception NO_MEMORY            ex_body;   // dynamic memory allocation failure
    exception IMP_LIMIT            ex_body;   // violated implementation limit
    exception COMM_FAILURE         ex_body;   // communication failure
    exception INV_OBJREF           ex_body;   // invalid object reference
    exception NO_PERMISSION        ex_body;   // no permission for attempted op.
    exception INTERNAL             ex_body;   // ORB internal error
    exception MARSHAL              ex_body;   // error marshalling param/result
    exception INITIALIZE           ex_body;   // ORB initialization failure
    exception NO_IMPLEMENT         ex_body;   // op. implementation unavailable
    exception BAD_TYPECODE         ex_body;   // bad typecode
    exception BAD_OPERATION        ex_body;   // invalid operation
    exception NO_RESOURCES         ex_body;   // insufficient resources for
                                   request
    exception NO_RESPONSE          ex_body;   // response to req. not yet
                                   available
    exception PERSIST_STORE        ex_body;   // persistent storage failure
    exception BAD_INV_ORDER        ex_body;   // routine invocations out
                                   of order
    exception TRANSIENT            ex_body;   // transient failure - reissue
                                   request
    exception FREE_MEM             ex_body;   // cannot free memory
    exception INV_IDENT            ex_body;   // invalid identifier syntax
    exception INV_FLAG             ex_body;   // invalid flag was specified
    exception INTF_REPOS           ex_body;   // error accessing interface
                                   repository
    exception CONTEXT              ex_body;   // error processing context object
    exception OBJ_ADAPTER          ex_body;   // failure detected by object adapter
    exception DATA_CONVERSION      ex_body;   // data conversion error
  };


ΓòÉΓòÉΓòÉ 5.2.4. Interface declarations ΓòÉΓòÉΓòÉ

The IDL specification for a class of objects must contain a declaration of the
interface these objects will support. Because, in SOM, objects are implemented
using classes, the interface name is always used as a class name as well.
Therefore, an interface declaration can be understood to specify a class name,
and its parent (direct base) class names. This is the approach used in the
following description of an interface declaration. In addition to the class
name and its parents names, an interface indicates new methods (operations),
and any constants, type definitions, and exception structures that the
interface exports. An interface declaration has the following syntax:

  interface class-name [: parent-class1, parent-class2, ...]
  {
  constant declarations     (optional)
  type declarations       (optional)
  exception declarations     (optional)
  attribute declarations     (optional)
  method declarations      (optional)
  implementation statement    (optional)
  };

Many class implementers distinguish a "class-name" by using an initial capital
letter, but that is optional. The "parent-class" (or base-class) names specify
the interfaces from which the interface of "class-name" instances is derived.
Parent-class names are required only for the immediate parent(s). Each parent
class must have its own IDL specification (which must be #included in the
subclass's .idl file). A parent class cannot be named more than once in the
interface statement header.

Note: In general, an "interface class-name" header must precede any subsequent
implementation that references "class-name." For more discussion of multiple
interface statements, refer to the later topic "Module declarations to define
multiple interfaces in an .idl file."

The following topics describe the various declarations/statements that can be
specified within the body of an interface declaration. The order in which these
declarations are specified is usually optional, and declarations of different
kinds can be intermixed. Although all of the declarations/statements are listed
above as "optional," in some cases using one of them may mandate another. For
example, if a method raises an exception, the exception structure must be
defined beforehand. In general, types, constants, and exceptions, as well as
interface declarations, must be defined before they are referenced, as in
C/C++.


ΓòÉΓòÉΓòÉ 5.2.5. Constant, type, and exception declarations ΓòÉΓòÉΓòÉ

The form of a constant, type, or exception declaration within the body of an
interface declaration is the same as described previously in this chapter.
Constants and types defined within an interface for a class are transferred by
the SOM Compiler to the binding files it generates for that class, whereas
constants and types defined outside of an interface are not.

Global types (such as, those defined outside of an interface and module) can be
emitted by surrounding them with the following #pragmas:

   #pragma somemittypes on
        typedef sequence <long,10> vec10;
        exception BAD_FLAG { long ErrCode; char Reason[80]; };
        typedef long long_t;
   #pragma somemittypes off

Types, constants, and exceptions defined in a parent class are also accessible
to the child class. References to them, however, must be unambiguous. Potential
ambiguities can be resolved by prefacing a name with the name of the class that
defines it, separated by the characters "::" as illustrated below:

   MyParentClass::myType

The child class can redefine any of the type, constant, and exception names
that have been inherited, although this is not advised. The derived class
cannot, however, redefine attributes or methods. It can only replace the
implementation of methods through overriding (as in example 3 of the Tutorial).
To refer to a constant, type, or exception "name" defined by a parent class and
redefined by "class-name," use the "parent-name::name" syntax as before.

Note: A name reference such as MyParentClass::myType required in IDL syntax is
equivalent to MyParentClass_myType in C/C++. For a full discussion of name
recognition in SOM, see "Scoping and name resolution" later in this chapter.


ΓòÉΓòÉΓòÉ 5.2.6. Attribute declarations ΓòÉΓòÉΓòÉ

Declaring an attribute as part of an interface is equivalent to declaring two
accessor methods: one to retrieve the value of the attribute (a "get" method,
named "_get_<attributeName>") and one to set the value of the attribute (a
"set" method, named "_set_<attributeName>").

Attributes are declared as follows:

      [ readonly ]  attribute  type-spec declarators ;

 where "type-spec" specifies any valid IDL type and "declarators" is a list of
 identifiers or pointer declarators, delimited by commas. (An array declarator
 cannot be used directly when declaring an attribute, but the type of an
 attribute can be a user-defined type that is an array.) The optional readonly
 keyword specifies that the value of the attribute can be accessed but not
 modified by client programs.  (In other words, a readonly attribute has no
 "set" method.) Below are examples of attribute declarations, which are
 specified within the body of an interface statement for a class:

     interface Goodbye: Hello, SOMObject
     {
       void  sayBye();

       attribute short xpos;
       attribute char c1, c2;
       readonly attribute float xyz;
     };

 The preceding attribute declarations are equivalent to defining the following
 methods:

     short _get_xpos();
     void  _set_xpos(in short xpos);
     char  _get_c1();
     void  _set_c1(in char c1);
     char  _get_c2();
     void  _set_c2(in char c2);
     float _get_xyz();

 Note: Although the preceding attribute declarations are equivalent to the
 explicit method declarations above, these method declarations are not legal
 IDL, because the method names begin with an `_'. All IDL identifiers must
 begin with an alphabetic character, not including `_'.

 Attributes are inherited from ancestor classes (indirect base classes). An
 inherited attribute name cannot be redefined to be a different type.


ΓòÉΓòÉΓòÉ 5.2.7. Method (operation) declarations ΓòÉΓòÉΓòÉ

Method (operation) declarations define the interface of each method introduced
by the class. A method declaration is similar to a C/C++ function definition:

      [oneway] type-spec identifier ( parameter-list)
      [raises-expr]  [context-expr] ;
 where "identifier" is the name of the method and "type-spec" is any valid IDL
 type (or the keyword void, indicating that the method returns no value).
 Unlike C and C++ procedures, methods that do not return a result must specify
 void as their return type. The remaining syntax of a method declaration is
 elaborated in the following subtopics.

 Note:  Although IDL does not allow methods to receive and return values whose
 type is a pointer to a function, it does allow methods to receive and return
 method names (as string values). Thus, rather than defining methods that pass
 pointers to functions (and that subsequently invoke those functions),
 programmers should instead define methods that pass method names (and
 subsequently invoke those methods using one of the SOM-supplied
 method-dispatching or method-resolution methods or functions, such as
 somDispatch).


ΓòÉΓòÉΓòÉ 5.2.7.1. Oneway keyword ΓòÉΓòÉΓòÉ

The optional oneway keyword specifies that when a client invokes the method,
the invocation semantics are "best-effort", which does not guarantee delivery
of the call. "Best-effort" implies that the method will be invoked at most
once. A oneway method should not have any output parameters and should have a
return type of void. A oneway method also should not include a "raises
expression" (see below), although it may raise a standard exception.

If the oneway keyword is not specified, then the method has "at-most-once"
invocation semantics if an exception is raised, and it has "exactly-once"
semantics if the method succeeds. This means that a method that raises an
exception has been executed zero or one times, and a method that succeeds has
been executed exactly once.

Note: Currently the "oneway" keyword, although accepted, has no effect on the
C/C++ bindings that are generated.


ΓòÉΓòÉΓòÉ 5.2.7.2. Parameter list ΓòÉΓòÉΓòÉ

The "parameter-list" contains zero or more parameter declarations for the
method, delimited by commas. (The target object for the method is not
explicitly specified as a method parameter in IDL, nor are the Environment or
Context parameters.)  If there are no explicit parameters, the syntax "( )"
must be used, rather than "(void)". A parameter declaration has the following
syntax:

{ in Γöé out Γöé inout }  type-spec  declarator

where "type-spec" is any valid IDL type and "declarator" is an identifier,
array declarator, or pointer declarator.

In, out, inout parameters:  The required inΓöéoutΓöéinout directional attribute
indicates whether the parameter is to be passed from client to server (in),
from server to client (out), or in both directions (inout). A method must not
modify an in  parameter. If a method raises an exception, the values of the
return result and the values of the out and inout parameters (if any) are
undefined. When an unbounded string or sequence is passed as an inout
parameter, the returned value must be no longer than the input value.

The following are examples of valid method declarations in SOM IDL:

   short meth1(in char c, out float f);
   oneway void meth2(in char c);
   float meth3();

Classes derived from SOMObject can declare methods that take a pointer to a
block of memory containing a variable number of arguments, using a final
parameter of type va_list. The va_list must use the parameter name "ap", as in
the following example:

   void MyMethod(in short numArgs, in va_list ap);

For in parameters of type array, C and C++ clients must pass the address of the
first element of the array. For in parameters of type struct,  union, sequence
or any, C/C++ clients must pass the address of a variable of that type, rather
than the variable itself.

For all IDL types except arrays, if a parameter of a method is out or inout,
then C/C++ clients must pass the address of a variable of that type (or the
value of a pointer to that variable) rather than the variable itself. (For
example, to invoke method "meth1" above, a pointer to a variable of type float
must be passed in place of parameter "f".) For arrays, C/C++ clients must pass
the address of the first element of the array.

If the return type of a method is a struct, union, sequence, or any type, then
for C/C++ clients, the method returns the value of the C/C++ struct
representing the IDL struct, union, sequence, or any. If the return type is
string, then the method returns a pointer to the first character of the string.
If the return type is array, then the method returns a pointer to the first
element of the array.

The pointers implicit in the parameter types and return types for IDL method
declarations are made explicit in SOM's C and C++ bindings. Thus, the stub
procedure that the SOM Compiler generates for method "meth1", above, has the
following signature:

   SOM_Scope short  SOMLINK meth1(char c, float *f)

For C and C++ clients, if a method has an out parameter of type  string
sequence, or any, then the method must allocate the storage for the string, for
the "_buffer" member of the struct that represents the sequence, or for the
"_value" member of the struct that represents the any. It is then the
responsibility of the client program to free the storage when it is no longer
needed. Similarly, if the return type of a method is string, sequence, array,
or any, then storage must be allocated by the method, and it will be the
responsibility of the client program to subsequently free it.

Note: The foregoing description also applies for the _get_ <attributeName>
method associated with an attribute of type string, sequence, any, or array.
Hence, the attribute should be specified with a "noget" modifier to override
automatic implementation of the attribute's "get" method.  Then, needed memory
can be allocated by the developer's "get" method implementation and
subsequently deallocated by the caller.  (The "noget" modifier is described in
Modifier statements.)


ΓòÉΓòÉΓòÉ 5.2.7.3. Raises expression ΓòÉΓòÉΓòÉ

The optional raises expression ("raises-expr") in a method declaration
indicates which exceptions the method may raise. (IDL exceptions are
implemented by simply passing back error information after a method call, as
opposed to the "catch/throw" model where an exception is implemented by a long
jump or signal.)  A raises expression is specified as follows:

      raises ( identifier1, identifier2,... )

 where each "identifier" is the name of a previously defined exception. In
 addition to the exceptions listed in the raises expression, a method may also
 signal any of the standard exceptions. Standard exceptions, however, should
 not appear in a raises expression. If no raises expression is given, then a
 method can raise only the standard exceptions. (See the earlier topic
 "Exception declarations" for information on defining exceptions and for the
 list of standard exceptions. See "Exceptions and error handling," in Chapter 3
 for information on using exceptions.)


ΓòÉΓòÉΓòÉ 5.2.7.4. Context expression ΓòÉΓòÉΓòÉ

The optional context expression ("context-expr") in a method declaration
indicates which elements of the client's context the method may consult. A
context expression is specified as follows:

      context ( identifier1, identifier2, ... )

 where each "identifier" is a string literal made up of alphanumeric
 characters, periods, underscores, and asterisks. (The first character must be
 alphabetic, and an asterisk can only appear as the last character, where it
 serves as a wildcard matching any characters. If convenient, identifiers may
 consist of period-separated valid identifier names, but that form is
 optional.)

 The Context is a special object that is specified by the CORBA standard.  It
 contains a properly list - a set of property-name/string-value pairs that the
 client can use to store information about its environment that methods may
 find useful.  It is used in much the same way as environment variables.  It is
 passed as an additional (third) parameter to CORBA-compliant methods that are
 defined as "context-sensitive" in IDL, along with the CORBA-defined
 Environment structure.

 The context expression of a mehod declaration in IDL specifies which property
 names the method uses.  If these properties are present in the Context object
 supplied by the client, they will be passed to the object implementation,
 which can access them via the get_values method of the Context object.
 However, the argument that is passed to the method having a context expression
 is a Context object, not the names of the properties.  The client program must
 either create a Context object and use the set_values or set_one_value method
 of the Context class to set the context properties, or use the
 get_default_context method. The client program then passes the Context object
 in the method invocation.  Note that the CORBA standard also allows properties
 in addition to those in the context expression to be passed in the Context
 object.

 In Chapter 3, "Using SOM Classes in Client Programs," the topic "Invoking
 Methods" describes the placement of a context parameter in a method call. See
 also Chapter 6 of The Common Object Request Broker: Architecture and
 Specification for a discussion of how clients associate values with context
 identifiers. A description of the Context class and its methods is contained
 in the SOM Programming Reference.


ΓòÉΓòÉΓòÉ 5.2.8. Implementation statements ΓòÉΓòÉΓòÉ

A SOM IDL interface statement for a class may contain an implementation
statement, which specifies information about how the class will be implemented
(version numbers for the class, overriding of inherited methods, what
resolution mechanisms the bindings for a particular method will support, and so
forth). If the implementation statement is omitted, default information is
assumed.

Because the implementation statement is specific to SOM IDL (and is not part of
the CORBA standard), the implementation statement should be preceded by an
"#ifdef _ _SOMIDL_ _" directive and followed by an "#endif" directive. (See
Example 3 in the SOM IDL Tutorial presented earlier.)

The syntax for the implementation statement is as follows:

   #ifdef __SOMIDL__
   implementation
   {
     implementation*
   };
   #endif

where each "implementation" can be a modifier  statement, a passthru statement,
or a declarator of an instance variable, terminated by a semicolon. These
constructs are described below. An interface statement may not contain multiple
implementation statements.


ΓòÉΓòÉΓòÉ 5.2.8.1. Modifier statements ΓòÉΓòÉΓòÉ

A modifier statement gives additional implementation information about IDL
definitions, such as interfaces, attributes, methods, and types. Modifiers can
be unqualified or qualified: An unqualified modifier is associated with the
interface it is defined in. An unqualified modifier statement has the following
two syntactic forms:

  modifier
  modifier = value
where "modifier" is either a SOM Compiler-defined identifier or a user-defined
identifier, and where "value" is an identifier, a string enclosed in double
quotes (" "), or a number.

For example:

   filestem = foo;
   nodata;
   persistent;
   dllname = "E:/som/dlls";

A qualified modifier is associated with a qualifier.  The qualified modifier
has the syntax:

   qualifier : modifier
   qualifier : modifier = value
   #pragma modifier qualifier : modifier
   #pragma modifier qualifier : modifier = value

where "qualifier" is the identifier of an IDL definition or is user defined. If
the "qualifier" is an IDL definition introduced in the current interface,
module, or global scope, then the modifier is attached to that definition.
Otherwise, if the qualifier is user defined, the modifier is attached to the
interface it occurs in. If a user-defined modifier is defined outside of an
interface body (by using #pragma modifier), then it is ignored.

For example, consider the following IDL file.  (Notice that qualified modifiers
can be defined with the "qualifier" and "modifier[=value]" in either order.
Also observe that additional modifiers can be included by separating them with
commas.)

   #include <somobj.idl>
   #include <somcls.idl>


   typedef long newInt;
   #pragma somemittypes on
   #pragma modifier newInt : nonportable;
   #pragma somemittypes off
   module M {
       typedef long long_t;
       module  N {
           typedef short short_t;
           interface M_I : SOMClass {
               implementation {
                   somInit : override;
               };
           };
           interface I : SOMObject {
               void op ();
               #pragma modifier op : persistent;


               typedef char char_t;
               implementation {
                   releaseorder : op;
                   metaclass = M_I;
                   callstyle = oidl;
                   mymod : a, b;
                   mymod : c, d;
                   e     : mymod;
                   f     : mymod;
                   op : persistent;
               };
          };
       };
    };

From the preceding IDL file, we associate modifiers with the following
definitions:

   TypeDef "::newInt"             1  modifier: nonportable
   InterfaceDef "::M::N::M_I"     1  modifier: override = somInit
   InterfaceDef "::M::N::I"       9 modifiers: metaclass = M_I,
                                               releaseorder = op
                                               callstyle = oidl
                                               mymod = a,b,c,d,e,f
                                               a = mymod
                                               b = mymod
                                               c = mymod
                                               d = mymod
                                               e = mymod
                                               f = mymod
   OperationDef "::M::N::I::op"   1 modifier: persistent

Notice, how the modifiers for the user-defined qualifier "mymod":

   mymod : a, b;
   mymod : c, d;
   e     : mymod;
   f     : mymod;

map onto:

   mymod = a,b,c,d,e,f
   a     = mymod
   b     = mymod
   c     = mymod
   d     = mymod
   e     = mymod
   f     = mymod

This enables users to look up the modifiers with "mymod", either by looking for
"mymod" or by using each individual value that uses "mymod". These user-defined
modifiers are available for Emitter writers and from the Interface Repository
(see Chapter 7, "The Interface Repository Framework").


ΓòÉΓòÉΓòÉ 5.2.8.2. SOM Compiler unqualified modifiers ΓòÉΓòÉΓòÉ

Unqualified modifiers (described below) include the SOM Compiler-defined
identifiers abstract, baseproxyclass, callstyle, classinit, directinitclasses,
dllname, filestem, functionprefix, majorversion, metaclass, memory_management,
minorversion, somallocte and somdeallocate.

 abstract                 Specifies that the class is intended for use as a
                          parent for subclass derivations, but not for creating
                          instances.

 baseproxyclass =class    Specifies the base proxy class to be used by DSOM
                          when dynamically creating a proxy class for the
                          current class. The base proxy class must be derived
                          from the class SOMDClientProxy  The SOMDClientProxy
                          class will be used if the baseproxyclass modifier is
                          unspecified. (See chapter 6, "distributed SOM," for a
                          discussion on customizing proxy classes.)

 callstyle = oidl         Specifies that the method stub procedures generated
                          by SOM's C/C++ bindings will not include the
                          CORBA-specified (Environment *ev) and (context *ctx)
                          parameters.

 classinit = procedure    Specifies a user-written procedure that will be
                          executed to complete the initialization of a class
                          object after it is created. The classinit modifier is
                          needed if something should happen exactly once when a
                          class is created. (That is, you want to define an
                          action that will not be inherited when subclasses are
                          created. One example of this is for staticdata
                          variables.) When the classinit modifier is specified
                          in the .idl file for a class, the implementation file
                          generated by the SOM Compiler provides a template for
                          the procedure, which includes a parameter that is a
                          pointer to the class. The class implementor can then
                          fill in the body of this procedure template. (For an
                          example, see the examples following the staticdata
                          modifier under "SOM Compiler qualified modifiers.")

 directinitclasses ="ancestor1, ancestor2, ..."


                          Specifies the ancestor class(es) whose initializers
                          (and destructors) will be directly invoked by this
                          class's initialization (and destruction) routines.
                          If this modifier is not explicitly specified, the
                          default setting is the parents of the class. For
                          further information, see "Initializing and
                          Uninitializing Objects" in Chapter 5, "Implementing
                          Classes in SOM."

 dllname = filename       Specifies the name of the library file that will
                          contain the class's implementation. If filename
                          contains special characters (e.g., periods,
                          backslashes), then filename should be surrounded by
                          double quotes (""). The filename specified can be
                          either a full pathname, or an unqualified (or
                          partially qualified) filename. In the latter cases,
                          the PATH environment variable is used to locate the
                          file.

 filestem  = stem         Specifies how the SOM Compiler will construct file
                          names for the binding files it generates (<stem>.h,
                          <stem>.c, etc.). The default stem is the file stem of
                          the .idl file for the class.

 functionprefix  = prefix Directs the SOM Compiler to construct
                          method-procedure names by prefixing method names with
                          "prefix". For example, "functionprefix = xx;"  within
                          an implementation statement would result in a
                          procedure name of xxfoo for method foo.  The default
                          for this attribute is the empty string. If an
                          interface is defined in a module, then the default
                          function prefix is the fully scooped interface name.
                          Tip:  Using a function prefix with the same name as
                          the class makes it easier to remember
                          method-procedure names when debugging.

                          When an .idl file defines multiple interfaces not
                          contained within a module, use of a function prefix
                          for each interface is essential to avoid name
                          collisions. For example, if one interface introduces
                          a method and another interface in the same .idl file
                          overrides it, then the implementation file for the
                          classes will contain two method procedures of the
                          same name (unless function prefixes are defined for
                          one of the classes), resulting in a name collision at
                          compile time.

 majorversion = number    Specifies the major version number of the current
                          class definition. The major version number of a class
                          definition usually changes only when a significant
                          enhancement or incompatible change is made to the
                          class. The "number" must be a positive integer less
                          than 2[32]-1.  If a non-zero major version number is
                          specified, SOM will verify that any code that
                          purports to implement the class has the same major
                          version number. The default major version number is
                          zero.

 memory_management = corba Specifies that all methods introduced by the class
                          follow the CORBA specification for parameter memory
                          management, except where a particular method has an
                          explicit modifier indicating otherwise (either
                          "object_owns_result" or "object_owns_parameters").
                          See the section in Chapter 6 entitled "Memory
                          Management" for a discussion of the CORBA
                          memory-management requirements.

 metaclass = class        Specifies the class's metaclass. The specified
                          metaclass (or one automatically derived from it at
                          run time) will be used to create the class object for
                          the class. If a metaclass is specified, its .idl file
                          (if separate) must be included in the include section
                          of the class's .idl file. If no metaclass is
                          specified, the metaclass will be defined
                          automatically.

 minorversion = number    Specifies the minor version number of the current
                          class definition. The minor version number of a class
                          definition changes whenever minor enhancements or
                          fixes are made to a class. Class implementers usually
                          maintain backward compatibility across changes in the
                          minor version number. The "number" must be a positive
                          integer less than 2[32]-1. If a non-zero minor
                          version number is specified, SOM will verify that any
                          code that purports to implement the class has the
                          same or a higher minor version number. The default
                          minor version number is zero.

 somallocate=procedure    Specifies a user-written procedure that will be
                          executed to allocate memory for class instances when
                          the somAllocate class method is invoked.

 somdeallocate=procedure  Specifies a user-written procedure that will be
                          executed to deallocate memory for class instances
                          when the somDeallocate class method is invoked.

 The following example illustrates the specification of unqualified interface
 modifiers:

     implementation
     {
       filestem = hello;
       functionprefix = hel;
       majorversion = 1;
       minorversion = 2;
       classinit = helloInit;
       metaclass = M_Hello;
     };


ΓòÉΓòÉΓòÉ 5.2.8.3. SOM Compiler qualified modifiers ΓòÉΓòÉΓòÉ

Qualified modifiers are categorized according to the IDL component (class,
attribute, method, or type) to which each modifier applies. Listed below are
the SOM Compiler-defined identifiers used as qualified modifiers, along with
the IDL component to which it applies. Descriptions of all qualified modifiers
are then given in alphabetical order. Recall that qualified modifiers are
defined using the syntax qualifier: modifier[=value].

 For classes:
      releaseorder

 For attributes:
      indirect, nodata, noget, noset

 For methods:
      caller_owns_parameters, caller_owns_result, const, init, method, migrate,
      namelookup, nocall, noenv, nonstatic, nooverride, noself,
      object_owns_parameters, object_owns_result, offset, override, procedure,
      reintroduce, and select

 For variables:
      staticdata

 For types:
      impctx

 caller_owns_parameters ="p1,p2,...,pn" Specifies the names of the method's
                          parameters whose ownership is retained by (in the
                          case of "in"parameters) or transferred to (for
                          "inout" or "out" parameters) the caller. This
                          modifier is only valid in the interface specification
                          of the method's introducing class.  This modifier
                          only makes sense for parameters whose IDL type is a
                          data item that can be freed (string, object, array,
                          pointer, or TypeCode), or a data item containing
                          memory that can be freed (for example, a sequence or
                          any), or a struct or union.

                          For parameters whose type is an object, ownership
                          applies to the object reference rather than to the
                          object (that is, the caller should invoke release on
                          the parameter, rather than somFree).

 caller_owns_result       Specifies that ownership of the return result of  the
                          method is transferred to the caller, and that the
                          caller is responsible for freeing the memory. This
                          modifier is only valid in the interface specification
                          of the method's introducing class. This modifier only
                          makes sense when the method's return type is a data
                          type that can be freed (string, object, array,
                          pointer, or TypeCode), or a data item containing
                          memory that can be freed (for example, a sequence or
                          any). For methods that return an object, ownership
                          applies to the object reference rather than to the
                          object (that is, the caller should invoke release on
                          the result, rather than somFree).

 const                    Indicates that implementations of the related method
                          should not modify their target argument. SOM provides
                          no way to verify or guarantee that implementations do
                          not modify the targets of such methods, and the
                          information provided by this modifier is not
                          currently of importance to any of the Toolkit
                          emitters. However, the information may prove useful
                          in the future. For example, since modifiers are
                          available in the Interface Repository, there may be
                          future uses of this information by DSOM.

 impctx                   Supports types that cannot be fully defined using
                          IDL. For full information, see "Using the tk_foreign
                          TypeCode" in Chapter 7, "The Interface Repository
                          Framework."

 indirect                 Directs the SOM Compiler to generate "get" and "set"
                          methods for the attribute that take and return a
                          pointer to the attribute's value, rather than the
                          attribute value itself. For example, if an attribute
                          x of type float is declared to be an indirect
                          attribute, then the "_get_x" method will return a
                          pointer to a float, and the input to the "_set_x"
                          method must be a pointer to a float. (This modifier
                          is provided for OIDL compatibility only.)

 init                     Indicates that a method is an initializer method.
                          For information concerning the use of this modifier,
                          see "Initializing and Uninitializing Objects: in
                          Chapter 5, "Implementing Classes in SOM"

 method or nonstatic or  procedure Indicates the category of method
                          implementation. Refer to the topic "The four kinds of
                          SOM methods" in Chapter 5, "Implementing Classes in
                          SOM," for an explanation of the meanings of these
                          different method modifiers.  If none of these
                          modifiers is specified, the default is method.
                          Methods with the procedure modifier cannot be invoked
                          remotely using DSOM.

 migrate  = ancestor      Indicates that a method originally introduced by this
                          interface has been moved upward to a specified
                          <ancestor> interface. When this is done, the method
                          introduction must be removed from this interface
                          (because the method is now inherited). However, the
                          original  releaseorder entry for the method should be
                          retained, and migrate should be used to assure that
                          clients compiled based on the original interface will
                          not require recompilation. The ancestor interface is
                          specified using a C-scoped interface name. For
                          example, "Module_InterfaceName", not
                          "Module::InterfaceName". See the later topic "Name
                          usage in client programs" for an explanation of
                          C-scoped names.

 namelookup               See "offset or namelookup."

 nocall                   Specifies that the related method should not be
                          invoked on an instance of this class even though it
                          is supported by the interface.

 nodata                   Directs the SOM Compiler not to define an instance
                          variable corresponding to the attribute. For example,
                          a "time" attribute would not require an instance
                          variable to maintain its value, because the value can
                          be obtained from the operating system.  The "get" and
                          "set" methods for "nodata" attributes must be defined
                          by the class implementer; stub method procedures for
                          them are automatically generated in the
                          implementation template for the class by the SOM
                          Compiler.

 noenv                    Indicates that a direct-call procedure does not
                          receive an environment as an argument.

 noget                    Directs the SOM Compiler not to automatically
                          generate a "get" method procedure for the attribute
                          in the .ih/.xih binding file for the class. Instead,
                          the "get" method must be implemented by the class
                          implementer. A stub method procedure for the "get"
                          method is automatically generated in the
                          implementation template for the class by the SOM
                          Compiler, to be filled in by the implementer.

 nonstatic                See "method or nonstatic or procedure."

 nooverride               Indicates that the method should not be overridden by
                          subclasses. The SOM Compiler will generate an error
                          if this method is overridden.

 noself                   Indicates that a direct-call procedure does not
                          receive a target object as an argument.

 noset                    Directs the SOM Compiler not  to automatically
                          generate a "set" method procedure for the attribute
                          in the .ih/.xih binding file for the class. Instead,
                          the "set" method must be implemented by the class
                          implementer. A stub method procedure for the "set"
                          method is automatically generated in the
                          implementation template for the class by the SOM
                          Compiler.

                          Note: The "set" method procedure that the SOM
                          Compiler generates by default for an attribute in the
                          .h/.xh binding file (when the noset modifier is not
                          used) does a shallow copy of the value that is passed
                          to the attribute. For some attribute types, including
                          strings and pointers, this may not be appropriate.
                          For instance, the "set" method for an attribute of
                          type string should perform a string copy, rather than
                          a shallow copy, if the attribute's value may be
                          needed after the client program has freed the memory
                          occupied by the string. In such situations, the class
                          implementer should specify the noset attribute
                          modifier and implement the attribute's "set" method
                          manually, rather than having SOM implement the "set"
                          method automatically.

 object_owns_parameters  ="p1, p2, ..., pn"


                          Specifies the names of the method's parameters whose
                          ownership is transferred to (in the case of "in"
                          parameters) or is retained by (for "inout" or "out"
                          parameters) the object.  For "in" parameters, the
                          object can free the parameter at any time after
                          receiving it. (Hence, the caller should not reuse the
                          parameter or pass it as any other object-owned
                          parameter in the same method call.) For "inout" and
                          "out" parameters, the object is responsible for
                          freeing the parameter sometime before the object is
                          destroyed. This modifier is only valid in the
                          interface specification of the method's introducing
                          class. This modifier only makes sense for parameters
                          whose IDL type is a data item that can be freed
                          (string, object, array, pointer, or TypeCode), or a
                          data item containing memory that can be freed (for
                          example, a sequence or any), or a struct or union.

                          For parameters whose type is an object, ownership
                          applies to the object reference  rather than to the
                          object (that is, the object will invoke release on
                          the parameter, rather than somFree). For "in" and
                          "out" parameters whose IDL-to-C/C++ mapping
                          introduces a pointer, ownership applies only to the
                          data item itself, and not to the introduced pointer.
                          (For example, even if an "out string" IDL parameter
                          (which becomes a "string *" C/C++ parameter) is
                          designated as "object-owned,"  the object assumes
                          ownership of the string, but not of the pointer to
                          the string.)

 object_owns_result       Specifies that the object retains ownership of the
                          return result of the method, and that the caller must
                          not free the memory. The object is responsible for
                          freeing the memory sometime before the object is
                          destroyed. This modifier is only valid in the
                          interface specification of the method's introducing
                          class. This modifier only makes sense when the
                          method's return type is a data type that can be freed
                          (string, object, array, pointer, or TypeCode), or a
                          data item containing memory that can be freed (for
                          example, a sequence or any). For methods that return
                          an object, ownership applies to the object reference
                          rather than to the object (that is, the object will
                          be responsible for invoking release on the result,
                          rather than somFree).

 offset or namelookup     Indicates whether the SOM Compiler should generate
                          bindings for invoking the method using offset
                          resolution or name lookup. Offset resolution requires
                          that the class of the method's target object be known
                          at compile time. When different methods of the same
                          name are defined by several classes, namelookup is a
                          more appropriate technique for method resolution than
                          is offset resolution. (See Chapter 3, the section
                          entitled "Invoking Methods.")  The default modifier
                          is offset.

 override                 Indicates that the method is one introduced by an
                          ancestor class and that this class will re-implement
                          the method. See also the related modifier, select.

 procedure                See "method or nonstatic or procedure."

 reintroduce              Indicates that this interface will "hide" a method
                          introduced by some ancestor interface, and will
                          replace it with another implementation. Methods
                          introduced as direct-call procedures or nonstatic
                          methods can be reintroduced. However, static methods
                          (the default implementation category for SOM met
                          hods) cannot be reintroduced.

 releaseorder: a, b, c, ... Specifies the order in which the SOM Compiler will
                          place the class's methods in the data structures it
                          builds to represent the class. Maintaining a
                          consistent release order for a class allows the
                          implementation of a class to change without requiring
                          client programs to be recompiled.

                          The release order should contain every method name
                          introduced by the class (private and nonprivate), but
                          should not include any inherited methods, even if
                          they are overridden by the class. The "get" and "set"
                          methods defined automatically for each new attribute
                          (named "_get_<attributeName>" and
                          "_set_<attributeName>") should also be included in
                          the release order list. The order of the names on the
                          list is unimportant except that once a name is on the
                          list and the class has client programs, it should not
                          be reordered or removed, even if the method is no
                          longer supported by the class, or the client programs
                          will require recompilation. New methods should be
                          added only to the end of the list. If a method named
                          on the list is to be moved up in the class hierarchy,
                          its name should remain on the current list, but it
                          should also be added to the release order list for
                          the class that will now introduce it.

                          If not explicitly specified, the release order will
                          be determined by the SOM Compiler, and a warning will
                          be issued for each missing method. If new methods or
                          attributes are subsequently added to the class, the
                          default release order might change; programs using
                          the class would then require recompilation. Thus, it
                          is advisable to explicitly give a release order.

 select = parent          Used in conjunction with the override modifier, this,
                          indicates that an inherited static method will use
                          the implementation inherited from the indicated
                          <parent> class. The parent is specified using the
                          C-scoped name. For example, "Module_InterfaceName",
                          not "Module:: InterfaceName". See the later topic
                          "Name usage in client programs" for an explanation of
                          C-scoped names.

 staticdata               Indicates that the declared variable is not stored
                          within objects, but, instead, that the ClassData
                          structure for the implementing class will contain a
                          pointer to the staticdata variable. This is similar
                          in concept to C++ static data members. The staticdata
                          variable must also be included in the releaseorder.
                          The class implementor has responsibility for
                          allocating the staticdata variable and for loading
                          the ClassData structure's pointer to the staticdata
                          variable during class initialization. (The pointer is
                          accessible as <className>ClassData.<variableName>.)
                          The implementor's responsibility can be facilitated
                          by writing a special class initialization function
                          and indicating its name using the classinit
                          unqualified modifier. (See also the examples that
                          follow.)

                          Note:  Attributes can be declared as staticdata. This
                                 is an important implementation technique that
                                 allows classes to introduce attributes whose
                                 backing storage is not inherited by
                                 subclasses.

 The following example illustrates the specification of qualified modifiers:

  implementation
  {
    releaseorder : op1, op3, op2, op5, op6, x, y, _set_z, _get_z;
    op1 : persistent;
    somDefaultInit : override, init;
    op2: reintroduce, procedure;
    op3: reintroduce, nonstatic;
    op4: override, select = ModuleName_parentInterfaceName;
    op5: migrate = ModuleName_ancestorInterfaceName;
    op6: procedure, noself, noenv;
    long x;
    x: staticdata;
    y: staticdata; // y and z are attributes
    _set_z: object_owns_parameters = "name";
    _get_z: object_owns_result;
    mymod : a, b;
  };

 As shown above for attribute z, separate modifiers can be declared for an
 attribute's _set and _get methods, using method modifiers. This capability may
 be useful for DSOM applications. (See the DSOM sample program "animal" that is
 distributed with the SOMobjects Toolkit.)

 The next example for classes "X" and "Y" illustrates the use of a staticdata
 modifier, along with its corresponding classinit modifier and the template
 procedure generated for classinit by the SOM Compiler.

  size='auto'.
  /* IDL for staticdata and classinit example: */

  #include <somobj.idl>

  interface X : SOMObject {
          attribute long staticAttribute;
          attribute long normalAttribute;
          implementation {
                  staticAttribute: staticdata;
                  classinit = Xinit;
                  releaseorder: staticAttribute,
                                      _get_staticAttribute,
                                      _set_staticAttribute,
                                      _get_normalAttribute,
                                      _set_normalAttribute;
          };
  };

  interface Y : X { };

  /* Template procedure for classInit: */

  #ifndef SOM_Module_classinit_Source
  #define SOM_Module_classinit_Source
  #endif
  #define X_Class_Source

  #include "classInit.ih"

  static long holdStaticAttribute = 1234;
  void SOMLINK Xinit(SOMClass *cls)
  {
          XClassData.staticAttribute = &holdStaticAttribute;
  }

  main()
  {
          X *x = XNew();
          Y *y = YNew();

          somPrintf("initial staticAttribute = x(%d) = y(%d)\n",
                                  _get_staticAttribute(x,0),
                                  _get_staticAttribute(y,0));

          _set_staticAttribute(x,0,42);
          _set_staticAttribute(y,0,4321);

          somPrintf("changed staticAttribute = x(%d) = y(%d)\n",
                                  _get_staticAttribute(x,0),
                                  _get_staticAttribute(y,0));
  }

  /* Program output:

          initial staticAttribute = x(1234) = y(1234)
          changed staticAttribute = x(4321) = y(4321)
          after setting normalAttribute, x(10) != y(20)
  */


ΓòÉΓòÉΓòÉ 5.2.8.4. Passthru statements ΓòÉΓòÉΓòÉ

A passthru statement (used within the body of an implementation statement,
described above) allows a class implementer to specify blocks of code (for
C/C++ programmers, usually only #include directives) that the SOM compiler will
pass into the header files it generates.

Passthru statements are included in SOM IDL primarily for backward
compatibility with the SOM OIDL language, and their use by C and C++
programmers should be limited to #include directives. C and C++ programmers
should use IDL type and constant declarations rather than passthru statements
when possible. (Users of other languages, however, may require passthru
statements for type and constant declarations.)

The contents of the passthru lines are ignored by the SOM compiler and can
contain anything that needs to placed near the beginning of a header file for a
class. Even comments contained in passthru lines are processed without
modification. The syntax for specifying passthru lines is one of the following
forms:

 passthru language_suffix      = literal+ ;
 passthru language_suffix_before = literal+ ;
 passthru language_suffix_after = literal+ ;

 where "language" specifies the programming language and "suffix" indicates
 which header files will be affected. The SOM Compiler supports suffixes h, ih,
 xh, and xih. For both C and C++, "language" is specified as C.

 Each "literal" is a string literal (enclosed in double quotes) to be placed
 verbatim into the specified header file.  [Double quotes within the passthru
 literal should be preceded by a backslash. No other characters escaped with a
 backslash will be interpreted, and formatting characters (newlines, tab
 characters, etc.) are passed through without processing.]  The last literal
 for a passthru statement must not end in a backslash (put a space or other
 character between a final backslash and the closing double quote).

 When either of the first two forms is used, passthru  lines are placed before
 the #include statements in the header file. When the third form is used,
 passthru lines are placed just after the #include statements in the header
 file.

 For example, the following passthru statement

     implementation
     {
       passthru C_h = "#include <foo.h>";
     };

 results in the directive #include <foo.h> being placed at the beginning of the
 .h C binding file that the SOM Compiler generates.

 For any given target file (as indicated by language_suffix), only one passthru
 statement may be defined within each implementation section. You may, however,
 define multiple #include statements in a single passthru. For legibility, each
 #include should begin on a new line, optionally with a blank line to precede
 and follow the #include list. For an example, see "Introducing non-IDL data
 types or classes" later in this section.


ΓòÉΓòÉΓòÉ 5.2.8.5. Declaring instance variables and staticdata variables ΓòÉΓòÉΓòÉ

Declarators are used within the body of an implementation statement (described
above) to specify the instance variables that are introduced by a class, and
the staticdata variables pointed to by the class's ClassData structure. These
variables are declared using ANSI C syntax for variable declarations,
restricted to valid SOM IDL types (see "Type and constant declarations,"
above). For example, the following implementation statement declares two
instance variables, x and y, and a staticdata variable, z, for class "Hello," :

implementation
{
  short x;
  long y;
  double z;
  z: staticdata;
};

Instance variables are normally intended to be accessed only by the class's
methods and not by client programs or subclasses' methods. For data to be
accessed by client programs or subclass methods, attributes should be used
instead of instance variables. (Note, however, that declaring an attribute has
the effect of also declaring an instance variable of the same name, unless the
"nodata" attribute modifier is specified.)

Staticdata variables, by contrast, are publicly available and are associated
specifically with their introducing class. They are, however, very different in
concept from class variables. Class variables are really instance variables
introduced by a metaclass, and are therefore present in any class that is an
instance of the introducing metaclass (or of any metaclass derived from this
metaclass). As a result, class variables present in any given class will also
be present in any class derived from this class (that is, class variables are
inherited). In contrast, staticdata variables are introduced by a class (not a
metaclass) and are (only) accessed from the class's ClassData structure - they
are not inherited.


ΓòÉΓòÉΓòÉ 5.2.8.6. Introducing non-IDL data types or classes ΓòÉΓòÉΓòÉ

On occasion, you may want a new .idl file to reference some element that the
SOM Compiler would not recognize, such as a user-defined class or an instance
variable or attribute with a user-defined data type. You can reference such
elements if they already exist in .h or .xh files that the SOM Compiler can
#include with your new .idl file, as follows:

     To introduce a non-IDL class, insert an interface statement that is a
      forward reference to the existing user-defined class. It must precede the
      interface statement for the new class in the .idl file.

     To declare an instance variable or attribute that is not a valid IDL
      type, declare a dummy typedef preceding the interface declaration.

     In each case above, in the implementation section use a passthru
      statement to pass an #include statement into the language-specific
      binding file(s) of the new .idl file (a) for the existing user-defined
      class or (b) for the real typedef.

 In the following example, the generic SOM type somToken is used in the .idl
 file for the user's types "myRealType" and "myStructType". The passthru
 statement then causes an appropriate #include statement to be emitted into the
 C/C++ binding file, so that the file defining types "myRealType" and
 "myStructType" will be included when the binding files process. In addition,
 an interface declaration for "myOtherClass" is defined as a forward reference,
 so that an instance of that class can be used within the definition of
 "myCurrentClass". The passthru statement also #includes the binding file for
 "myOtherClass":

  typedef somToken myRealType;
  typedef somToken myStructType;

  interface myOtherClass;

  interface myCurrentClass : SOMObject {
  . . .
          implementation {
                  . . .
                  myRealType myInstVar;
                  attribute myStructType st1;
                  passthru C_h =
                          ""
                          "#include <myTypes.h>"
                          "#include <myOtherClass.h>"
                          "";
          };
  };

 Note: See also the section "Using the tk_foreign TypeCode" in Chapter 7, "The
 Interface Repository Framework."


ΓòÉΓòÉΓòÉ 5.2.9. Comments within a SOM IDL file ΓòÉΓòÉΓòÉ

SOM IDL supports both C and C++ comment styles. The characters "//" start a
line comment, which finishes at the end of the current line. The characters
"/*" start a block comment that finishes with the "*/". Block comments do not
nest. The two comment styles can be used interchangeably.

Comments in a SOM IDL specification must be strictly associated with particular
syntactic elements, so that the SOM Compiler can put them at the appropriate
place in the header and implementation files it generates. Therefore, comments
may appear only in these locations (in general, following the syntactic unit
being commented):

     At the beginning of the IDL specification

     After a semicolon

     Before or after the opening brace of a module, interface statement,
      implementation statement, structure definition, or union definition

     After a comma that separates parameter declarations or enumeration
      members

     After the last parameter in a prototype (before the closing parenthesis)

     After the last enumeration name in an enumeration definition (before the
      closing brace)

     After the colon following a case label of a union definition

     After the closing brace of an interface statement

 Numerous examples of the use of comments can be found in the Tutorial of
 Chapter 2.

 Because comments appearing in a SOM IDL specification are transferred to the
 files that the SOM Compiler generates, and because these files are often used
 as input to a programming language compiler, it is best within the body of
 comments to avoid using characters that are not generally allowed in comments
 of most programming languages. For example, the C language does not allow "*/"
 to occur within a comment, so its use is to be avoided, even when using C++
 style comments in the .idl file.

 SOM IDL also supports throw-away comments. They may appear anywhere in an IDL
 specification, because they are ignored by the SOM Compiler and are not
 transferred to any file it generates. Throw-away comments start with the
 string "//#" and end at the end of the line. Throw-away comments can be used
 to "comment out" portions of an IDL specification.

 To disable comment processing (that is, to prevent the SOM Compiler from
 transferring comments from the IDL specification to the binding files it
 generates), use the -c option of the sc command when running the SOM Compiler
 (See Section 4.3, "The SOM Compiler").  When comment processing is disabled,
 comment placement is not restricted and comments can appear anywhere in the
 IDL specification.


ΓòÉΓòÉΓòÉ 5.2.10. Designating `private' methods and attributes ΓòÉΓòÉΓòÉ

To designate methods or attributes within an IDL specification as "private,"
the declaration of the method or attribute must be surrounded with the
preprocessor commands #ifdef __PRIVATE__ (with two leading underscores and two
following underscores) and #endif. For example, to declare a method "foo" as a
private method, the following declaration would appear within the interface
statement:

   #ifdef __PRIVATE__
   void foo();
   #endif

Any number of methods and attributes can be designated as private, either
within a single #ifdef or in separate ones.

When compiling an .idl file, the SOM Compiler normally recognizes only public
(nonprivate) methods and attributes, as that is generally all that is needed.
To generate header files for client programs that do need to access private
methods and attributes, or for use when implementing a class library containing
private methods, the -p option should be included when running the SOM
Compiler. The resulting header files will then include bindings for private, as
well as public, methods and attributes. Both the implementation bindings (.ih
or .xih file) and the usage bindings to be #included in the implementation (.h
or .xh file) should be generated under the -p option. The -p option is
described in the topic "Running the SOM Compiler" later in this chapter.

The SOMobjects Toolkit also provides a pdl (Public Definition Language) emitter
that can be used with the SOM Compiler to generate a copy of an .idl file which
has the portions designated as private removed. The next main section of this
chapter describes how to invoke the SOM Compiler and the various emitters.


ΓòÉΓòÉΓòÉ 5.2.11. Module declarations to define multiple interfaces in a .idl file ΓòÉΓòÉΓòÉ

A single .idl file can define multiple interfaces. This allows, for example, a
class and its metaclass to be defined in the same file. When a file defines two
(or more) interfaces that reference one another, forward declarations can be
used to declare the name of an interface before it is defined. This is done as
follows:

interface class-name ;

The actual definition of the interface for "class-name" must appear later in
the same .idl file.

If multiple interfaces are defined in the same .idl file, and the classes are
not a class-metaclass pair, they can be grouped into modules, by using the
following syntax:

module module-name { definition+ };

where each "definition" is a type declaration, constant declaration, exception
declaration, interface statement, or nested module statement. Modules are used
to scope identifiers (see below).

Alternatively, multiple interfaces can be defined in a single .idl file without
using a module to group the interfaces. Whether or not a module is used for
grouping multiple interfaces, the languages bindings produced from the .idl
file will include support for all of the defined interfaces.

Note:  When multiple interfaces are defined in a single .idl file and a module
       statement is not used for grouping these interfaces, it is necessary to
       use the functionprefix modifier to assure that different names exist for
       functions that provide different implementations for a method.  In
       general, it is a good idea to always use the functionprefix modifier,
       but in this case it is essential.


ΓòÉΓòÉΓòÉ 5.2.12. Scoping and name resolution ΓòÉΓòÉΓòÉ

A .idl file forms a naming scope (or scope). Modules, interface statements,
structures, unions, methods, and exceptions form nested scopes. An identifier
can only be defined once in a particular scope. Identifiers can be redefined in
nested scopes.

Names can be used in an unqualified form within a scope, and the name will be
resolved by successively searching the enclosing scopes. Once an unqualified
name is defined in an enclosing scope, that name cannot be redefined.

Fully qualified names are of the form:

scoped-name::identifier

For example, method name "meth" defined within interface "Test" of module "M1"
would have the fully qualified name:

  M1::Test::meth

A qualified name is resolved by first resolving the "scoped-name" to a
particular scope S, then locating the definition of "identifier" within that
scope. Enclosing scopes of S are not searched.

Qualified names of the form:

      ::identifier

 These names are resolved by locating the definition of "identifier" within the
 smallest enclosing module.

 Every name defined in an IDL specification is given a global name, constructed
 as follows:

     Before the SOM Compiler scans an .idl file, the name of the current root
      and the name of the current scope are empty. As each module is
      encountered, the string "::" and the module name are appended to the name
      of the current root. At the end of the module, they are removed.

     As each interface, struct, union, or exception definition is encountered,
      the string "::" and the associated name are appended to the name of the
      current scope. At the end of the definition, they are removed. While
      parameters of a method declaration are processed, a new unnamed scope is
      entered so that parameter names can duplicate other identifiers.

     The global name of an IDL definition is then the concatenation of the
      current root, the current scope, a "::", and the local name for the
      definition.

 The names of types, constants, and exceptions defined by the parents of a
 class are accessible in the child class. References to these names must be
 unambiguous. Ambiguities can be resolved  by using a scoped name (prefacing
 the name with the name of the class that defines it and the characters "::",
 as in "parent-class::identifier). Scope names can also be used to refer to a
 constant, type, or exception name defined by a parent class but redefined by
 the child class.


ΓòÉΓòÉΓòÉ 5.2.12.1. Name usage in client programs ΓòÉΓòÉΓòÉ

Within a C or C++ program, the global name for a type, constant, or exception
corresponding to an IDL scoped name is derived by converting the string "::" to
an underscore ("_") and removing the leading underscore. Such names are
referred to as C-scoped names. This means that types, constants, and exceptions
defined within the interface statement for a class can be referenced in a C/C++
program by prepending the class name to the name of the type, constant, or
exception. For example, consider the types defined in the following IDL
specification:

   typedef sequence<long,10> mySeq;
   interface myClass : SOMObject
   {
      enum color {red, white, blue};
      typedef string<100> longString;
      ...
   }

These types could be accessed within a C or C++ program with the following
global names:

mySeq,
myClass_color,
myClass_red,
myClass_white,
myClass_blue, and
myClass_longString.

Type, constant, and exception names defined within modules similarly have the
module name prepended. When using SOM's C/C++ bindings, the short form of type,
constant, and exception names (such as, color, longString) can also be used
where unambiguous, except that enumeration names must be referred to using the
long form (for example, myClass_red and not simply red).

Because replacing "::" with an underscore to create global names can lead to
ambiguity if an IDL identifier contains underscores, it is best to avoid the
use of underscores when defining IDL identifiers.


ΓòÉΓòÉΓòÉ 5.2.13. Extensions to CORBA IDL permitted by SOM IDL ΓòÉΓòÉΓòÉ

The following topics describe several SOM-unique extensions of the standard
CORBA syntax that are permitted by SOM IDL for convenience. These constructs
can be used in an .idl file without generating a SOM Compiler error.

If you want to verify that an IDL file contains only standard CORBA
specifications, the SOM Compiler option -mcorba turns off each of these
extensions and produces compiler errors wherever non-CORBA specifications are
used. (The SOM Compiler command and options are described in the topic "Running
the SOM Compiler" later in this chapter.)


ΓòÉΓòÉΓòÉ 5.2.13.1. Pointer `*' types ΓòÉΓòÉΓòÉ

In addition to the base CORBA types, SOM IDL permits the use of pointer types
(`*'). As well as increasing the range of base types available to the SOM IDL
programmer, using pointer types also permits the construction of more complex
data types, including self-referential and mutually recursive structures and
unions.

If self-referential structures and unions are required, then, instead of using
the CORBA approach for IDL sequences, such as the following:

   struct X {
       ...
       sequence <X> self;
       ...
   };

it is possible to use the more typical C/C++ approach. For example:

   struct X {
       ...
       X *self;
       ...
   };

SOM IDL does not permit an explicit `*' in sequence declarations. If a sequence
is required for a pointer type, then it is necessary to typedef the pointer
type before use.  For example:

   sequence <long *> long_star_seq;        // error.

   typedef long * long_star;
   sequence <long_star> long_star_seq;      // OK.


ΓòÉΓòÉΓòÉ 5.2.13.2. Unsigned types ΓòÉΓòÉΓòÉ

SOM IDL permits the syntax "unsigned <type>", where <type> is a previously
declared type mapping onto "short" or "long".  (Note that CORBA permits only
"unsigned short" and "unsigned long".)


ΓòÉΓòÉΓòÉ 5.2.13.3. Implementation section ΓòÉΓòÉΓòÉ

SOM IDL permits an implementation section in an IDL interface specification to
allow the addition of instance variables, method overrides, metaclass
information, passthru information, and "pragma-like" information, called
modifiers, for the emitters. See the topic "Implementation statements" earlier
in this chapter.


ΓòÉΓòÉΓòÉ 5.2.13.4. Comment processing ΓòÉΓòÉΓòÉ

The SOM IDL Compiler by default does not remove comments in the input source;
instead, it attaches them to the nearest preceding IDL statement. This facility
is useful, since it allows comments to be emitted in header files, C template
files, documentation files, and so forth. However, if this capability is
desired, this does mean that comments cannot be placed with quite as much
freedom as with an ordinary IDL compiler. To turn off comment processing so
that you can compile .idl files containing comments placed anywhere, you can
use the compiler option -c or use "throw-away" comments throughout the .idl
file (that is, comments preceded by //#); as a result, no comments will be
included in the output files.


ΓòÉΓòÉΓòÉ 5.2.13.5. Generated header files ΓòÉΓòÉΓòÉ

CORBA expects one header file, <file>.h, to be generated from <file>.idl.
However, SOM IDL permits use of a class modifier, filestem, that changes this
default output file name.  (See "Running the SOM Compiler" later in this
chapter.)


ΓòÉΓòÉΓòÉ 5.3. The SOM Compiler ΓòÉΓòÉΓòÉ

The SOM Compiler translates the IDL definition of a SOM class into a set of
"binding files" appropriate for the language that will implement the class's
methods and the language(s) that will use the class. These bindings make it
more convenient for programmers to implement and use SOM classes. The SOM
Compiler currently produces binding files for the C and C++ languages.

Important Note: C and C++ bindings can not both be generated during the same
execution of the SOM compiler.


ΓòÉΓòÉΓòÉ 5.3.1. Generating binding files ΓòÉΓòÉΓòÉ

The SOM Compiler operates in two phases:

     A precompile phase, in which a precompiler analyzes an OIDL or IDL class
      definition, and

     An emission phase, in which one or more  emitter programs produce
      binding files.

 Each binding file is generated by a separate emitter program. Setting the
 SMEMIT environment variable determines which emitters will be used, as
 described below. Note: In the discussion below, the <filesystem> is determined
 by default from the name of the source .idl file with the ".idl" extension
 removed. Otherwise, a "filestem" modifier can be defined in the .idl file to
 specify another file name (see Modifier statements).

 Note: If changes to definitions in the .idl file later become necessary, the
 SOM Compiler should be rerun to update the current implementation template
 file, provided that the c or xc emitter is specified (either with the -s
 option or the SMEMIT environment variable, as described below). For more
 information on generating updates, see "Running incremental updates of the
 implementation template file" later in this chapter.

 The emitters for the C language produce the following binding files:

 <filestem>.c             (produced by the c emitter)

                          This is a template for a C source program that
                          implements a class's methods. This will become the
                          primary source file for the class. (The other binding
                          files can be generated from the .idl file as needed.)
                          This template implementation file contains "stub"
                          procedures for each method introduced or overridden
                          by the class. (The stub procedures are empty of code
                          except for required initialization and debugging
                          statements.)

                          After the class implementer has supplied the code for
                          the method procedures, running the c emitter again
                          will update the implementation file to reflect
                          changes made to the class definition (in the .idl
                          file). These updates include adding new stub
                          procedures, adding comments, and changing method
                          prototypes to reflect changes made to the method
                          definitions in the IDL specification. Existing code
                          within method procedures is not disturbed, however.

                          The .c file contains an #include directive for the
                          .ih file, described below.

                          The contents of the C source template is controlled
                          by the Emitter Framework file
                          <SOMBASE>/include/ctm.efw. This file can be
                          customized to change the template produced.

 <filestem>.h             (produced by the h emitter)

                          This is the header file to be included by C client
                          programs (programs that use the class). It contains
                          the C usage bindings for the class, including macros
                          for accessing the class's methods and a macro for
                          creating new instances of the class. This header file
                          includes the header files for the class's parent
                          classes and its metaclass, as well as the header file
                          that defines SOM's generic C bindings, som.h.

 <filestem>.ih            (produced by the ih emitter)

                          This is the header file to be included in the
                          implementation file (the file that implements the
                          class's methods-the .c file). It contains the
                          implementation bindings for the class, including:

                              a struct defining the class's instance
                               variables,
                              macros for accessing instance variables,
                              macros for invoking parent methods the class
                               overrides,
                              the <className>GetData  macro used by the method
                               procedures in the <filestem>.c file (see "Stub
                               procedures for methods" in Chapter 5.)
                              a <className>NewClass procedure for constructing
                               the class object at run time, and
                              any IDL types and constants defined in the IDL
                               interface.

 The emitters for the C++ language produce the following binding files:

 <filestem>.cpp           (produced by the xc emitter)

                          This is a template for a C++ source program that
                          implements a class's methods. This will become the
                          primary source file for the class. (The other binding
                          files can be generated from the .idl file as needed.)
                          This template implementation file contains "stub"
                          procedures for each method introduced or overridden
                          by the class. (The stub procedures are empty of code
                          except for required initialization and debugging
                          statements.)

                          After the class implementer has supplied the code for
                          the method procedures, running the xc emitter again
                          will update this file to reflect changes made to the
                          class definition (in the .idl file). These updates
                          include adding new stub procedures, adding comments,
                          and changing method prototypes to reflect changes
                          made to the method definitions in the IDL
                          specification. Existing code within method procedures
                          is not disturbed, however.

                          The C++ implementation file contains an #include
                          directive for the .xih file, described below.

                          The contents of the C++ source template is controlled
                          by the Emitter Framework file
                          <SOMBASE>/include/ctm.efw. This file can be
                          customized to change the template produced.

 <filestem>.xh            (produced by the xh emitter)

                          This is the header file to be included by C++ client
                          programs that use the class. It contains the usage
                          bindings for the class, including a C++ definition of
                          the class, macros for accessing the class's methods,
                          and the new operator for creating new instances of
                          the class. This header file includes the header files
                          for the class's parent classes and its metaclass, as
                          well as the header file that defines SOM's generic
                          C++ bindings, som.xh.

 <filestem>.xih           (produced by the xih emitter)

                          This is the header file to be included in the
                          implementation file (the file that implements the
                          class's methods). It contains the implementation
                          bindings for the class, including:

                              a struct defining the class's instance
                               variables,
                              macros for accessing instance variables,
                              macros for invoking parent methods the class
                               overrides,
                              the <className>GetData  macro (see section 5.4),
                              a <className>NewClass procedure for constructing
                               the class object at run time, and
                              any IDL types and constants defined in the IDL
                               interface.

 Other files the SOM Compiler generates:

 <filestem>.hh            (produced by the hh emitter)

                          This file is a DirectToSOM C++ header file that
                          describes a SOMobjects class in a way appropriate to
                          DTS C++. When running this emitter, you must include
                          the noqualitytypes command-line modifier for the -m
                          optionof the SOM Compiler command sc or somc.

 <filestem>.pdl           (produced by the pdl emitter) This file is the same
                          as the .idl file from which it is produced except
                          that all items within the .idl file that are marked
                          as "private" have been removed.  (an item is marked
                          as private by surrounding it with "#ifdef_PRIVATE_"
                          and "#endif" directives.  Thus, the pdl (Public
                          Definition Lnguage) emitter can be used to generate a
                          "public" version of the .idl file.

 <filestem>.def           (produced by the def emitter)

                          This file is used by the linker to package a class as
                          a library. To combine several classes into a single
                          library, you must merge the exports statements from
                          each of their .def  files into a single .def file for
                          the entire library. When packaging multiple classes
                          in a single library, you must also write a simple C
                          procedure named SOMInitModule and add it to the
                          export list. This procedure should call the routine
                          <className>NewClass for each class packaged in the
                          library. The SOMInitModule procedure is called by the
                          SOM Class Manager when the library is dynamically
                          loaded.

 The Interface Repository (produced by the ir emitter)

                          See Chapter 7 for a discussion on the Interface
                          Repository.
 Note: The C/C++ bindings generated by the SOM Compiler have the following
 limitation: If two classes named "ClassName" and "ClassNameC" are defined, the
 bindings for these two classes will clash. That is, if a client program uses
 the C/C++ bindings (includes the .h/.xh header file) for both classes, a name
 conflict will occur. Thus, class implementers should keep this limitation in
 mind when naming their classes.

 SOM users can extend the SOM Compiler to generate additional files by writing
 their own emitters.

 Note re:  porting SOM classes: The header files (binding files) that the SOM
 Compiler generates will only work on the platform (operating system) on which
 they were generated. Thus, when porting SOM classes from the platform where
 they were developed to another platform, the header files must be regenerated
 from the .idl file by the SOM Compiler on that target platform.


ΓòÉΓòÉΓòÉ 5.3.2. Environment variables affecting the SOM Compiler ΓòÉΓòÉΓòÉ

To execute the SOM Compiler on one or more files that contain IDL
specifications for one or more classes, use the sc. as follows:

      sc [-options] files

 where "files" specifies one or more .idl files.

 Available "-options" for the command are detailed in the next topic. The
 operation of the SOM Compiler (whether it produces C binding files or C++
 binding files, for example) is also controlled by a set of environment
 variables that can be set before the sc command is issued. The applicable
 environment variables are as follows:

 SMEMIT              Determines which output files the SOM Compiler produces.
                     Its value consists of a list of items separated by
                     semicolons Each item designates an emitter to execute.
                     For example, the statement:

                     SET SMEMIT=c;h;ih             (for C binding files)

                     directs the SOM Compiler to produce the C binding files
                     "hello.c", "hello.h", and"hello.ih" from the "hello.idl"
                     input specification. By comparison,

                     SET SMEMIT=xc;xh;xih

                     directs the SOM Compiler to produce C++ binding files

                                           "hello.cpp" (for OS/2)
                     , "hello.xh", and "hello.xih" from the "hello.idl" input
                     specification.

                     By default, all output files are placed in the same
                     directory as the input file. If the SMEMIT environment
                     variable is not set, then a default value of "h;ih" is
                     assumed.

 SMINCLUDE           Specifies where the SOM Compiler should look for .idl
                     files #included by the .idl file being compiled. Its value
                     should be one or more directory names separated by a
                     semicolon Directory names can be specified with absolute
                     or relative pathnames. For example:

                     SET SMINCLUDE=.;..\MYSCDIR;C:\TOOLKT20\C\INCLUDE;

                     The default value of the SMINCLUDE environment variable is
                     the "include" subdirectory of the directory into which SOM
                     has been installed.

 SMTMP               Specifies the directory that the SOM Compiler should use
                     to hold intermediate output files. This directory should
                     not coincide with the directory of the input or output
                     files. AIX, the default setting of SMTMP is /tmp; for
                     OS/2, the default setting of SMTMP is the root directory
                     of the current drive. For example:

                                             SET SMTMP=..\MYSCDIR\GARBAGE

                     tells the SOM Compiler to place the temporary files in the
                     GARBAGE directory.


                     Or, :

                                             SET SMTMP=%TMP%

                     tells the SOM Compiler to use the same directory for
                     temporary files as given by the setting of the TMP
                     environment variable (the defult location for temporary
                     system files).

 SMKNOWNEXTS         Specifies additional emitters to which the SOM Compiler
                     should add a header. For example, if you were to write a
                     new emitter for Pascal, called "emitpas", then by default
                     the SOM Compiler would not add any header comments to it.
                     However, by setting SMKNOWNEXTS=pas, as shown:

                       set SMKNOWNEXTS=pas

                     the SOM Compiler will add a header to files generated with
                     the "emitpas" emitter. The "header" added is a SOM
                     Compiler-generated message plus any comments, such as
                     copyright statements, that appear at the head of your .idl
                     input file.

 SOMIR               Specifies the name (or list of names) of the Interface
                     Repository file. The ir emitter, if run, creates the
                     Interface Repository, or checks it for consistency if it
                     already exists. If the -u option is specified when
                     invoking the SOM Compiler, the ir emitter also updates an
                     existing Interface Repository.

 SMADDSTAR           When defined, causes all interface references to have a
                     "*" added to them for the C bindings. The command-line
                     options -maddstar and -mnoaddstar supercede and override
                     the SMADDSTAR setting, however.

 Note:  Environment variables that affect the SOM Compiler can be set for any
        -m options of the SOM Compiler command. See the -m option in the
        following topic, "Running the SOM Compiler." Also, the -E option in the
        following topic can be used to set an environment variable.


ΓòÉΓòÉΓòÉ 5.3.3. Running the SOM Compiler ΓòÉΓòÉΓòÉ

The syntax of the command for running the SOM Compiler takes the forms:

      sc  [-options] files

 The "files" specified in the sc command denote one or more files containing
 the IDL class definitions to be compiled. If no extension is specified, .idl
 is assumed. By default, the <filestem> of the .idl file determines the
 filestem of each emitted file. Otherwise, a "filestem" modifier can be defined
 in the .idl file to specify another name (see Modifier statements).

 Selected "-options" can be specified individually, as a string of option
 characters, or as a combination of both. Any option that takes an argument
 either must be specified individually or must appear as the final option in a
 string of option characters. Available options and their purposes are as
 follows:

 -C n        Sets the maximum allowable size for a simple comment in the .idl
             file (default: 32767). This is only needed for very large single
             comments.

 -D name[=def] Defines name as in a #define directive. The default def is 1.
             This option is the same as the -D option for the C compiler. Note:
             This option can be used to define __PRIVATE__ so that the SOM
             Compiler will also compile any methods and attributes that have
             been defined as private using the directive  #ifdef__PRIVATE__;
             however, the -p option does the same thing more easily. When a
             class contains private methods or attributes, both the
             implementation bindings and the usage bindings to be #included in
             the implementation should be generated using either the -p or
             -D_PRIVATE_ option.

 -E variable=value Sets an environment variable. (See the previous topic for a
             discussion of the available environment variables: SMADDSTAR,
             SMEMIT, SMINCLUDE, SMTMP, SMKNOWNEXTS, and SOMIR.)

 -I dir      When looking for #included files, looks first in dir, then in the
             standard directories (same as the C compiler -I option).

 -S n        Sets the total allowable amount of unique string space used in the
             IDL specification for names and passthru lines (default: 32767).
             This is only needed for very large .idl files.

 -U name     Removes any initial definition (via a #define preprocessor
             directive) of symbol name.

 -V          Displays version information about the SOM Compiler.

 -c          Turns off comment processing. This allows comments to appear
             anywhere within an IDL specification (rather than in restricted
             places), and it causes comments not to be transferred to the
             output files that the SOM Compiler produces.

 -d  directory Specifies a directory where all output files should be placed.
             If the -d option is not used, all output files are placed in the
             same directory as the input file.

 -h or -?    Produces a listing of this option list. (This option is typically
             used in an sc or command that does not include a .idl file name)

 -i filename Specifies the name of the class definition file. Use this option
             to override the built-in assumption that the input file will have
             a .idl extension. Any filename supplied with the -i option is used
             exactly as it is specified.

 -m name[=value] Adds a global modifier. (See the following Note on the -m
             options, which explains how to convert any "-m name" modifier to
             an environment variable.)

             Note:  All command-line -m modifier options can be specified in
                    the environment by changing them to UPPERCASE and
                    prepending "SM" to them. For example, if you want to always
                    set the options "-mnotc" and "-maddstar", set corresponding
                    environment variables as follows:

                                            set SMNOTC=1
                                            set SMADDSTAR=1

             The currently supported global modifiers for the -m name[=value]
             option are as follows:

             addprefixes         Adds `functionprefixes' to the method
                                 procedure prototypes during an incremental
                                 update of the implementation template file.
                                 This option applies only when rerunning the c
                                 or xc emitter on an IDL file that previously
                                 did not specify a functionprefix. A class
                                 implementor who later decides to use prefixes
                                 should add a line in the `implementation'
                                 section of the .idl file containing the
                                 specification:

                                 functionprefix = prefix

                                 (as described in Modifier statements) and then
                                 rerun the c or xc emitter using the
                                 -maddprefixes option. The method procedure
                                 prototypes in the implementation file will
                                 then be updated so that each method name
                                 includes the assigned prefix. (This option
                                 does not support changes to existing prefix
                                 names, nor does it apply for OIDL files.)

             addstar             This option causes all interface references to
                                 have a `*' added to them for the C bindings.
                                 See the earlier section entitled "Object
                                 types" for further details.

             comment=comment string where comment string can be either of the
                                 designations: "/*"  or "//". This option
                                 indicates that comments marked in the
                                 designated manner in the .idl file are to be
                                 completely ignored by the SOM Compiler and
                                 will not be included in the output files.
                                 Note: Comments on lines beginning with "//#"
                                 are always ignored by the SOM Compiler.

             corba               This option directs the SOM Compiler to
                                 compile the input definition according to
                                 strict CORBA-defined IDL syntax. This means,
                                 for example, that comments may appear anywhere
                                 and that pointers are not allowed. When the
                                 -mcorba option is used, parts of a .idl file
                                 surrounded by #ifdef__SOMIDL__ and #endif
                                 directives are ignored. This option can be
                                 used to determine whether all nonstandard
                                 constructs (those specific to SOM IDL) are
                                 properly protected by #ifdef__SOMIDL__ and
                                 #endif directives.

             csc                 This option forces the OIDL compiler to be
                                 run. This is required only if you want to
                                 compile an OIDL file that does not have an
                                 extension of .csc or .sc.

             emitappend          This option causes emitted files to be
                                 appended at the end of existing files of the
                                 same name.

             noaccessors         This option turns off the automatic creation
                                 of OperationDef entries in the Interface
                                 Repository for attribute accessors (that is,
                                 for an attribute's _set and _get methods).

             noaddstar           This option ensures that interface references
                                 will not have a "*" added to them for the C
                                 bindings. This is the default setting; it is
                                 the opposite of the -m compiler option
                                 addstar.

             noint               This option directs the SOM Compiler not to
                                 warn about the portability problems of using
                                 int's in the source.

             nolock              This option causes the Interface Repository
                                 Emitter emitir (see Chapter 7, "Interface
                                 Repository Framework") to leave the IR
                                 unlocked when updates are made to it. This can
                                 improve performance on networked file systems.
                                 By not locking the IR, however, there is the
                                 risk of multiple processes attempting to write
                                 to the same IR, with unpredictable results.
                                 This option should only be used when you know
                                 that only one process is updating an IR at
                                 once.

             nopp                This option directs the SOM Compiler not to
                                 run the SOM preprocessor on the .idl input
                                 file.

             noqualifytypes      This option prevents the use od C-scoped names
                                 in emitter output, and is used in conjunction
                                 with the .hh emitter.

             notc                This option directs the SOM Compiler not to
                                 create TypeCode information when emitting IDL
                                 files that contain some undeclared types. This
                                 option is only used when compiling converted
                                 .csc files (that is, OIDL files originally)
                                 that have not had typing information added.

             nouseshort          This option directs the SOM Compiler not to
                                 generate short forms for type names in the .h
                                 and .xh public header files. This can be
                                 useful to save disk space.

             pbl                 This option tells the SOM Compiler that, in
                                 declarations containing a linkage specifier,
                                 the "*" will appear before the linkage
                                 specifier. This is required when using any C++
                                 compiler (Watcom is a known example) that
                                 cannot handle declarations in the default
                                 format where the "*" follows the linkage
                                 specifier. A default example is the
                                 declaration:

                                                                  typedef void (SOMLINK * somTD_SOMObject_somFree)
                                                                               (SOMObject *somSelf);

                                 Under the -mpbl option of the SOM Compiler
                                 command, the same example would be declared
                                 as:

                                                                  typedef void (* SOMLINK somTD_SOMObject_somFree)
                                                                               (SOMObject *somSelf);

             pp=preprocessor     This option directs the SOM Compiler to use
                                 the specified preprocessor as the SOM
                                 preprocessor, rather than the default
                                 "somcpp". Any standard C/C++ preprocessor can
                                 be used as a preprocessor for IDL
                                 specifications.

             tcconsts            This option directs the SOM Compiler to
                                 generate TypeCode constants in the h and .xh
                                 public header files. Please refer to the
                                 Interface Repository (described in Chapter 7)
                                 for more details.

 -p          Causes the "private" sections of the IDL file to be included in
             the compilation (that is, sections preceded by #ifdef __PRIVATE__
             that contain private methods and attributes). Note: If -p is used,
             it must be applied for both the implementation bindings (.ih or
             .xih file) and the usage bindings (.h or .xh file) to be #included
             in the implementation.

 -r          Checks that all names specified in the release order statement are
             valid method names (default: FALSE).

 -s string   Substitutes string in place of the contents of the SMEMIT
             environment variable for the duration of the current sc command.
             This determines which emitters will be run and, hence, which
             output files will be produced. (If a list of values is given, the
             list must be enclosed in double quotes.)

             The -s option is a convenient way to override the SMEMIT
             environment variable. The command:

                             > SC -s"h;c" EXAMPLE

             is equivalent to the following sequence of commands:

                             > SET OLDSMEMIT=%SMEMIT%
                             > SET SMEMIT=H;C
                             > SC EXAMPLE
                             > SET SMEMIT=%OLDSMEMIT%

 -u          Updates the Interface Repository (default: no update). With this
             option, the Interface Repository will be updated even if the ir
             emitter is not explicitly requested in the SMEMIT environment
             variable or the -s option.

 -v          Uses verbose mode to display informational messages (default:
             FALSE). This option is primarily intended for debugging purposes
             and for writers of emitters.

 -w          Suppresses warning messages (default: FALSE).

 The following sample commands illustrate various options for the sc command:

 sc -sc hello.idl         Generates file "hello.c".

 sc -hV                   Generates a help message and displays the version of
                          the SOM Compiler currently available.

 sc -vsh";"ih hello.idl   Generates "hello.h" and "hello.ih" with informational
                          messages.

 sc -sxc -doutdir hello.idl Generates "hello.xc" in directory "outdir".


ΓòÉΓòÉΓòÉ 5.4. The `pdl' Facility ΓòÉΓòÉΓòÉ

As discussed earlier in this chapter, the SOM Compiler provides a pdl (Public
Definition Language) emitter. This emitter generates a file that is the same as
the .idl file from which it is produced, except that it removes all items
within the .idl file that are marked as "private." An item is marked as private
by surrounding it with "#ifdef_ _PRIVATE_ _" and "#endif" directives. Thus, the
pdl emitter can be used to generate a "public" version of a .idl file.
(Generally, client programs will need only the "public" methods and
attributes.)

The SOMobjects Toolkit also provides a separate program, pdl, which performs
the same function as the pdl emitter, but can be invoked independently of the
SOM Compiler. In addition, the pdl program can remove any kind of items in the
.idl file that are preceded by a user-specified "#ifdef" directive and followed
by an "#endif" directive:  The pdl  program is invoked as follows:

        pdl [ -c | d | f | h | s |/]  files

 where "files" specifies one or more .idl files whose specified "#ifdef"
 sections are to be removed.  Filenames must be completely specified (with the
 .idl extension).  If no "#ifdef" directive is specified (by including a
 -/<string> option), then the "#ifdef_ _PRIVATE_ __"sections will be removed by
 default.

 The pdl command supports the following options.  (Selected options can be
 specified individually, as a string of option characters, or as a combination
 of both. Any option that takes an argument either must be specified
 individually or must appear as the final option in a string of option
 characters.)

 -c cmd      Specifies that, for each .idl file, the pdl program is to run the
             specified system command. This command may contain a single
             occurrence of the string "%s", which will be replaced with the
             source file name before the command is executed. For example the
             option -c"sc -sh %s" has the same effect as issuing the sc command
             with the -sh option.

 -d dir      Specifies a directory in which the output files are to be placed.
             (The output files are given the same name as the input files.) If
             no directory is specified, the output files are named
             <fileStem>.pdl (where fileStem is the file stem of the input file)
             and are placed in the current working directory.

 -h          Requests this description of the pdl command syntax and options.

 -f          Specifies that output files are to replace existing files with the
             same name, even if the existing files are read-only. By default,
             files are replaced only if they have write access.

 -s smemit   Specifies the SMEMIT variable with which the pdl program is to
             invoke the SOM Compiler.

 -/ <string> Specifies the "#ifdef" pattern for which the pdl program will
             strip out .idl statements. The default is "#ifdef _ _PRIVATE_ _".

 For example, to install public versions of the .idl files in the directory
 "pubinclude", type:

     pdl -d pubinclude *.idl


ΓòÉΓòÉΓòÉ 6. Implementing Classes in SOM ΓòÉΓòÉΓòÉ

This chapter begins with a more in-depth discussion of SOM concepts and the SOM
run-time environment than was appropriate in Tutorial for Implementing SOM
Classes. Subsequent sections then provide information about completing an
implementation template file, updating the template file, compiling and
linking, packaging classes in libraries, and other useful topics for class
implementors. During this process, you can refer to Chapter 4, "SOM IDL and the
SOM Compiler," if you want to read the reference information or see the full
syntax related to topics discussed in this chapter. The current chapter ends
with topics describing how to customize SOMobjects execution in various ways.


ΓòÉΓòÉΓòÉ 6.1. The SOM Run-Time Environment ΓòÉΓòÉΓòÉ

The SOMobjects Developer Toolkit provides

     The SOM Compiler, used when creating SOM class libraries, and

     The SOM run-time library, for using SOM classes at execution time.

 The SOM run-time library provides a set of functions used primarily for
 creating objects and invoking methods on them. The data structures and objects
 that are created, maintained, and used by the functions in the SOM run-time
 library constitute the SOM run-time environment.

 A distinguishing characteristic of the SOM run-time environment is that SOM
 classes are represented by run-time objects; these objects are called class
 objects.  By contrast, other object-oriented languages such as C++ treat
 classes strictly as compile-time structures that have no properties at run
 time. In SOM, however, each class has a corresponding run-time object. This
 has three advantages:  First, application programs can access information
 about a class at run time, including its relationships with other classes, the
 methods it supports, the size of its instances, and so on. Second, because
 much of the information about a class is established at run time rather than
 at compile time, application programs needn't be recompiled when this
 information changes. Finally, because class objects can be instances of
 user-defined classes in SOM, users can adapt the techniques for subclassing
 and inheritance in order to build object-oriented solutions to problems that
 are otherwise not easily addressed within an OOP context.


ΓòÉΓòÉΓòÉ 6.1.1. Run-time environment initialization ΓòÉΓòÉΓòÉ

When the SOM run-time environment is initialized, four primitive SOM objects
are automatically created. Three of these are class objects (SOMObject,
SOMClass, and SOMClassMgr), and one is an instance of SOMClassMgr, called the
SOMClassMgrObject. Once loaded, application programs can invoke methods on
these class objects to perform tasks such as creating other objects, printing
the contents of an object, freeing objects, and the like. These four primitive
objects are discussed below.

In addition to creating the four primitive SOM objects, initialization of the
SOM run-time environment also involves initializing global variables to hold
data structures that maintain the state of the environment.  Other functions in
the SOM run-time library rely on these global variables.

For application programs written in C or C++ that use the language-specific
bindings provided by SOM, the SOM run-time environment is automatically
initialized the first time any object is created.  Programmers using other
languages must initialize the run-time environment explicitly by calling the
somEnvironmentNew function (provided by the SOM run-time library) before using
any other SOM functions or methods.


ΓòÉΓòÉΓòÉ 6.1.1.1. SOMObject class object ΓòÉΓòÉΓòÉ

SOMObject is the root class for all SOM classes. It defines the essential
behavior common to all SOM objects.  All user-defined SOM classes are derived,
directly or indirectly, from this class. That is, every SOM class is a subclass
of SOMObject or of some other class derived from SOMObject. SOMObject has no
instance variables, thus objects that inherit from SOMObject incur no size
increase. They do inherit a suite of methods that provide the behavior required
of all SOM objects.


ΓòÉΓòÉΓòÉ 6.1.1.2. SOMClass class object ΓòÉΓòÉΓòÉ

Because SOM classes are run-time objects, and since all run-time objects are
instances of some class, it follows that a SOM class object must also be an
instance of some class. The class of a class is called a metaclass. Hence, the
instances of an ordinary class are individuals (nonclasses), while the
instances of a metaclass are class objects.

In the same way that the class of an object defines the "instance methods" that
the object can perform, the metaclass of a class defines the "class methods"
that the class itself can perform. Class methods (sometimes called factory
methods or constructors) are performed by class objects. Class methods perform
tasks such as creating new instances of a class, maintaining a count of the
number of instances of the class, and other operations of a "supervisory"
nature. Also, class methods facilitate inheritance of instance methods from
parent classes. For information on the distinction between parent classes and
metaclasses, see the section "Parent Class vs. metaclass," later in this
chapter.

SOMClass is the root class for all SOM metaclasses.  That is, all SOM
metaclasses must be subclasses of SOMClass or of some metaclass derived from
SOMClass. SOMClass defines the essential behavior common to all SOM class
objects.  In particular, SOMClass provides:

     Six class methods for creating new class instances: somNew, somNewNoInit,
      somRenew, somRenewNoInit, somRenewNoZero  and somRenewNoInitNoZero.

     A number of class methods that dynamically obtain or update information
      about a class and its methods at run time, including:

        -  somInitMIClass, for implementing multiple inheritance from parent
           classes,

        -  somOverrideSMethod, for overriding inherited methods, and

        -  somAddStaticMethod and somAddDynamicMethod, for including new
           methods.

 SOMClass is a subclass (or child) of SOMObject. Hence, SOM class objects can
 also perform the same set of basic instance methods common to all SOM objects.
 This is what allows SOM classes to be real objects in the SOM run-time
 environment. SOMClass also has the unique distinction of being its own
 metaclass (that is, SOMClass defines its own class methods).

 A user-defined class can designate as its metaclass either SOMClass or another
 user-written metaclass descended from SOMClass. If a metaclass is not
 explicitly specified, SOM determines one automatically.


ΓòÉΓòÉΓòÉ 6.1.1.3. SOMClassMgr class object and SOMClassMgrObject ΓòÉΓòÉΓòÉ

The third primitive SOM class is SOMClassMgr. A single instance of the
SOMClassMgr class is created automatically during SOM initialization. This
instance is referred to as the SOMClassMgrObject, because it is pointed to by
the global variable SOMClassMgrObject. The object SOMClassMgrObject has the
responsibility to

     Maintain a registry (a run-time directory) of all SOM classes that exist
      within the current process, and to

     Assist in the dynamic loading and unloading of class libraries.

 For C/C++ application programs using the SOM C/C++ language bindings, the
 SOMClassMgrObject automatically loads the appropriate library file and
 constructs a run-time object for the class the first time an instance of a
 class is created. For programmers using other languages, SOMClassMgr provides
 a method, somFindClass, for directing the SOMClassMgrObject to load the
 library file for a class and create its class object.

 Again, the primitive classes supplied with SOM are SOMObject, SOMClass, and
 SOMClassMgr.  During SOM initialization, the latter class generates an
 instance called SOMClassMgrObject. The SOMObject class is the parent class of
 SOMClass and SOMClassMgr. The SOMClass class is the metaclass of itself, of
 SOMObject, and of SOMClassMgr, which are all class objects at run time.
 SOMClassMgr is the class of SOMClassMgrObject.


ΓòÉΓòÉΓòÉ 6.1.2. Parent class vs. metaclass ΓòÉΓòÉΓòÉ

There is a distinct difference between the notions of "parent" (or base) class
and "metaclass." Both notions are related to the fact that a class defines the
methods and variables of its instances, which are therefore called instance
methods and instance variables.

A parent of a given class is a class from which the given class is derived  by
subclassing. (Thus, the given class is called a child or a subclass  of the
parent.) A parent class is a class from which instance methods and instance
variables are inherited. For example, the parent of class "Dog" might be class
"Animal". Hence, the instance methods and variables introduced by "Animal"
(such as methods for breathing and eating, or a variable for storing an
animal's weight) would also apply to instances of "Dog", because "Dog" inherits
these from "Animal", its parent class. As a result, any given dog instance
would be able to breath and eat, and would have a weight.

A metaclass is a class whose instances are class objects, and whose instance
methods and instance variables (as described above) are therefore the methods
and variables of class objects. For this reason, a metaclass is said to define
class methods-the methods that a class object performs. For example, the
metaclass of "Animal" might be "AnimalMClass", which defines the methods that
can be invoked on class "Animal" (such as, to create Animal instances-objects
that are not classes, like an individual pig or cat or elephant or dog).

Note: It is important to distinguish the methods of a class object (that is,
the methods that can be invoked on the class object, which are defined by its
metaclass) from the methods that the class defines for its instances.

To summarize: the parent of a class provides inherited methods that the class's
instances can perform; the metaclass of a class provides class methods that the
class itself can perform.  These distinctions are further summarized below: The
distinctions between parent class and metaclass are summarized in A class has
both parent classes and a metaclass.


A class has both parent classes and a metaclass

Any class "C" has both a metaclass and one or more parent class(es).

     The parent class(es) of "C" provide the inherited instance methods that
      individual instances (objects "O{i}") of class "C" can perform. Instance
      methods that an instance "O{i}" performs might include (a) initializing
      itself, (b) performing computations using its instance variables, (c)
      printing its instance variables, or (d) returning its size. New instance
      methods are defined by "C" itself, in addition to those inherited from
      C's parent classes.

     The metaclass "M"defines the class methods that class "C" can perform.
      For example, class methods defined by metaclass "M" include those that
      allow "C" to (a) inherit its parents' instance methods and instance
      variables, (b) tell its own name, (c) create new instances, and (d) tell
      how many instance methods it supports. These methods are inherited from
      SOMClass. Additional methods supported by "M" might allow "C" to count
      how many instances it creates.

     Each class "C" has one or more parent classes and exactly one metaclass.
      (The single exception is SOMObject, which has no parent class.)  Parent
      class(es) must be explicitly identified in the IDL declaration of a
      class. (SOMObject is given as a parent if no subsequently-derived class
      applies.)  If a metaclass is not explicitly listed, the SOM run time will
      determine an applicable metaclass.

     An instance of a metaclass is always another class object. For example,
      class "C" is an instance of metaclass "M".  SOMClass is the SOM-provided
      metaclass from which all subsequent metaclasses are derived.

 A metaclass has its own inheritance hierarchy (through its parent classes)
 that is independent of its instances' inheritance hierarchies. For example,
 suppose a series of classes is defined (or derived), stemming from SOMObject.
 The child class (or subclass) at the end of this line ("C[2]") inherits
 instance methods from all of its ancestor classes (here, SOMObject  and
 "C[1]"). An instance created by "C2" can perform any of these instance
 methods.  In an analogous manner, a line of metaclasses can be defined,
 stemming from SOMClass. Just as a new class is derived from an existing class
 (such as SOMObject), a new metaclass is derived from an existing metaclass
 (such as SOMClass).


  Parent classes and metaclasses each have their own independent inheritance hierrarchies


ΓòÉΓòÉΓòÉ 6.1.3. SOM-derived metaclasses ΓòÉΓòÉΓòÉ

As previously discussed, a class object can perform any of the class methods
that its metaclass defines. New metaclasses are typically created to modify
existing class methods or introduce new class method(s). Chapter 8, "Metaclass
Framework," discusses metaclass programming.

Three factors are essential for effective use of metaclasses in SOM:

     First, every class in SOM is an object that is implemented by a
      metaclass.

     Second, programmers can define and name new metaclasses, and can use
      these metaclasses when defining new SOM classes.

     Finally, and most importantly, metaclasses cannot interfere with the
      fundamental guarantee required of every OOP system: specifically, any
      code that executes without method-resolution error on instances of a
      given class will also execute without method-resolution errors on
      instances of any subclass of this class.

 Surprisingly, SOM is currently the only OOP system that can make this final
 guarantee while also allowing programmers to explicitly define and use named
 metaclasses. This is possible because SOM automatically determines an
 appropriate metaclass that supports this guarantee, automatically deriving new
 metaclasses by subclassing at run time when this is necessary. As an example,
 suppose class "A" is an instance of metaclass "AMeta".

 Assume that "AMeta" supports a method "bar" and that "A" supports a method
 "foo" that uses the expression "_bar( _somGetClass(somSelf ) )." That is,
 method "foo" invokes "bar" on the class of the object on which "foo" is
 invoked. For example, when method "foo" is invoked on an instance of class "A"
 (say, object "O{1}"), this in turn invokes "bar" on class "A" itself.

 Now consider what happens if class "A" were subclassed by "B," a class that
 has the explicit metaclass "BMeta" declared in its SOM IDL source file (and
 assuming "BMeta" is not derived from "AMeta"). Also assume that object "O{2}"
 is an instance of class "B."

 Recall that "AMeta" supports method "bar" and that class "A" supports method
 "foo" (which incorporates "bar" in its definition). Given the hierarchy
 described above, an invocation of "foo" on "O {2}" would fail, because
 metaclass "BMeta" does not support the "bar" method.


  Example of Metaclass Incompatibility

 There is only one way that "BMeta" can support this specific method-by
 inheriting it from "AMeta" ("BMeta" could introduce another method named
 "bar", but this would be a different method from the one introduced by
 "AMeta"). Therefore, in this example, because "BMeta" is not a subclass of
 "AMeta", "BMeta" cannot be allowed to be the metaclass of "B". That is,
 "BMeta" is not compatible with the requirements placed on "B" by the
 fundamental principle of OOP referred to above. This situation is referred to
 as metaclass incompatibility.

 SOM does not allow hierarchies with metaclass incompatibilities. Instead, SOM
 automatically builds derived metaclasses when this is necessary. For example,
 SOM would create a "DerivedMeta" metaclass that has both "AMeta" and "BMeta"
 as parents. This ensures that the invocation of method "foo" on instances of
 class "B" will not fail, and also ensures that the desired class methods
 provided by "BMeta" will be available on class "B".


  Example of a Derived Metaclass

 There are three important aspects of SOM's approach to derived metaclasses:

     First, the creation of SOM-derived metaclasses is integrated with
      programmer-specified metaclasses. If a programmer-specified metaclass
      already supports all the class methods and variables needed by a new
      class, then the programmer-specified metaclass will be used as is.

     Second, if SOM must derive a different metaclass than the one explicitly
      indicated by the programmer (in order to support all the necessary class
      methods and variables), then the SOM-derived metaclass inherits from the
      explicitly indicated metaclass first. As a result, the method procedures
      defined by the specified metaclass take precedence over other
      possibilities (see the following section on inheritance and the
      discussion of resolution of ambiguity in the case of multiple
      inheritance).

     Finally, the class methods defined by the derived metaclass invoke the
      appropriate initialization methods of its parents to ensure that the
      class variables of its instances are correctly initialized.

 As further explanation for the automatic derivation of metaclasses, consider
 the following multiple-inheritance example. Class "C" (derived from classes
 "A" and "B") does not have an explicit metaclass declaration in its SOM IDL,
 yet its parents "A" and "B" do. As a result, class "C" requires a derived
 metaclass. (If you still have trouble following the reasoning behind derived
 metaclasses, ask yourself the following question: What class should "C" be an
 instance of? After a bit of reflection, you will conclude that, if SOM did not
 build the derived metaclass, you would have to do so yourself.)


  Multiple Inheritance requires Derived Metaclasses

 In summary, SOM allows and encourages the definition and explicit use of named
 metaclasses. With named metaclasses, programmers can not only affect the
 behavior of class instances by choosing the parents of classes, but they can
 also affect the behavior of the classes themselves by choosing their
 metaclasses.  Because the behavior of classes in SOM includes the
 implementation of inheritance itself, metaclasses in SOM provide an extremely
 flexible and powerful capability allowing classes to package solutions to
 problems that are otherwise very difficult to address within an OOP context.

 At the same time, SOM is unique in that it relieves programmers of the
 responsibility for avoiding metaclass incompatibility when defining a new
 class. At first glance, this might seem to be merely a useful (though very
 important) convenience. But, in fact, it is absolutely essential, because SOM
 is predicated on binary compatibility with respect to changes in class
 implementations.

 A programmer might, at one point in time, know the metaclasses of all ancestor
 classes of a new subclass, and, as a result, be able to explicitly derive an
 appropriate metaclass for the new class. Nevertheless, SOM must guarantee that
 this new class will still execute and perform correctly when any of its
 ancestor class's implementations are changed (which could even include
 specifying different metaclasses). Derived metaclasses allow SOM to make this
 guarantee.  A SOM programmer need never worry about the problem of metaclass
 incompatibility; SOM does this for the programmer. Instead, explicit
 metaclasses can simply be used to "add in" whatever behavior is desired for a
 new class. SOM automatically handles anything else that is needed. Chapter 8
 provides useful examples of such metaclasses. A SOM programmer should find
 numerous applications for the techniques that are illustrated there.


ΓòÉΓòÉΓòÉ 6.2. Inheritance ΓòÉΓòÉΓòÉ

One of the defining aspects of an object model is its characterization of
inheritance. This section describes SOM's model for inheritance.

A class in SOM defines an implementation for objects that support a specific
interface:

     The interface defines the methods supported by objects of the class, and
      is specified using SOM IDL.

     The implementation  defines what instance variables implement an object's
      state and what procedures implement its methods.

 New classes are derived (by subclassing) from previously existing classes
 through inheritance, specialization, and addition.  Subclasses inherit
 interface from their parent classes: any method available on instances of a
 class is also available on instances of any class derived from it (either
 directly or indirectly).  Subclasses also inherit implementation (the
 procedures that implement the methods) from their parent classes unless the
 methods are overridden (redefined or specialized). In addition, a subclass may
 introduce new instance methods and instance variables that will be inherited
 by other classes derived from it.

 SOM also supports multiple inheritance. That is, a class may be derived from
 (and may inherit interface and implementation from) multiple parent classes.
 Note: Multiple inheritance is available only to SOM classes whose interfaces
 are specified in IDL, and not to SOM classes whose interfaces are specified in
 SOM's earlier interface definition language, OIDL. See Appendix B for
 information on how to automatically convert existing OIDL files to IDL.

 It is possible under multiple inheritance to encounter potential conflicts or
 ambiguities with respect to inheritance. All multiple inheritance models must
 face these issues, and resolve the ambiguities in some way. For example, when
 multiple inheritance is allowed, it is possible that a class will inherit the
 same method or instance variable from different parents (because each of these
 parents has some common ancestor that introduces the method or instance
 variable). In this situation, a SOM subclass inherits only one implementation
 of the method or instance variable. (The implementation of an instance
 variable within an object is just the location where it is stored. The
 implementation of a method is a procedure pointer, stored within a method
 table.) The following illustration addresses the question of which method
 implementation would be inherited.

 Consider this situation: Class "W" defines a method "foo", implemented by
 procedure "proc1". Class "W" has two subclasses, "X" and "Y". Subclass "Y"
 overrides the implementation of "foo" with procedure "proc2". Subclass "X"
 does not override "foo". In addition, classes "X" and "Y" share a common
 subclass, "Z". That is, the IDL interface statement for class "Z" lists its
 parents as "X" and "Y" in that order. (These relationships form a diamond
 shape, with class "W" at the top.)

 The question is thus: which implementation of method "foo" does class "Z"
 inherit-procedure "proc1" defined by class "W", or procedure "proc2" defined
 by class "Y"? The procedure for performing inheritance that is defined by
 SOMClass resolves this ambiguity by using the left path precedence rule: when
 the same method is inherited from multiple ancestors, the procedure used to
 support the method is the one used by the leftmost ancestor from which the
 method is inherited. (The ordering of parent classes is determined by the
 order in which the class implementor lists the parents in the IDL
 specification for the class.)

 Class "Z" inherits the implementation of method "foo" defined by class "W"
 (procedure "proc1"), rather than the implementation defined by class "Y"
 (procedure "proc2"), because "X" is the leftmost ancestor of "Z" from which
 the method "foo" is inherited. This rule may be interpreted as giving priority
 to classes whose instance interfaces are mentioned first in IDL interface
 definitions.

 If a class implementor decides that the default inherited implementation is
 not appropriate (for example, procedure "proc2" is desired), then SOM IDL
 allows the class designer to select the parent whose implementation is
 desired. For more information concerning this approach, see the Select
 modifier, which is documented in the topic "Modifier statements" in Chapter 4,
 "SOM IDL and the SOM Compiler."

 Note:  Alternatively, an explicit metaclass for "Z" could be introduced to
        change the way methods are inherited. However, this would be a fairly
        serious step to take-it would also affect the semantics of inheritance
        for all of Z's descendant classes.

 Another conflict that may arise with the use of multiple inheritance is when
 two ancestors of a class define different methods (in general, with different
 signatures) of the same name. For example, suppose Class "X" defines a method
 "bar" with type T1, and class "Y" defines a method "bar" with type T2. Class
 "Z" is derived from both "X" and "Y", and "Z" does not override method "bar".

 This example illustrates a method name that is "overloaded"-that is, used to
 name two entirely different methods (note that overloading is completely
 unrelated to overriding). This is not necessarily a difficult problem to
 handle. Indeed, the run-time SOM API allows the construction of a class that
 supports the two different "bar" methods. (They are implemented using two
 different method-table entries, each of which is associated with its
 introducing class.)

 However, the interface to instances of such classes can not be defined using
 IDL. IDL specifically forbids the definition of interfaces in which method
 names are overloaded.  Furthermore, within SOM itself, the use of such classes
 can lead to anomalous behavior unless care is taken to avoid the use of
 name-lookup method resolution (discussed in the following section), since, in
 this case, a method name alone does not identify a unique method. For this
 reason, (statically declared) multiple-inheritance classes in SOM are
 currently restricted to those whose instance interfaces can be defined using
 IDL. Thus, the above example cannot be constructed with the aid of the SOM
 Compiler.


ΓòÉΓòÉΓòÉ 6.3. Method Resolution ΓòÉΓòÉΓòÉ

Method resolution is the step of determining which procedure to execute in
response to a method invocation. For example, consider this scenario:

     Class "Dog" introduces a method "bark", and

     A subclass of "Dog", called "BigDog", overrides "bark", and

     A client program creates an instance of either "Dog" or "BigDog"
      (depending on some run-time criteria) and invokes method "bark" on that
      instance.

 Method resolution is the process of determining, at run time, which method
 procedure to execute in response to the method invocation (either the method
 procedure for "bark" defined by "Dog", or the method procedure for "bark"
 defined by "BigDog"). This determination depends on whether the receiver of
 the method (the object on which it is invoked) is an instance of "Dog" or
 "BigDog" (or perhaps depending on some other criteria).

 SOM allows class implementors and client programs considerable flexibility in
 deciding how SOM performs method resolution. In particular, SOM supports three
 mechanisms for method resolution, described in order of increased flexibility
 and increased computational cost: offset resolution, name-lookup resolution,
 and dispatch-function resolution.


ΓòÉΓòÉΓòÉ 6.3.1. Offset resolution ΓòÉΓòÉΓòÉ

When using SOM's C and C++ language bindings, offset resolution is the default
way of resolving methods, because it is the fastest. For those familiar with
C++, it is roughly equivalent to the C++ "virtual function" concept.

Although offset resolution is the fastest technique for method resolution, it
is also the most constrained. Specifically, using offset resolution requires
these constraints:

     The name of the method to be invoked must be known at compile time,

     The name of the class that introduces the method must be known at compile
      time (although not necessarily by the programmer), and

     The method to be invoked must be part of the introducing class's static
      (IDL) interface definition.

 To perform offset method resolution, SOM first obtains a method token from a
 global data structure associated with the class that introduced the method.
 This data structure is called the ClassData structure. It includes a method
 token for each method the class introduces. The method token is then used as
 an "index" into the receiver's method table, to access the appropriate method
 procedure. Because it is known at compile time which class introduces the
 method and where in that class's ClassData structure the method's token is
 stored, offset resolution is quite efficient. The cost of offset method
 resolution is currently about twice the cost of calling a C function using a
 pointer loaded with the function address.

 An object's method table is a table of pointers to the procedures that
 implement the methods that the object supports. This table is constructed by
 the object's class and is shared among the class instances. The method table
 built by class (for its instances) is referred to as the class's instance
 method table. This is useful terminology, since, in SOM, a class is itself an
 object with a method table (created by its metaclass) used to support method
 calls on the class.

 Usually, offset method resolution is sufficient; however, in some cases, the
 more flexible name-lookup resolution is required.


ΓòÉΓòÉΓòÉ 6.3.2. Name-lookup resolution ΓòÉΓòÉΓòÉ

Name-lookup resolution is similar to the method resolution techniques employed
by Objective-C and Smalltalk. It is currently about five times slower than
offset resolution. It is more flexible, however. In particular, name-lookup
resolution, unlike offset resolution, can be used when:

     The name of the method to be invoked isn't known until run time, or

     The method is added to the class interface at run time, or

     The name of the class introducing the method isn't known until run time.

 For example, a client program may use two classes that define two different
 methods of the same name, and it might not be known until run time which of
 the two methods should be invoked (because, for example, it will not be known
 until run time which class's instance the method will be applied to).

 Name-lookup resolution is always performed by a class, so it requires a method
 call. (Offset resolution, by contrast, requires no method calls.) To perform
 name-lookup method resolution, the class of the intended receiver object
 obtains a method procedure pointer for the desired method  that is appropriate
 for its instances. In general, this will require a name-based search through
 various data structures maintained by ancestor classes.

 Offset and name-lookup resolution achieve the same net effect (that is, they
 select the same method procedure); they just achieve it differently (via
 different mechanisms for locating the method's method token). Offset
 resolution is faster, because it does not require searching for the method
 token, but name-lookup resolution is more flexible.

 When defining (in SOM IDL) the interface to a class of objects, the class
 implementor can decide, for each method, whether the SOM Compiler will
 generate usage bindings that support name-lookup resolution for invoking the
 method. Regardless of whether this is done, however, application programs
 using the class can have SOM use either technique, on a per-method-call basis.
 Chapter 3, "Using SOM Classes in Client Programs," describes how client
 programs invoke methods.


ΓòÉΓòÉΓòÉ 6.3.3. Dispatch-function resolution ΓòÉΓòÉΓòÉ

Dispatch-function resolution is the slowest, but most flexible, of the three
method-resolution techniques. Dispatch functions permit method resolution to be
based on arbitrary rules associated with the class of which the receiving
object is an instance. Thus, a class implementor has complete freedom in
determining how methods invoked on its instances are resolved.

With both offset and name-lookup resolution, the net effect is the same-the
method procedure that is ultimately selected is the one supported by the class
of which the receiver is an instance. For example, if the receiver is an
instance of class "Dog", then Dog's method procedure will be selected; but if
the receiver is an instance of class "BigDog", then BigDog's method procedure
will be selected.

By contrast, dispatch-function resolution allows a class of instances to be
defined such that the method procedure is selected using some other criteria.
For example, the method procedure could be selected on the basis of the
arguments to the method call, rather than on the receiver. For more information
on dispatch-function resolution, see the description and examples for the
somDispatch and somOverrideMTab methods in the SOM Programming Reference.


ΓòÉΓòÉΓòÉ 6.3.4. Customizing Method Resolution ΓòÉΓòÉΓòÉ

Customizing method resolution requires the use of metaclasses that override
SOMClass methods.  This is not recommended without use of the Cooperative
Metaclass that guarantees correct operation of SOMobjects in conjunction with
such metaclasses.  SOMobjects users who require this functionality should
request access to the experimental Cooperative Metaclass used to implement the
SOMobjects Metaclass Framework. Metaclasses implemented using the Cooperative
Metaclass may have to be reprogrammed in the future when SOMobjects introduces
an oficially supported Cooperative Metaclass.


ΓòÉΓòÉΓòÉ 6.3.5. The four kinds of SOM methods ΓòÉΓòÉΓòÉ

SOM supports four different kinds of methods:  static methods, nonstatic
methods, dynamic methods, and direct-call procedures. The following paragraphs
explain these four method categories and the kinds of method resolution
available for each.


ΓòÉΓòÉΓòÉ 6.3.5.1. Static methods ΓòÉΓòÉΓòÉ

These are similar in concept to C++ virtual functions. Static methods are
normally invoked using offset resolution via a method table, as described
above, but all three kinds of method resolution are applicable to static
methods. Each different static method available on an object is given a
different slot in the object's method table. When SOMobjects Toolkit language
bindings are used to implement a class, the SOM IDL method modifier can be
specified to indicate that a given method is static; however, this modifier is
rarely used since it is the default for SOM methods.

Static methods introduced by a class can be overridden (redefined) by any
descendant classes of the class. When SOMobjects language bindings are used to
implement a class, the SOM IDL override modifier is specified to indicate that
a class overrides a given inherited method. When a static method is resolved
using offset resolution, it is not important which interface is accessing the
method - the actual class of the object determines the method procedure that is
selected.

Note:  All SOM IDL modifiers are described in the topic "Modifier statements"
in Chapter 4, "SOM IDL and the SOM Compiler."


ΓòÉΓòÉΓòÉ 6.3.5.2. Nonstatic methods ΓòÉΓòÉΓòÉ

These methods are similar in concept to C++ nonstatic member functions (that
is, C++ functions that are not virtual member functions and are not static
member functions). Nonstatic methods are normally invoked using offset
resolution, but all three kinds of method resolution are applicable  to
nonstatic methods. When the SOMobjects language bindings are used to implement
a class, the SOM IDL nonstatic modifier is used to indicate that a given method
is nonstatic.

Like static methods, nonstatic methods are given individual positions in method
tables. However, nonstatic methods cannot be overridden. Instead, descendants
of a class that introduces a nonstatic method can use the SOM IDL reintroduce
modifier to "hide" the original nonstatic method with another (nonstatic or
static) method of the same name. When a nonstatic method is resolved, selection
of the specific method procedure is determined by the interface that is used to
access the method.


ΓòÉΓòÉΓòÉ 6.3.5.3. Dynamic methods ΓòÉΓòÉΓòÉ

These methods are not declared when specifying an object interface using IDL.
Instead, they are registered with a class object at run time using the method
somAddDynamicMethod. Because there is no way for SOM to know about dynamic
methods before run time, offset resolution is not available for dynamic
methods. Only name-lookup or dispatch-function resolution  can be used to
invoke dynamic methods. Dynamic methods cannot be overridden.


ΓòÉΓòÉΓòÉ 6.3.5.4. Direct-call procedures ΓòÉΓòÉΓòÉ

These are similar in concept to C++ static member functions. Direct-call
procedures are not given positions in SOM method tables, but are accessed
directly from a class's ClassData structure. Strictly speaking, none of the
previous method-resolution approaches apply for invoking a direct-call
procedure, although SOMobjects language bindings provide the same invocation
syntax for direct-call procedures as for static or nonstatic methods.
Direct-call procedures cannot be overridden, but they can be reintroduced. When
SOMobjects language bindings are used to implement a class, the SOM IDL
procedure modifier is used to indicate that a given method is a direct-call
procedure. Note: Methods having the procedure modifier cannot be invoked
remotely using DSOM.


ΓòÉΓòÉΓòÉ 6.4. Implementing SOM Classes ΓòÉΓòÉΓòÉ

The interface to a class of objects contains the information that a client must
know to use an object - namely, the signatures of its methods and the names of
its attributes. The interface is described in a formal language independent of
the programming language used to implement the object's methods. In SOM, the
formal language used to define object interfaces is the Interface Definition
Language (described in Chapter 4, "SOM IDL and the SOM Compiler").

The implementation of a class of objects  (that is, the procedures that
implement the methods and the instance variables that store an object's state)
is written in the implementor's preferred programming language. This language
can be object-oriented (for instance, C++) or procedural (for instance, C).

A completely implemented class definition, then, consists of two main files:

     An IDL specification of the interface to instances of the class - the
      interface definition file (or .idl file) and

     Method procedures written in the implementor's language of choice - the
      implementation file.

 The SOM Compiler provides the link between those two files: To assist users in
 implementing classes, the SOM Compiler produces a template implementation file
 - a type-correct guide for how the implementation of a class should look.
 Then, the class implementor modifies this template file to fully implement the
 class's methods.  That process is the subject of the remainder of this
 chapter.

 The SOM Compiler can also update the implementation file to reflect changes
 subsequently made to a class's interface definition file (the .idl file).
 These incremental updates include adding new stub procedures, adding comments,
 and changing method prototypes to reflect changes made to the method
 definitions in the IDL specification. These updates to the implementation
 file, however, do not disturb existing code in the method procedures. These
 updates are discussed further in "Running incremental updates of the
 implementation template file" later in this section.

 For C programmers, the SOM Compiler generates a <filestem>.c file. For C++
 programmers, the SOM Compiler generates a <filestem>.cpp file. To specify
 whether the SOM Compiler should generate a C or C++ implementation template,
 set the value of the SMEMIT environment variable, or use the -s option when
 running the SOM Compiler. (See "The SOM Compiler" in Chapter 4, "SOM IDL and
 the SOM Compiler.")

 Note:  As this chapter describes, a SOM class can be implemented by using C++
        to define the instance variables introduced by the class and to define
        the procedures that implement the overridden and introduced methods of
        the class. Be aware, however, that the C++ class defined by the C++
        usage bindings for a SOM class (described in Chapter 3) cannot be
        subclassed in C++ to create new C++ or SOM classes. [The reason why the
        C++ implementation of a SOM class involves the definition of C++
        procedures (not C++ methods) to support SOM methods is that there is no
        language-neutral way to call a C++ method. Only C++  code can call C++
        methods, and this calling code must be generated by the same compiler
        that generates the method code. In contrast, the method procedures that
        implement SOM methods must be callable from any language, without
        knowledge on the part of the object client as to which language is used
        to implement the resolved method procedure]


ΓòÉΓòÉΓòÉ 6.4.1. The implementation template ΓòÉΓòÉΓòÉ

Consider the following IDL description of the "Hello" class:

   #include <somobj.idl>

   interface Hello : SOMObject
   {
       void sayHello();
       // This method outputs the string "Hello, World!".
   };

From this IDL description, the SOM Compiler generates the following C
implementation template, hello.c (a C++ implementation template, or hello.cpp,
is identical except that the #included file is <hello.xih> rather than
<hello.ih>):

   #define Hello_Class_Source
   #include <hello.ih>

   /*
    *  This method outputs the string "Hello, World!".
    */

   SOM_Scope void   SOMLINK sayHello(Hello somSelf, Environment *ev)
   {
       /* HelloData *somThis = HelloGetData(somSelf); */
       HelloMethodDebug("Hello","sayHello");
   }

The first line defines the "Hello_Class_Source" symbol, which is used in the
SOM-generated implementation header files for C to determine when to define
various functions, such as "HelloNewClass."  For interfaces defined within a
module, the directive "#define <className>_Class_Source" is replaced by the
directive "#define SOM_Module_<moduleName>_Source".

The second line (#include <hello.ih> for C, or #include <hello.xih> for C++)
includes the SOM-generated implementation header file. This file defines a
struct holding the class's instance variables, macros for accessing instance
variables, macros for invoking parent methods, and so forth.


ΓòÉΓòÉΓòÉ 6.4.1.1. Stub procedures for methods ΓòÉΓòÉΓòÉ

For each method introduced or overridden by the class, the implementation
template includes a stub procedure-a procedure that is empty except for an
initialization statement, a debugging statement, and possibly a return
statement. The stub procedure for a method is preceded by any comments that
follow the method's declaration in the IDL specification.

For method "sayHello" above, the SOM Compiler generates the following prototype
of the stub procedure:

   SOM_Scope void SOMLINK sayHello(Hello somSelf, Environment *ev)

The "SOM_Scope" symbol is defined in the implementation header file as either
"extern" or "static," as appropriate. The term "void" signifies the return type
of method "sayHello". The "SOMLINK" symbol is defined by SOM; it represents the
keyword needed to link to the C or C++ compiler, and its value is
system-specific. Using the "SOMLINK" symbol allows the code to work with a
variety of compilers without modification.

Following the "SOMLINK" symbol is the name of the procedure that implements the
method. If no functionprefix modifier has been specified for the class, then
the procedure name is the same as the method name. If a functionprefix modifier
is in effect, then the procedure name is generated by prepending the specified
prefix to the method name. For example, if the class definition contained the
following statement:

   functionprefix = xx_;

then the prototype of the stub procedure for method "sayHello" would be:

   SOM_Scope void SOMLINK xx_sayHello(Hello somSelf, Environment *ev)

The functionprefix can not be

   <classname>_

since this is used in method invocation macros defined by the C usage bindings.

Following the procedure name is the formal parameter list for the method
procedure. Because each SOM method always receives at least one argument (a
pointer to the SOM object that responds to the method), the first parameter
name in the prototype of each stub procedure is called somSelf. (The macros
defined in the implementation header file rely on this convention.) The somSelf
parameter is a pointer to an object that is an instance of the class being
implemented (here, class "Hello") or an instance of a class derived from it.

Unless the IDL specification of the class included the callstyle=oidl modifier,
then the formal parameter list will include one or two additional parameters
before the parameters declared in the IDL specification: an (Environment *ev)
input/output parameter, which permits the return of exception information, and,
if the IDL specification of the method includes a context specification, a
(Context *ctx) input parameter. These parameters are prescribed by the CORBA
standard.  For more information on using the Environment  and Context
parameters, see the section entitled "Exceptions and error handling" in Chapter
3, "Using SOM Classes in Client Programs," and the book The Common Object
Request Broker: Architecture and Specification, published by Object Management
Group and X/Open.

The first statement in the stub procedure for method "sayHello" is the
statement:

   /* HelloData *somThis = HelloGetData(somSelf); */

This statement is enclosed in comments only when the class does not introduce
any instance variables. The purpose of this statement, for classes that do
introduce instance variables, is to initialize a local variable (somThis) that
points to a structure representing the instance variables introduced by the
class. The somThis pointer is used by the macros defined in the "Hello"
implementation header file to access those instance variables. (These macros
are described below.)  In this example, the "Hello" class introduces no
instance variables, so the statement is commented out. If instance variables
are later added to a class that initially had none, then the comment characters
can be removed if access to the variable is required.

The "HelloData" type and the "HelloGetData" macro used to initialize the
somThis pointer are defined in the implementation header file. Within a method
procedure, class implementers can use the somThis pointer to access instance
data, or they can use the convenience macros defined for accessing each
instance variable, as described below.

To implement a method so that it can modify a local copy of an object's
instance data without affecting the object's real instance data, declare a
variable of type <className>Data (for example, "HelloData") and assign to it
the structure that somThis points to; then make the somThis pointer point to
the copy. For example:

   HelloData myCopy = *somThis;
   somThis = &myCopy;

Next in the stub procedure for method "sayHello" is the statement:

   HelloMethodDebug("Hello", "sayHello");

This statement facilitates debugging. The "HelloMethodDebug" macro is defined
in the implementation header file. It takes two arguments, a class name and a
method name. If debugging is turned on (that is, if global variable
SOM_TraceLevel is set to one in the calling program), the macro produces a
message each time the method procedure is entered. (See the next Chapter 3,
"Using SOM Classes in Client Programs," for information on debugging with SOM.)

Debugging can be permanently disabled (regardless of the setting of the
SOM_TraceLevel setting in the calling program) by redefining the
<className>MethodDebug macro to be SOM_NoTrace(c,m) following the #include
directive for the implementation header file. (This can yield a slight
performance improvement.) For example, to permanently disable debugging for the
"Hello" class, insert the following lines in the hello.c implementation file
following the line "#include hello.ih" (or "#include hello.xih," for classes
implemented in C++):

   #undef HelloMethodDebug
   #define HelloMethodDebug(c,m) SOM_NoTrace(c,m)

The way in which the stub procedure ends is determined by whether the method is
a new or an overriding method.

     For non-overriding (new) methods, the stub procedure ends with a return
      statement (unless the return type of the method is void). The class
      implementer should customize this return statement.

     For overriding methods, the stub procedure ends by making a "parent
      method call" for each of the class's parent classes. If the method has a
      return type that is not void, the last of these parent method calls is
      returned as the result of the method procedure. The class implementer can
      customize this return statement if needed (for example, if some other
      value is to be returned, or if the parent method calls should be made
      before the method procedure's own processing). See the next section for a
      discussion of parent method calls.

 If a classinit modifier was specified to designate a user-defined procedure
 that will initialize the "Hello" class object, as in the statement:

     classinit = HInit;

 then the implementation template file would include the following stub
 procedure for "HInit", in addition to the stub procedures for Hello's methods:

     void  SOMLINK HInit(SOMClass *cls)
     {

     }

 This stub procedure is then filled in by the class implementer. If the class
 definition specifies a functionprefix modifier, the classinit procedure name
 is generated by prepending the specified prefix to the specified classinit
 name, as with other stub procedures.


ΓòÉΓòÉΓòÉ 6.4.2. Extending the implementation template ΓòÉΓòÉΓòÉ

To implement a method, add code to the body of the stub procedure.  In addition
to standard C or C++ code, class implementers can also use any of the
functions, methods, and macros SOM provides for manipulating classes and
objects. Chapter 3, "Using SOM Classes in Client Programs," discusses these
functions, methods, and macros.

In addition to the functions, methods, and macros SOM provides for both class
clients and class implementers, SOM provides two facilities especially for
class implementers. They are for (1) accessing instance variables of the object
responding to the method and (2) making parent method calls, as follows.


ΓòÉΓòÉΓòÉ 6.4.2.1. Accessing internal instance variables ΓòÉΓòÉΓòÉ

To access internal instance variables, class implementers can use either of the
following forms:

      _variableName

      somThis->variableName

 To access internal instance variables "a", "b", and "c", for example, the
 class implementer could use either _a, _b, and _c, or somThis->a, somThis->b,
 and somThis->c. These expressions can appear on either side of an assignment
 statement. The somThis  pointer must be properly initialized in advance using
 the <className>GetData  procedure, as shown above.

 Instance variables can be accessed only within the implementation file of the
 class that introduces the instance variable, and not within the implementation
 of subclasses or within client programs. (To allow access to instance data
 from a subclass or from client programs, use an attribute rather than an
 instance variable to represent the instance data.) For C++ programmers, the
 _variableName form is available only if the macro  VARIABLE_MACROS is defined
 (that is, #define VARIABLE_MACROS) in the implementation file prior to
 including the .xih file for the class.


ΓòÉΓòÉΓòÉ 6.4.2.2. Making parent method calls ΓòÉΓòÉΓòÉ

In addition to macros for accessing instance variables, the implementation
header file that the SOM Compiler generates also contains definitions of macros
for making "parent method calls." When a class overrides a method defined by
one or more of its parent classes, often the new implementation simply needs to
augment the functionality of the existing implementation(s). Rather than
completely re-implementing the method, the overriding method procedure can
conveniently invoke the procedure that one or more of the parent classes uses
to implement that method, then perform additional computation (redefinition) as
needed. The parent method call can occur anywhere within the overriding method.
(See Example 3 of the SOM IDL tutorial.)

The SOM-generated implementation header file defines the following macros for
making parent-method calls from within an overriding method:

<className>_parent_<parentClassName>_<methodName>

             (for each parent class of the class overriding
the method), and

<className>_parents_<methodName>.

For example, given class "Hello" with parents "File" and "Printer" and
overriding method somInit (the SOM method that initializes each object), the
SOM Compiler defines the following macros in the implementation header file for
"Hello":

   Hello_parent_Printer_somInit
   Hello_parent_File_somInit
   Hello_parents_somInit

Each macro takes the same number and type of arguments as <methodName>. The
<className>_parent_<parentClassName>_<methodName> macro invokes the
implementation of <methodName> inherited from <parentClassName>. Hence, using
the macro "Hello_parent_File_somInit" invokes the File's implementation of
somInit.

The <className>_parents_<  methodName> macro invokes the parent method for each
parent of the child class that supports <methodName>. That is,
"Hello_parents_somInit" would invoke File's implementation of somInit, followed
by Printer's implementation of somInit. The <className>_parents_<methodName>
macro is redefined in the binding file each time the class interface is
modified, so that if a parent class is added or removed from the class
definition, or if <methodName> is added to one of the existing parents, the
macro <className>_parents_<methodName > will be redefined appropriately.


ΓòÉΓòÉΓòÉ 6.4.2.3. Converting C++ classes to SOM classes ΓòÉΓòÉΓòÉ

For C++ programmers implementing SOM classes, SOM provides a macro that
simplifies the process of converting C++ classes to SOM classes. This macro
allows the implementation of one method of a class to invoke another new or
overriding method of the same class on the same receiving object by using the
following shorthand syntax:

   _methodName(arg1, arg2, ...)

For example, if class X introduces or overrides methods m1 and m2, then the C++
implementation of method m1 can invoke method m2 on its somSelf argument using
_m2(arg, arg2, ...), rather than somSelf->m2(arg1, arg2, ...), as would
otherwise be required. (The longer form is also available.) Before the
shorthand form in the implementation file is used, the macro METHOD_MACROS must
be defined (that is, use #define METHOD_MACROS) prior to including the xih file
for the class.


ΓòÉΓòÉΓòÉ 6.4.2.4. Running incremental updates of the implementation template file ΓòÉΓòÉΓòÉ

Refining the .idl file for a class is typically an iterative process. For
example, after running the IDL source file through the SOM Compiler and writing
some code in the implementation template file, the class implementer realizes
that the IDL class interface needs another method or attribute, a method needs
a different parameter, or any such changes.

As mentioned earlier, the SOM Compiler (when run using the c or xc emitter)
assists in this development by reprocessing the .idl file and making
incremental updates to the current implementation file. This modify-and-update
process may in fact be repeated several times before the class declaration
becomes final. Importantly, these updates do not disturb existing code for the
method procedures. Included in the incremental update are these changes:

     Stub procedures are inserted into the implementation file for any new
      methods added to the .idl file.

     New comments in the .idl file are transferred to the implementation file,
      reformatted appropriately.

     If the interface to a method has changed, a new method procedure
      prototype is placed in the implementation file. As a precaution, however,
      the old prototype is also preserved within comments. The body of the
      method procedure is left untouched (as are the method procedures for all
      methods).

     Similarly left intact are preprocessor directives, data declarations,
      constant declarations, non-method functions, and additional comments-in
      essence, everything else in the implementation file.

 Some changes to the .idl file are not reflected automatically in the
 implementation file after an incremental update. The class implementer must
 manually edit the implementation file after changes such as these in the .idl
 file:

     Changing the name of a class or a method.

     Changing the parents of a class (see also "If you change the parents of a
      class..." later in this topic).

     Changing a functionprefix class modifier statement.

     Changing the content of a passthru statement directed to the
      implementation (.c, or cpp) file. As previously emphasized, however,
      passthru statements are primarily recommended only for placing #include
      statements in a binding file (.ih, xih, .h, or .xh file) used as a header
      file in the implementation file or in a client program.

     If the class implementer has placed "forward declarations" of the method
      procedures in the implementation file, those are not updated. Updates
      occur only for method prototypes that are part of the method procedures
      themselves.


ΓòÉΓòÉΓòÉ 6.4.2.5. Considerations to ensure that updates work ΓòÉΓòÉΓòÉ

To ensure that the SOM Compiler can properly update method procedure prototypes
in the implementation file, class implementers should avoid editing changes
such as the following:

     A method procedure name should not be enclosed in parentheses in the
      prototype.

     A method procedure name must appear in the first line of the prototype,
      excluding comments and white space. Thus, a new line must not be inserted
      before the procedure name.

 Error messages occur while updating an existing implementation file if it
 contains syntax that is not ANSI C. For example, "old style" method
 definitions such as the example on the left generate errors:

 Invalid "old" syntax     Required  ANSI C
 void foo(x)              void foo(short x) {
 short x;                 ...
 {                        }
 ...
 }

 Similarly, error messages occur if anything in the .idl file would produce an
 implementation file that is not syntactically valid for C/C++ (such as nested
 comments). If update errors occur, either the .idl file or the implementation
 file may be at fault. One way to track down the problem is to run the
 implementation file through the C/C++ compiler. Or, move the existing
 implementation file to another directory, generate a completely new one from
 the .idl file, and then run it through the C/C++ compiler. One of these steps
 should pinpoint the error, if the compiler is strict ANSI.

 Conditional compilation (using #if and #ifdef directives) in the
 implementation file can be another source of errors, because the SOM Compiler
 does not invoke the preprocessor (it simply recognizes and ignores those
 directives). The programmer should be careful when using conditional
 compilation, to avoid a situation such as shown below; here, with apparently
 two open braces and only one closing brace, the c or xc emitter would report
 an unexpected end-of-file:

 Invalid syntax           Required matching braces

 #ifdef FOOBAR            #ifdef FOOBAR
  {                        {
 ...                      ...
 #else                     }
 {                        #else
 ...                       {
 #endif                   ...
  }                        }
                          #endif


ΓòÉΓòÉΓòÉ 6.4.2.6. If you change the parents of a class... ΓòÉΓòÉΓòÉ

Because the implementation-file emitters never change any existing code within
a previously generated implementation file, changing the parents of a class
requires extremely careful attention by the programmer. For example, for
overridden methods, changing a class's parents may invalidate previous
parent-method calls provided by the template, and require the addition of new
parent-method calls. Neither of these issues is addressed by the incremental
update of previously generated method-procedure templates.

The greatest danger from changing the parents of a class, however, concerns the
ancestor-initializer calls provided in the stub procedures for initializer
methods. (For further information on ancestor initializer calls, see
"Initializing and Uninitializing Objects" later in this chapter.) Unlike
parent-method calls, ancestor-initializer calls are not optional - they must be
made t all classes specified in a directinitclasses modifier, and these calls
should always include the parents of the class (the default when no
directinitclasses modifier is given). When the parents of a class are changed,
however, the ancestor-initializer calls (which must be made in a specific
order) are not updated.

The easiest way to deal with this problem is to change the method name of the
previously generated initializer stub procedure in the implementation template
file. Then, the SOM Compiler can correctly generate a completely new
initializer stub procedure (while ignoring the renamed procedure). Once this is
done, your customization code from the renamed initializer procedure can be
"merged" into the newly generated one, after which the renamed initializer
procedure can be deleted.


ΓòÉΓòÉΓòÉ 6.4.3. Compiling and linking ΓòÉΓòÉΓòÉ

After you fill in the method stub procedures, the implementation template file
can be compiled and linked with a client program as follows. (In these
examples, the environment variable SOMBASE represents the directory in which
SOM has been installed.)

Note: If you are building an application that uses a combination of C and C++
compiled object modules, then the C++ linker must be used to link them.

When the client program (main.c) and the implementation file (hello.c) are in
C:

   > set LIB=%SOMBASE%\lib;%LIB%
   > icc -I. -I%SOMBASE%\include -Fe .*hello
   hello.exe main.c hello.c somtk.lib

When the client program and the implementation file are written in C++:

   > set LIB=%SOMBASE%\lib;%LIB%
   > icc -I. -I%SOMBASE%\include -Fe .*hello
   hello.exe main.cpp hello.cpp somtk.lib

If the class definition (in the .idl file) changes, run the SOM Compiler again.
This will generate new header files, and it will update the implementation file
to include any:

     New comments,

     Stub procedures for any new methods, and

     Revised method procedure prototypes for methods whose signatures have
      been changed in the .idl file.

 After rerunning the SOM Compiler, add to the implementation file the code for
 any newly added method procedures, and recompile the implementation file with
 the client program.


ΓòÉΓòÉΓòÉ 6.5. Initializing and Uninitializing Objects ΓòÉΓòÉΓòÉ

This section discusses the initialization and uninitialization of SOM objects.
Subsequent topics introduce the methods and capabilities that the SOMobjects
Developer Toolkit provides to facilitate this.

Object creation is the act that enables the execution of methods on an object.
In SOM, this means storing a pointer to a method table into a word of memory.
This single act converts raw memory into an (uninitialized) SOM object that
starts at the location of the method table pointer.

Object initialization, on the other hand, is a separate activity from object
creation in SOM. Initialization is a capability supported by certain methods
available on an object. An object's class determines the implementation of the
methods available on the object, and thus determines its initialization
behavior.

The instance variables encapsulated by a newly created object must be brought
into a consistent state before the object can be used. This is the purpose of
initialization methods. Because, in general, every ancestor of an object's
class contributes instance data to an object, it is appropriate that each of
these ancestors contribute to the initialization of the object.

SOM thus recognizes initializers as a special kind of method. One advantage of
this approach is that special metaclasses are not required for defining
constructors (class methods) that take arguments. Furthermore, a class can
define multiple initializer methods, thus enabling its different objects to be
initialized supporting different characteristics or capabilities. This results
in simpler designs and more efficient programs.

The SOMobjects Toolkit provides an overall framework that class designers can
easily exploit in order to implement default or customized initialization of
SOM objects. This framework is fully supported by the SOM Toolkit emitters that
produce the implementation template file. The following sections describe the
declaration, implementation, and use of initializer (and uninitializer)
methods.

Important: All code written prior to SOMobjects Release 2.1 using documented
guidelines for the earlier initialization approach based on the somInit method
(as well as all existing class binaries) continues to be fully supported and
useful.


ΓòÉΓòÉΓòÉ 6.5.1. Initializer methods ΓòÉΓòÉΓòÉ

As noted above, in the SOMobjects Toolkit each ancestor of an object
contributes to the initialization of that object. Initialization of an object
involves a chain of ancestor-method calls that, by default, are automatically
determined by the SOM Compiler emitters. The SOMobjects framework for
initialization of objects is based on the following approach:

   1. SOMobjects recognizes initializers as a special kind of method, and
      supports a special mechanism for ordering the execution of
      ancestor-initializer method procedures. The SOMObject class introduces an
      initializer method, somDefaultInit that uses this execution mechanism.

   2. The SOM Compiler's emitters provide special support for methods that are
      declared as initializers in the .idl file. To supplement the
      somDefaultInit method, SOM class designers can also declare additional
      initializers in their own classes.

 Two SOM IDL modifiers are provided for declaring initializer methods and
 controlling their execution, init and directinitclasses:

     The init modifier is required in order to designate a given method is a
      initializer; that is, to indicate that the method both uses and supports
      the object-initialization protocol described here.

     The  directinitclasses modifier can be used to control the order of
      execution of initializer method procedures provided by the different
      ancestors of the class of an object.

     For full definitions of init and directinitclasses, see the topic
      "Modifier statements" in Chapter 4,"SOM IDL and the SOM Compiler."

 Every SOM class has a list that defines (in sequential order) the ancestor
 classes whose initializer method procedures the class should invoke. If a
 class's IDL does not specify an explicit directinitclasses modifier, the
 default for this list is simply the class's parents - in left-to-right order.

 Using the directinitclasses list and the actual run-time class hierarchy above
 itself, each class inherits from SOMClass the ability to create a data
 structure of type somInitCtrl. This structure is used to control the execution
 of initializers. Moreover, it represents a particular visit-ordering that
 reaches each class in the transitive closure of directinitclasses exactly
 once. To initialize a given object, this visit-ordering occurs as follows:
 While recursively visiting each ancestor class whose initializer method
 procedure should be run, SOMobjects first runs the initializer method
 procedures of all of that class's directinitclasses if they have not already
 been run by another class's initializers, with ancestor classes always taken
 in left-to-right order.

 The code that deals with the somInitCtrl data structure is generated
 automatically within the implementation bindings for a class, and need not
 concern a class implementor.

 When an instance of a given class (or some descendant class) is initialized,
 only one of the given class's initializers will be executed, and this will
 happen exactly once (under control of the ordering determined by the class of
 the object being initialized).

 The somInitCtrl structure solves a problem originally created by the add
 ition of multiple inheritance to SOMobjects 2.0. With multiple inheritance,
 any class can appear at the top of a multiple inheritance diamond. Previously,
 whenever this happened, the class could easily receive multiple initialization
 calls. In the current version of the SOMobjects Toolkit, however, the
 somInitCtrl structure prevents this from happening.


ΓòÉΓòÉΓòÉ 6.5.1.1. Declaring new initializers in SOM IDL ΓòÉΓòÉΓòÉ

When defining SOMobjects classes, programmers can easily declare and implement
new initializers. Classes can have as many initializers as desired, and
subclassers can invoke whichever of these they want. When introducing new
initializers, developers must adhere to the following rules:

     All initializer methods take a somInitCtrl data structure as an initial
      inout parameter (its type is defined in the SOMobjects header file
      somapi.h), and

     All initializers return void.

 Accordingly, the somDefaultInit initializer introduced by SOMObject takes a
 somInitCtrl structure as its (only) argument, and returns void. Here is the
 IDL syntax for this method, as declared in somobj.idl:

  void  somDefaultInit (inout somInitCtrl ctrl);

 When introducing a new initializer, it is also necessary to specify the init
 modifier in the implementation section. The init modifier is what tells
 emitters that the new method is actually an initializer, so the method can be
 properly supported from the language bindings. As described below, this
 support includes the generation of special initializer stub procedures in the
 implementation template file, as well as bindings containing
 ancestor-initialization macros and object constructors that invoke the class
 implementor's new initializers.

 It is a good idea to begin the names of initializer methods with the name of
 the class (or some other string that can be unique for the class). This is
 important because all initializers available on a class must be newly
 introduced by that class (that is, you cannot override initializers - except
 for somDefaultInit). Using a class-unique name means that subclasses will not
 be unnecessarily constrained in their choice of initializer names.

 Here are two classes that introduce new initializers:

  interface Example1 : SOMObject
  {
     void Example1_withName (inout somInitCtrl ctrl,in string name);
     void Example1_withSize (inout somInitCtrl ctrl,in long size);
     void Example1_withNandS(inout somInitCtrl ctrl,in string name,
                                                     in long size);
          implementation {
                  releaseorder: Example1_withName,
                                Example1_withSize,
                                Example1_withNandS;
                  somDefaultInit: override, init;
                  somDestruct: override;
                  Example1_withName: init;
                  Example1_withSize: init;
                  Example1_withNandS: init;
          };
  };

  interface Example2 : Example1
  {
     void Example2_withName(inout somInitCtrl ctrl, in string name);
     void Example2_withSize(inout somInitCtrl ctrl, in long size);
          implementation {
                  releaseorder: Example2_withName,
                                Example2_withSize;
                  somDefaultInit: override, init;
                  somDestruct: override;
                  Example2_withName: init;
                  Example2_withSize: init;
          };
  };

 Here, interface "Example1" declares three new initializers. Notice the use of
 inout somInitCtrl as the first argument of each initializer, and also note
 that the init modifier is used in the implementation section. These two things
 are required to declare initializers. Any number of initializers can be
 declared by a class. "Example2" declares two initializers.

 "Example1" and "Example2" both override the somDefaultInit initializerThis
 initializer method is introduced by SOMObject and is special for two resons:
 First, somDefaultInit is the only initializer that can be overridden And,
 second, SOMobjects arranges that this initializer will always be available on
 any class (as further explained below).

 Historically in the SOMobjects Toolkit, object#initialization methods by
 default have invoked the somInit method, which class implementors could
 override to customize initialization as appropriate. SOMobjects continues to
 support this approach, so that existing code (and class binaries) will execute
 correctly. However, the somDefaultInit method is now the preferred form of
 initialization because it offers greatly improved efficiency.

 Even if no specialized initialization is needed for a class, you should still
 override the somDefaultInit method in the interest of efficiency. If you do
 not override somDefaultInit, then a generic (and therefore less efficient)
 somDefaultInit method procedure will be used for your class. This generic
 method procedure first invokes somDefaultInit on the appropriate ancestor
 classes. Then (for consistency with earlier versions of SOMobjects), it checks
 to determine if the class overrides somInit and, if so, calls any customized
 somInit code provided by the class.

 When you override somDefaultInit, the emitter's implementation template file
 will include a stub procedure similar to those used for other initializers,
 and you can fill it in as appropriate (or simply leave it as is). Default
 initialization for your class will then run much faster than with the generic
 method procedure. Examples of initializer stub procedures (and customizations)
 are given below.

 In summary, the initializers available for any class of objects are
 somDefaultInit (which you should always override) plus any new initializers
 explicitly declared by the class designer. Thus, "Example1" objects may be
 initialized using any of four different initializers (the three that are
 explicitly declared, plus somDefaultInit). Likewise, there are three
 initializers for the "Example2" objects. Some examples of using initializers
 are provided below.


ΓòÉΓòÉΓòÉ 6.5.1.2. Considerations for 'somInit' initialization from earlier SOM releases ΓòÉΓòÉΓòÉ

To re-emphasize: All code written prior to SOMobjects Release 2.1 using
documented guidelines for the earlier initialization approach based on the
somInit method (as well as all existing class binaries) continues to be fully
supported and useful.

Prior to SOMobjects 2.1, initialization was done with initializer methods that
would simply "chain" parent-method calls upward, thereby allowing the execution
of initializer method procedures contributed by all ancestors of an object's
class. This chaining of initializer calls was not supported in any special way
by the SOM API. Parent-method calls are simply one of the possible idioms
available to users of OOP in SOM, easily available to a SOM class designer as a
result of the support provided by the SOMobjects Toolkit emitters for
parent-method calls.

So, SOM did not constrain initialization to be done in any particular way or
require the use of any particular ordering of the method procedures of ancestor
classes.  But, SOM did provide an overall framework that class designers could
easily utilize in order to implement default initialization of SOM objects.
This framework is provided by the somInit object-initializatio method
introduced by the SOMobject class and supported by the SOM Toolkit emitters.
The emitters create an implementation template file with stub procedures for
overridden methods that automatically chain parent-method calls upward through
parent classes. Many of the class methods that perform object creation called
somInit automatically.

Note:  These will now call somDefaultInit, which in turn calls somInit for
       legacy code, as described in the previous topic.

 Because it takes no arguments, somInit best served the purpose of a default
 initializer. SOM programmers also had the option of introducing additional
 "non-default" initialization methods that took arguments. In addition, by
 using metaclasses, they could introduce new class methods as object
 constructors that first create an object (generally using somNewNoInit.) and
 then invoke some non-default initializer on the new object.

 For a number of reasons, the somInit framework has been augmented by
 recognizing initializers special kind of method in SOMobjects. One advantage
 of this approach is that special metaclasses are no longer required for
 defining constructors that take arguments. Instead, because the init modifier
 identifies initializers, usage-binding emitters can now provide these
 constructors. This results in simpler designs and more efficient programs.

 Although somDefaultInit replaces somInit  as the no-argument initializer used
 for SOM objects, all previous use of somInit is still supported by the
 SOMobjects Developers Toolkit. You may continue to use somInit  although this
 is somewhat less efficient than using somDefaultInit.

 However, you cannot use both methods. In particular, if a class overrides both
 somDefaultInit  and somInit, its somInit code will never be executed. It is
 recommended that you always override  somDefaultInit for object
 initialization. For one thing, it is likely that when SOMobjects is ported to
 new systems, somInit (and somUninit) may not be supported on those systems.
 Thus, code written using these (obsolete) methods will be less portable.


ΓòÉΓòÉΓòÉ 6.5.1.3. Implementing initializers ΓòÉΓòÉΓòÉ

When new initializers are introduced by a class, as in the preceding examples,
the implementation template file generated by the SOM Toolkit C and C++
emitters automatically contains an appropriate stub procedure for each
initializer method, for the class implementor's use. The body of an initializer
stub procedure consists of two main sections:

     The first section performs calls to ancestors of the class to invoke
      their initializers.

     The second section is used by the programmer to perform any "local"
      initializations appropriate to the instance data of the class being
      defined.

 In the first section, by default, the parents of the new class are the
 ancestors whose initializers are called. When something else is desired, the
 IDL directinitclasses modifier can be used to explicitly designate the
 ancestors whose initializer methods should be invoked by a new class's
 initializers.

 Important:  Under no circumstances can the number or the ordering of ancestor
 initializer calls in the first section of an initializer stub procedure be
 changed. The control masks used by initializers are based on these orderings.
 (If you want to change the number or ordering of ancestor initializer calls,
 you must use the directinitclasses modifier.) The ancestor initializer calls
 themselves can be modified as described below.

 Each call to an ancestor initializer is made using a special macro (much like
 a parent call) that is defined for this purpose within the implementation
 bindings. These macros are defined for all possible ancestor initialization
 calls. Initially, an initializer stub procedure invokes the default ancestor
 initializers provided by somDefaultInit. However, a class implementor can
 replace any of these calls with a different initializer call, as long as it
 calls the same ancestor (see the example in the next topic). Non-default
 initializer calls generally take other arguments in addition to the control
 argument.

 In the second section of an initializer stub procedure, the programmer
 provides any class-specific code that may be needed for initialization. For
 example, the "Example2_withName" stub procedure is shown below. As with all
 stub procedures produced by the SOMobjects implementation-template emitters,
 this code requires no modification to run correctly.

  SOM_Scope void SOMLINK Example2_withName(Example2 *somSelf,
                                           Environment *ev,
                                           somInitCtrl* ctrl,
                                           string name)
  {
      Example2Data *somThis; /* set by BeginInitializer */
      somInitCtrl globalCtrl;
      somBooleanVector myMask;
      Example2MethodDebug("Example2","withName");

      /*
       * first section -- calls to ancestor initializers
       */
       Example2_BeginInitializer_Example2_withName;
       Example2_Init_Example1_somDefaultInit(somSelf, ctrl);

      /*
       * second section -- local Example2 initialization code
       */
  }

 In this example, notice that the "Example2_withName" initializer is an IDL
 callstyle method, so it  receives an Environment argument. In contrast,
 somDefaultInit is introduced by the SOMObject class (so it has an OIDL
 callstyle initializer, without an environment).

 Important: If a class is defined where multiple initializers have exactly the
 same signature, then the C++ usage bindings will not be able to differentiate
 among them. That is, if there are multiple initializers defined with
 environment and long arguments, for example, then C++ clients would not be
 able to make a call using only the class name and arguments, such as:


  new Example2(env, 123);

 Rather, C++ users would be forced to first invoke the somNewNoInit method on
 the class to create an uninitialized object, and then separately invoke the
 desired initializer method on the object. This call would pass a zero for the
 control argument, in addition to passing values for the other arguments. For
 further discussion of client usage, see "Using initializers when creating new
 objects" later in this chapter.


ΓòÉΓòÉΓòÉ 6.5.1.4. Selecting non-default ancestor initializer calls ΓòÉΓòÉΓòÉ

Often, it will be appropriate (in the first section of an initializer stub
procedure) to change the invocation of an ancestor's somDefaultInit initializer
to some other initializer available on the same class. The rule for making this
change is simple: Replace somDefaultInit with the name of the desired ancestor
initializer, and add any new arguments that are required by the replacement
initializer. Important: Under no circumstances can you change anything else in
the first section.

This example shows how to change an ancestor-initializer call correctly. Since
there is a known "Example1_withName" initializer, the following default
ancestor-initializer call (produced within the stub procedure for
"Example2_withName") can be changed from


Example2_Init_Example1_somDefaultInit(somSelf, ctrl);

to

Example2_Init_Example1_Example1_withName(somSelf, ev, ctrl, name)

Notice that the revised ancestor-initializer call includes arguments for an
Environment and a name, as defined by the "Example1_withname" initializer.


ΓòÉΓòÉΓòÉ 6.5.1.5. Using initializers when creating new objects ΓòÉΓòÉΓòÉ

There are several ways that client programs can take advantage of the
somDefaultInit object initialization. If desired, clients can use the SOM API
directly (rather than taking advantage of the usage bindings). Also, the
general object constructor, somNew, can always be invoked on a class to create
and initialize objects. This call creates a new object and then invokes
somDefaultInit on it

To use the SOM API directly, the client code should first invoke the
somNewNoInit method on the desired class object to create a new, uninitialized
object. Then, the desired initializer is invoked on the new object, passing a
null (that is, 0) control argument in addition to whatever other arguments may
be required by the initializer. For example:


/* first make sure the Example2 class object exists */
Example2NewClass(Example2_MajorVersion, Example2_MinorVersion);


/* then create a new, uninitialized Example2 object */
myObject = _somNewNoInit(_Example2);

(null)
/* then initialize it with the desired initializer */
Example2_withName(myObject, env, 0, "MyName");

Usage bindings hide the details associated with initializer use in various ways
and make calls more convenient for the client. For example, the C usage
bindings for any given class already provide a convenience macro,
<className>New, that first assures existence of the class object, and then
calls somNew on it to create and initialize a new object. As explained above,
somNew will use somDefaultInit to initialize the new object.

Also, the C usage bindings provide object-construction macros that use
somNewNoInit and then invoke non-default initializers. These  macros are named
using the form <className>New_<initializerName>. For example, the C usage
bindings for "Example2" allow using the following expression to create,
initialize, and return a new "Example2" object:


Example2New_Example2_withName(env, "AnyName");

In the C++ bindings, initializers are represented as overloaded C++
constructors. As a result, there is no need to specify the name of the
initializer method. For example, using the C++ bindings, the following
expressions could be used to create a new "Example2" object:


new Example2;                   // will use somDefaultInit
new Example2();                 // will use somDefaultInit
new Example2(env,"A.B.Normal"); // will use Example2_withName
new Example2(env,123);          // will use Example2_withSize

Observe that if multiple initializers in a class have exactly the same
signatures, the C++ usage bindings would be unable to differentiate among the
calls, if made using the forms illustrated above. In this case, a client could
use somNewNoInit first, and then invoke the specific initializer, as described
in the preceding paragraphs.


ΓòÉΓòÉΓòÉ 6.5.2. Uninitialization ΓòÉΓòÉΓòÉ

An object should always be uninitialized before its storage is freed. This is
important because it also allows releasing resources and freeing storage not
contained within the body of the object.  SOMobjects handles uninitialization
in much the same way as for initializers: An uninitializer takes a control
argument and is supported with stub procedures in the implementation template
file in a manner similar to initializers.

Only a single uninitialization method is needed, so SOMObject introduces the
method that provides this function: somDestruct. As with the default
initializer method, a class designer who requires nothing special in the way of
uninitialization need not be concerned about modifying the default somDestruct
method procedure. However, your code will execute faster if the .idl file
overrides somDestruct so that a non-generic stub-procedure code can be provided
for the class. Note that somDestruct was overridden by "Example1" and
"Example2" above No specific IDL modifiers other than override are required for
this.

Like an initializer template, the stub procedure for somDestruct consists of
two sections: The first section is used by the programmer for performing any
"local" uninitialization that may be required. The second section (which
consists of a single EndDestructor macro invocation) invokes somDestruct on
ancestors The second section must not be modified or removed by the programmer.
It must be the final statement executed in the destructor.


ΓòÉΓòÉΓòÉ 6.5.2.1. Using `somDestruct' ΓòÉΓòÉΓòÉ

It is rarely necessary to invoke the somDestruct method explicitly This is
because object uninitialization is normally done just before freeing an
object's storage, and the mechanisms provided by SOMobjects for this purpose
will automatically invoke somDestruct. For example, if an object were created
using somNew or somNewNoInit, or by using a convenience macro provided by the C
languag bindings, then the somFree method can be invoked on the object to
delete the object. This automatically calls somDestruct before freeing storage.

C++ users can simply use the delete operator provided by the C++ bindings. This
destructor calls somDestruct before the C++ delete  operator frees the object's
storage.

On the other hand, if an object is initially created by allocating memory in
some special way and subsequently some somRenew methods are used, somFree (or
C++ delete) is probably not appropriate. Thus, the somDestruct method should be
explicitly called to uninitialize the object before freeing memory.


ΓòÉΓòÉΓòÉ 6.5.3. A complete example ΓòÉΓòÉΓòÉ

The following example illustrates the implementation and use of initializers
and destructors from the C++ bindings. The first part shows the IDL for three
classes with initializers. For variety, some of the classes use callstyle OIDL
and others use callstyle IDL.

#include <somobj.idl>

interface A : SOMObject {
        readonly attribute long a;
        implementation {
                releaseorder: _get_a;
                functionprefix = A;
                somDefaultInit: override, init;
                somDestruct: override;
                somPrintSelf: override;
        };
};


(null)
interface B : SOMObject {
        readonly attribute long b;
        void BwithInitialValue(inout somInitCtrl ctrl,
                               in long initialValue);
        implementation {
                callstyle = oidl;
                releaseorder: _get_b, BwithInitialValue;
                functionprefix = B;
                BwithInitialValue: init;
                somDefaultInit: override, init;
                somDestruct: override;
                somPrintSelf: override;
        };
};


(null)
interface C : A, B      {
        readonly attribute long c;
        void CwithInitialValue(inout somInitCtrl ctrl,
                               in long initialValue);
        void CwithInitialString(inout somInitCtrl ctrl,
                                in string initialString);
        implementation {
                releaseorder: _get_c, CwithInitialString,
                              CwithInitialValue;
                functionprefix = C;
                CwithInitialString: init;
                CwithInitialValue: init;
                somDefaultInit: override;
                somDestruct: override;
                somPrintSelf: override;
        };
};


ΓòÉΓòÉΓòÉ 6.5.3.1. Implementation code ΓòÉΓòÉΓòÉ

Based on the foregoing class definitions, the next example illustrates several
important aspects of initializers. The following code is a completed
implementation template and an example client program for the preceding
classes. Code added to the original template is given in bold.

/*
 *  This file generated by the SOM Compiler and Emitter Framework.
 *  Generated using:
 *      SOM Emitter emitxtm.dll: 2.22
 */

#define SOM_Module_ctorfullexample_Source
#define VARIABLE_MACROS
#define METHOD_MACROS
#include <ctorFullExample.xih>
#include <stdio.h>

SOM_Scope void SOMLINK AsomDefaultInit(A *somSelf,
                                       somInitCtrl* ctrl)
{
    AData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    AMethodDebug("A","somDefaultInit");

    A_BeginInitializer_somDefaultInit;
    A_Init_SOMObject_somDefaultInit(somSelf, ctrl);
    /*
     * local A initialization code added by programmer
     */
_a = 1;
}

SOM_Scope void SOMLINK AsomDestruct(A *somSelf, octet doFree,
                                    somDestructCtrl* ctrl)
{
    AData *somThis; /* set by BeginDestructor */
    somDestructCtrl globalCtrl;
    somBooleanVector myMask;
    AMethodDebug("A","somDestruct");
    A_BeginDestructor;

    /*
     * local A deinitialization code added by programmer
     */
    A_EndDestructor;
}

SOM_Scope SOMObject*  SOMLINK AsomPrintSelf(A *somSelf)
{
    AData *somThis = AGetData(somSelf);
    AMethodDebug("A","somPrintSelf");
    somPrintf("{an instance of %s at location %X with (a=%d)}\n",
             _somGetClassName(),somSelf,__get_a((Environment*)0));
    return (SOMObject*)((void*)somSelf);
}

SOM_Scope void SOMLINK BBwithInitialValue(B *somSelf,
                                          somInitCtrl* ctrl,
                                          long initialValue)
{
    BData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    BMethodDebug("B","BwithInitialValue");

    B_BeginInitializer_withInitialValue;
    B_Init_SOMObject_somDefaultInit(somSelf, ctrl);

    /*
     * local B initialization code added by programmer
     */
 _b = initialValue;
}

SOM_Scope void SOMLINK BsomDefaultInit(B *somSelf,
                                       somInitCtrl* ctrl)
{
    BData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    BMethodDebug("B","somDefaultInit");

    B_BeginInitializer_somDefaultInit;
    B_Init_SOMObject_somDefaultInit(somSelf, ctrl);

    /*
     * local B initialization code added by programmer
     */
 _b = 2;
}

SOM_Scope void SOMLINK BsomDestruct(B *somSelf, octet doFree,
                                    somDestructCtrl* ctrl)
{
    BData *somThis; /* set by BeginDestructor */
    somDestructCtrl globalCtrl;
    somBooleanVector myMask;
    BMethodDebug("B","somDestruct");
    B_BeginDestructor;

    /*
     * local B deinitialization code added by programmer
     */

    B_EndDestructor;
}

SOM_Scope SOMObject*  SOMLINK BsomPrintSelf(B *somSelf)
{
    BData *somThis = BGetData(somSelf);
    BMethodDebug("B","somPrintSelf");
    printf("{an instance of %s at location %X with (b=%d)}\n",
           _somGetClassName(),somSelf,__get_b());
    return (SOMObject*)((void*)somSelf);
}

Note: The following initializer for a C object accepts a string as an argument,
converts this to an integer, and uses this for ancestor initialization of "B."
This illustrates how a default ancestor initializer call is replaced with a
non-default ancestor initializer call.

SOM_Scope void SOMLINK CCwithInitialString(C *somSelf,
                                           Environment *ev,
                                           somInitCtrl* ctrl,
                                           string initialString)
{
    CData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    CMethodDebug("C","CwithInitialString");

    C_BeginInitializer_withInitialString;
    C_Init_A_somDefaultInit(somSelf, ctrl);
    C_Init_B_BwithInitialValue(somSelf, ctrl,
                               atoi(initialString)-11);

    /*
     * local C initialization code added by programmer
     */
    _c = atoi(initialString);
}

SOM_Scope void SOMLINK CsomDefaultInit(C *somSelf,
                                       somInitCtrl* ctrl)
{
    CData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    CMethodDebug("C","somDefaultInit");

    C_BeginInitializer_somDefaultInit;
    C_Init_A_somDefaultInit(somSelf, ctrl);
    C_Init_B_somDefaultInit(somSelf, ctrl);

    /*
     * local C initialization code added by programmer
     */
_c = 3;
}

SOM_Scope void SOMLINK CsomDestruct(C *somSelf, octet doFree,
                                    somDestructCtrl* ctrl)
{
    CData *somThis; /* set by BeginDestructor */
    somDestructCtrl globalCtrl;
    somBooleanVector myMask;
    CMethodDebug("C","somDestruct");
    C_BeginDestructor;

    /*
     * local C deinitialization code added by programmer
     */

    C_EndDestructor;
}


SOM_Scope SOMObject*  SOMLINK CsomPrintSelf(C *somSelf)
{
    CData *somThis = CGetData(somSelf);
    CMethodDebug("C","somPrintSelf");
    printf("{an instance of %s at location %X with"
           " (a=%d, b=%d, c=%d)}\n",
        _somGetClassName(),somSelf,
        __get_a((Environment*)0),
        __get_b(),
        __get_c((Environment*)0));
    return (SOMObject*)((void*)somSelf);
}

SOM_Scope void SOMLINK CCwithInitialValue(  C *somSelf,
                                        Environment *ev,
                                        somInitCtrl* ctrl,
                                        long initialValue)
{
    CData *somThis; /* set by BeginInitializer */
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    CMethodDebug("C","CwithInitialValue");

    C_BeginInitializer_withInitialValue;
    C_Init_A_somDefaultInit(somSelf, ctrl);
    C_Init_B_BwithInitialValue(somSelf, ctrl, initialValue-11);

    /*
     * local C initialization code added by programmer
     */
    _c = initialValue;
}

Here is a C++ program that creates instances of "A", "B", and "C" using the
initializers defined above.

main()
{
    SOM_TraceLevel = 1;

    A *a = new A;
    a->somPrintSelf();
    delete a;
    printf("\n");

    B *b = new B();
    b->somPrintSelf();
    delete b;
    printf("\n");

    b = new B(22);
    b->somPrintSelf();
    delete b;
    printf("\n");

    C *c = new C;
    c->somPrintSelf();
    delete c;
    printf("\n");

    c = new C((Environment*)0, 44);
    c->somPrintSelf();
    delete c;
    printf("\n");

    c = new C((Environment*)0, "66");
    c->somPrintSelf();
    delete c;
}

The output from the preceding program is as follows:

"ctorFullExample.C": 18:        In A:somDefaultInit
"ctorFullExample.C": 48:        In A:somPrintSelf
"./ctorFullExample.xih": 292:     In A:A_get_a
{an instance of A at location 20063C38 with (a=1)}
"ctorFullExample.C": 35:        In A:somDestruct

"ctorFullExample.C": 79:        In B:somDefaultInit
"ctorFullExample.C": 110: In B:somPrintSelf
"./ctorFullExample.xih": 655:     In B:B_get_b
{an instance of B at location 20064578 with (b=2)}
"ctorFullExample.C": 97:        In B:somDestruct

"ctorFullExample.C": 62:        In B:BwithInitialValue
"ctorFullExample.C": 110: In B:somPrintSelf
"./ctorFullExample.xih": 655:     In B:B_get_b
{an instance of B at location 20064578 with (b=22)}
"ctorFullExample.C": 97:        In B:somDestruct

"ctorFullExample.C": 150: In C:somDefaultInit
"ctorFullExample.C": 18:        In A:somDefaultInit
"ctorFullExample.C": 79:        In B:somDefaultInit
"ctorFullExample.C": 182: In C:somPrintSelf
"./ctorFullExample.xih": 292:     In A:A_get_a
"./ctorFullExample.xih": 655:     In B:B_get_b
"./ctorFullExample.xih": 1104:    In C:C_get_c
{an instance of C at location 20065448 with (a=1, b=2, c=3)}
"ctorFullExample.C": 169: In C:somDestruct
"ctorFullExample.C": 35:        In A:somDestruct
"ctorFullExample.C": 97:        In B:somDestruct

"ctorFullExample.C": 196: In C:CwithInitialValue
"ctorFullExample.C": 18:        In A:somDefaultInit
"ctorFullExample.C": 62:        In B:BwithInitialValue
"ctorFullExample.C": 182: In C:somPrintSelf
"./ctorFullExample.xih": 292:     In A:A_get_a
"./ctorFullExample.xih": 655:     In B:B_get_b
"./ctorFullExample.xih": 1104:      In C:C_get_c
{an instance of C at location 20065448 with (a=1, b=33, c=44)}
"ctorFullExample.C": 169: In C:somDestruct
"ctorFullExample.C": 35:        In A:somDestruct
"ctorFullExample.C": 97:        In B:somDestruct



"ctorFullExample.C": 132: In C:CwithInitialString
"ctorFullExample.C": 18:        In A:somDefaultInit
"ctorFullExample.C": 62:        In B:BwithInitialValue
"ctorFullExample.C": 182: In C:somPrintSelf
"./ctorFullExample.xih": 292:     In A:A_get_a
"./ctorFullExample.xih": 655:     In B:B_get_b
"./ctorFullExample.xih": 1104:      In C:C_get_c
{an instance of C at location 20065448 with (a=1, b=55, c=66)}
"ctorFullExample.C": 169: In C:somDestruct
"ctorFullExample.C": 35:        In A:somDestruct
"ctorFullExample.C": 97:        In B:somDestruct


ΓòÉΓòÉΓòÉ 6.5.4. Customizing the initialization of class objects ΓòÉΓòÉΓòÉ

As described previously, the somDefaultInit method can be overridden to
customize the initialization of objects. Because classes are objects,
somDefaultInit is also invoked on classes when they are first created
(generally by invoking the somNew method on a metaclass). For a class object,
however, somDefaultInit normally just sets the name of the class to "unknown,"
after which the somInitMIClass method must be used for the major portion of
class initialization. Of course, metaclasses can override somDefaultInit to
initialize introduced class variables that require no arguments for their
initialization.

Note: Because somNew does not call somInitMIClass, class objects returned from
invocations of somNew on a metaclass are not yet useful class objects.

The somInitMIClass method (introduced by SOMClass) is invoked on a new class
object using arguments to indicate the class name and the parent classes from
which inheritance is desired (among other arguments). This invocation is made
by whatever routine is used to initialize the class. (For SOM classes using the
C or C++ implementation bindings, this is handled by the somBuildClass
procedure, which is called by the implementation bindings automatically.) The
somInitMIClass method is often overridden by a metaclass to influence
initialization of new classes in some way. Typically, the overriding procedure
begins by making parent method calls, and then performs additional actions
thereafter.

However, without use of the Cooperative Metaclass to guarantee correct
operation of SOMobjects in general, none of the methods introduced by SOMClass
should be overridden. As a result, customizing the initialization of class
objects (other than through overriding somDefaultInit for initialization of
class variables) is not recommended in SOMobjects 2.1.  Users whose
applications require this should request access to the experimental Cooperative
Metaclass used to implement the SOMobjects Metaclass Framework.  But,
metaclasses implemented using the experimental Cooperative Metaclass may
require reprogramming when SOMobjects introduces an officially supported
Cooperative Metaclass.


ΓòÉΓòÉΓòÉ 6.6. Creating a SOM Class Library ΓòÉΓòÉΓòÉ

One of the principal advantages of SOM is that it makes "black box" (or binary)
reusability possible. Consequently, SOM classes are frequently packaged and
distributed as class libraries. A class library  holds the actual
implementation of one or more classes and can be dynamically loaded and
unloaded as needed by applications. Importantly, class libraries can also be
replaced independently of the applications  that use them and, provided that
the class implementor observes simple SOM guidelines for preserving binary
compatibility, can evolve and expand over time.


ΓòÉΓòÉΓòÉ 6.6.1. General guidelines for class library designers ΓòÉΓòÉΓòÉ

One of the most important features of SOM is that it allows you to build and
distribute class libraries in binary form. Because there is no "fragile base
class" problem in SOM, client programs that use your libraries (by subclassing
your classes or by invoking the methods in your classes) will not need to be
recompiled if you later produce a subsequent version of the library, provided
you adhere to some simple restrictions.

   1. You should always maintain the syntax and the semantics of your existing
      interfaces. This means that you cannot take away any exposed
      capabilities, nor add or remove arguments for any of your public methods.

   2. Always maintain the releaseorder list, so that it never changes except
      for additions to the end. The releaseorder should contain all of your
      public methods, the one or two methods that correspond to each public
      attribute, and a placeholder for each private method (or private
      attribute method).

   3. Assign a higher minorversion number for each subsequent release of a
      class, so that client programmers can determine whether a new feature is
      present or not. Change the majorversion number only when you deliberately
      wish to break binary compatibility. (See the topic "Modifier statements"
      in Chapter 4, "SOM IDL and the SOM Compiler" for explanations of the
      releaseorder, minorversion and majorversion modifiers.)

   4. Define attributes that return pointers to structures, or define methods
      that take an out  parameter for passing a structure back to the caller.
      In Windows NT and Windows 95, Microsoft has provided the --stdcall
      convention. However it is not always faithfully implemented across
      compilers. So, it is best to avoid struct returns.

      Note that you can always avoid this problem in classes of your own
      design. However, some of the attributes and methods in the frameworks
      that come with the SOMobjects Toolkit do  return structures. Many of
      these are dictated by the OMG CORBA standard, and could not be avoided.

   5. With each new release of your class library, you have significant degrees
      of freedom to change much of the implementation detail. You can add to or
      reorganize your instance variables, add new public or private methods,
      inject new base classes into your class hierarchies, change metaclasses
      to more derived ones, and relocate the implementation of methods upward
      in your class hierarchies. Provided you always retain the same
      capabilities and semantics that were present in your first release, none
      of these changes will break the client programs that use your libraries.


ΓòÉΓòÉΓòÉ 6.6.2. Types of class libraries ΓòÉΓòÉΓòÉ

Since class libraries are not programs, users cannot execute them directly. To
enable users to make direct use of your classes, you must also provide one or
more programs that create the classes and objects that the user will need. This
section describes how to package your classes in a SOM class library and what
you must do to make the contents of the library accessible to other programs.

On AIX, class libraries are actually produced as AIX shared libraries, whereas
on OS/2 or Windows they appear as dynamically-linked libraries (or DLLs). The
term "DLL" is sometimes used to refer to either an AIX, an OS/2, or a Windows
class library, and (by convention only) the file suffix ".dll" is used for SOM
class libraries on all platforms.

A program can use a class library containing a given class or classes in one of
two ways:

   1. If the programmer employs the SOM bindings to instantiate the class and
      invoke its methods, the resulting client program contains static
      references to the class. The operating system will automatically resolve
      those references when the program is loaded, by also loading the
      appropriate class library.

   2. If the programmer uses only the dynamic SOM mechanisms for finding the
      class and invoking its methods (for example, by invoking somFindClass,
      somFindMethod, somLookupMethod, somDispatch, somResolveByName,and so
      forth), the resulting client program does not contain any static
      references to the class library. Thus, SOM will load the class library
      dynamically during execution of the program. Note: For SOM to be able to
      load the class library, the dllname modifier must be set in the .idl
      file. (See the topic "Modifier statements" in Chapter 4, "SOM IDL and the
      SOM Compiler.")

 Because the provider of a class library cannot predict which of these ways a
 class will be used, SOM class libraries must be built such that either usage
 is possible. The first case above requires the class library to export the
 entry points needed by the SOM bindings, whereas the second case requires the
 library to provide an initialization function to create the classes it
 contains. The following topics discuss each case.


ΓòÉΓòÉΓòÉ 6.6.3. Building export files ΓòÉΓòÉΓòÉ

The SOM Compiler provides a "def" emitter to produce the necessary exported
symbols for each class. For example, to generate the necessary exports for a
class "A", issue the sc command with the following -s option. (For a discussion
of the sc command and options, see "Running the SOM Compiler" in Chapter 4,
"SOM IDL and the SOM Compiler.")

This command generates an "a.def" file:

   sc -sdef a.idl

Typically, a class library contains multiple classes. To produce an appropriate
export file for each class that the library will contain, you can create a new
export file for the library itself by combining the exports from each "def"
file into a single file. The following example of a combined export "def" file
a class library composed of three classes, "A", "B", and "C".

 "def" file:

   LIBRARY abc INITINSTANCE
   DESCRIPTION 'abc example class library'
   PROTMODE
   DATA MULTIPLE NONSHARED LOADONCALL
   EXPORTS
     ACClassData
     AClassData
     ANewClass
     BCClassData
     BClassData
     BNewClass
     CCClassData
     CClassData
     CNewClass

Other symbols in addition to those generated by the "def" emitter can be
included if needed, but this is not required by SOM. One feature of SOM is that
a class library needs no more than three exports per class (by contrast, many
OOP systems require externals for every method as well). One required export is
the name of a procedure to create the class (<className>NewClass), and the
others are two external data structures that are referenced by the SOM
bindings.


ΓòÉΓòÉΓòÉ 6.6.4. Specifying the initialization function ΓòÉΓòÉΓòÉ

An initialization function for the class library must be provided to support
dynamic loading of the library by the SOM Class Manager. The SOM Class Manager
expects that, whenever it loads a class library, the initialization function
will create and register class objects for all of the classes contained in the
library.

These classes are then managed as a group (called an affinity group). One class
in the affinity group has a privileged position-namely, the class that was
specifically requested when the library was loaded. If that class (that is, the
class that caused loading to occur) is subsequently unregistered, the SOM Class
Manager will automatically unregister all of the other classes in the affinity
group as well, and will unload the class library. Similarly, if the SOM Class
Manager is explicitly asked to unload the class library, it will also
automatically unregister and free all of the classes in the affinity group.

It is the responsibility of the class-library creator to supply the
initialization function. The interface to the initialization function is given
by the following C/C++ prototype:

   #ifdef __IBMC__
     #pragma linkage (SOMInitModule, system)
   #endif

   SOMEXTERN void  SOMLINK SOMInitModule ( long majorVersion,
                                           long minorVersion,
                                           string className);

The parameters provided to this function are the className and the major/minor
version numbers of the class that was requested when the library was loaded
(that is, the class that caused loading). The initialization function is free
to use or to disregard this information; nevertheless, if it fails to create a
class object with the required name, the SOM Class Manager considers the load
to have failed. As a rule of thumb, however, if the initialization function
invokes a <className>NewClass procedure for each class in the class library,
this condition will always be met. Consequently, the parameters supplied to the
initialization function are not needed in most cases.

Here is a typical class-library initialization function, written in C, for a
library with three classes ("A", "B", and "C"):

   #include "a.h"
   #include "b.h"
   #include "c.h"
   #ifdef __IBMC__
     #pragma linkage (SOMInitModule, system)
   #endif

   SOMEXTERN void  SOMLINK SOMInitModule (long majorVersion,
                             long minorVersion, string className)
   {
       SOM_IgnoreWarning (majorVersion);  /* This function makes */
       SOM_IgnoreWarning (minorVersion);  /* no use of the passed */
       SOM_IgnoreWarning (className);     /* arguments.   */
       ANewClass (A_MajorVersion, A_MinorVersion);
       BNewClass (B_MajorVersion, B_MinorVersion);
       CNewClass (C_MajorVersion, C_MinorVersion);
   }

The source code for the initialization function can be added to one of the
implementation files for the classes in the library, or you can put it in a
separate file and compile it independently.


ΓòÉΓòÉΓòÉ 6.6.5. Creating the import library ΓòÉΓòÉΓòÉ

Finally, for each of your class libraries, you should create an import library
that can be used by client programs (or by other class libraries that use your
classes) to resolve the references to your classes.

Here is an example illustrating all of the steps required to create a class
library ("abc.dll") that contains the three classes "A", "B", and "C".

   1. Compile all of the implementation files for the classes that will be
      included in the library. Include the initialization function also.

      Written in C:

               icc -I. -I%SOMBASE%\include -Ge- -c a.c
               icc -I. -I%SOMBASE%\include -Ge- -c b.c
               icc -I. -I%SOMBASE%\include -Ge- -c c.c
               icc -I. -I%SOMBASE%\include -Ge- -c initfunc.c

      Note:  The "-Ge" option is used only with the IBM compiler.  It indicates
             that the object files will go into a DLL.

      Written in C++:

               icc -I. -I%SOMBASE%\include -Ge- -c a.cpp
               icc -I. -I%SOMBASE%\include -Ge- -c b.cpp
               icc -I. -I%SOMBASE%\include -Ge- -c c.cpp
               icc -I. -I%SOMBASE%\include -Ge- -c initfunc.cpp

      Note:  The "-Ge" option is used only with the IBM compiler.  It indicates
             that the object files will go into a DLL.

   2. Produce an export file for each class.

               sc -sdef a.idl b.idl c.idl

   3. Manually combine the exported symbols into a single file.

      Create a file "abc.def" from "a.def", "b.def", and  c.def". Include the
      initialization function (SOMInitModule) in the export list, so that all
      classes will be initialized automatically, unless your initialization
      function does not need arguments and you explicitly invoke it yourself
      from an DLL initialization    routine.

   4. Create an import library э/export.obj∙ that corresponds to the class
      library, so that programs and other class libraries can use (import) your
      classes.

              ilib /geni abc.def

      The filename ("abc.def") specifies the exported symbols to include in the
      import library. An import library ("abc.lib") will be created. Another
      file ("abc.exp") will also be created. This is the "export.obj" which
      will be used in the link step.

   5. Using the object files and the export file, produce a binary class
      library.

               set LIB=%SOMBASE%\lib;%LIB%
                 ilink a.obj b.obj c.obj initfunc.obj /OUT:abc.dll somtk.lib\ abc.exp

      If your classes make use of classes in other class libraries, include the
      names of their import libraries immediately after "somtk" (before the
      next comma).


ΓòÉΓòÉΓòÉ 6.7. Customizing Memory Management ΓòÉΓòÉΓòÉ

SOM is designed to be policy free and highly adaptable. Most of the SOM
behavior can be customized by subclassing the built-in classes and overriding
their methods, or by replacing selected functions in the SOM run-time library
with application code. This chapter contains more advanced topics describing
how to customize the following aspects of SOM behavior: memory management,
dynamic class loading and unloading, character output, error handling, and
method resolution. Information on customizing Distributed SOM is provided in
Chapter 6.

The memory management functions used by the SOM run-time environment are a
subset of those supplied in the ANSI C standard library. They have the same
calling interface and return the equivalent types of results as their ANSI C
counterparts, but include some supplemental error checking. Errors detected in
these functions result in the invocation of the error-handling function to
which SOMError points.

The correspondence between the SOM memory-management function variables and
their ANSI standard library equivalents is given in the table below.

Memory-Management Functions

ΓöîΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö¼ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÉ
Γöé SOM FUNCTION   Γöé ANSI STANDARD C Γöé RETURN TYPE  Γöé ARGUMENT TYPES   Γöé
Γöé VARIABLE     Γöé LIBRARY FUNC-  Γöé        Γöé           Γöé
Γöé         Γöé TION       Γöé        Γöé           Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMCalloc    Γöé calloc( )    Γöé somToken   Γöé size_t, size_t   Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMFree     Γöé free( )     Γöé void     Γöé somToken      Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMMalloc    Γöé malloc( )    Γöé somToken   Γöé size_t       Γöé
Γö£ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö╝ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöñ
Γöé SOMRealloc    Γöé realloc( )    Γöé somToken   Γöé somToken, size_t  Γöé
ΓööΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓö┤ΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÇΓöÿ

An application program can replace SOM's memory management functions with its
own memory management functions to change the way SOM allocates memory (for
example, to perform all memory allocations as suballocations in a shared memory
heap). This replacement is possible because SOMCalloc, SOMMalloc, SOMRealloc,
and SOMFree are actually global variables that point to SOM's default memory
management functions, rather than being the names of the functions themselves.
Thus, an application program can replace SOM's default memory management
functions by assigning the entry-point address of the user-defined memory
management function to the appropriate global variable. For example, to replace
the default free procedure with the user-defined function MyFree (which must
have the same signature as the ANSI C free function), an application program
would require the following code:

#include <som.h>
/* Define a replacement routine: */

#ifdef __OS2__                  /* not for SOM 3.0 */
#pragma linkage(myFree, system) /* not for SOM 3.0 */
#endif                          /* not for SOM 3.0 */

void SOMLINK myFree (somToken memPtr)
{
     (Customized code goes here)
}
...
SOMFree = myFree;

Note:  In general, all of these routines should be replaced as a group. For
       instance, if an application supplies a customized version of SOMMalloc,
       it should also supply correponding SOMCalloc, SOMFree, and SOMRealloc
       functions that conform to this same style of memory management.


ΓòÉΓòÉΓòÉ 6.8. Customizing Class Loading and Unloading ΓòÉΓòÉΓòÉ

SOM uses three routines that manage the loading and unloading of class
libraries (referred to here as DLLs). These routines are called by the
SOMClassMgrObject  as it dynamically loads and registers classes. If
appropriate, the rules that govern the loading and unloading of DLLs can be
modified, by replacing these functions with alternative implementations.


ΓòÉΓòÉΓòÉ 6.8.1. Customizing class initialization ΓòÉΓòÉΓòÉ

The SOMClassInitFuncName function has the following signature:

string  (*SOMClassInitFuncName) ( );

This function returns the name of the function that will initialize (create
class objects for) all of the classes that are packaged together in a single
class library. (This function name applies to all class libraries loaded by the
SOMClassMgrObject.) The SOM-supplied version of SOMClassInitFuncName returns
the string " SOMInitModule".  The interface to the library initialization
function is described under the topic "Creating a SOM Class Library" earlier in
this chapter.


ΓòÉΓòÉΓòÉ 6.8.2. Customizing DLL loading ΓòÉΓòÉΓòÉ

To dynamically load a SOM class, the SOMClassMgrObject  calls the function
pointed to by the global variable SOMLoadModule to load the DLL containing the
class. The reason for making public the SOMLoadModule function (and the
following SOMDeleteModule function) is to reveal the boundary where SOM touches
the operating system. Explicit invocation of these functions is never required.
However, they are provided to allow class implementors to insert their own code
between the operating system and SOM, if desired. The  SOMLoadModule function
has the following signature:

 long  (*SOMLoadModule)   ( string className,

                                                      string fileName,

                                                     string functionName,

                                                     long majorVersion,

                                                     long minorVersion,

                                                     somToken *modHandle);

 This function is responsible for loading the DLL containing the SOM class
 className and returning either the value zero (for success) or a nonzero
 system-specific error code. The output argument modHandle is used to return a
 token that can subsequently be used by the DLL-un loading routine (described
 below) to unload the DLL. The default DLL-loading routine returns the DLL's
 module handle in this argument. The remaining arguments are used as follows:

 Argument                 Usage

 fileName                 The file name of the DLL to be loaded, which can be
                          either a simple name or a full path name.

 functionName             The name of the routine to be called after the DLL is
                          successfully loaded by the SOMClassMgrObject. This
                          routine is responsible for creating the class objects
                          for the class(es) contained in the DLL. Typically,
                          this argument has the value "SOMInitModule", which is
                          obtained from the function SOMClassInitFuncName
                          described above. If no SOMInitModule entry exists in
                          the DLL, the default DLL-loading routine looks in the
                          DLL for a procedure with the name <className>NewClass
                          instead. If neither entry point can be found, the
                          default DLL-loading routine fails.

 majorVersion             The major version number to be passed to the class
                          initialization function in the DLL (specified by the
                          functionName argument).

 minorVersion             The minor version number to be passed to the class
                          initialization function in the DLL (specified by the
                          FunctionName argument).

 An application program can replace the default DLL-loading routine by
 assigning the entry point address of the new DLL-loading function (such as
 MyLoadModule) to the global variable SOMLoadModule, as follows:

  #include <som.h>
  /* Define a replacement routine: */
  long myLoadModule (string className, string fileName,
                     string functionName, long majorVersion,
                     long minorVersion, somToken *modHandle)
  {
       (Customized code goes here)
  }
  ...
  SOMLoadModule = MyLoadModule;


ΓòÉΓòÉΓòÉ 6.8.3. Customizing DLL unloading ΓòÉΓòÉΓòÉ

To unload a SOM class, the SOMClassMgrObject calls the function pointed to by
the global variable SOMDeleteModule. The SOMDeleteModule function has the
following signature:

long  (*SOMDeleteModule)  (in somToken modHandle);

This function is responsible for unloading the DLL designated by the modHandle
parameter and returning either zero (for success) or a nonzero system-specific
error code. The parameter modHandle contains the value returned by the DLL
loading routine (described above) when the DLL was loaded.

An application program can replace the default DLL-unloading routine by
assigning the entry point address of the new DLL-unloading function (such as,
MyDeleteModule) to the global variable SOMDeleteModule, as follows:

#include <som.h>
/* Define a replacement routine: */
long myDeleteModule (somToken modHandle)
{
     (Customized code goes here)
}
...
SOMDeleteModule = MyDeleteModule;


ΓòÉΓòÉΓòÉ 6.9. Customizing Character Output ΓòÉΓòÉΓòÉ

The SOM character-output function is invoked by all of the SOM error-handling
and debugging macros whenever a character must be generated (see "Debugging"
and "Exceptions and error handling" in Using SOM Classes in Client Programs,
"Using SOM Classes in Client Programs"). The default character-output routine,
pointed to by the global variable SOMOutCharRoutine, simply writes the
character to "stdout", then returns 1 if successful, or 0 otherwise.

For convenience, SOMOutCharRoutine is supplemented by the somSetOutChar
function.  The somSetOutChar function enables each task to have a customized
character output routine, thus it is often preferred for changing the output
routine called by somPrintf (because SOMOutChrRoutine would remain in effect
for subsequent tasks).  On Windows, the somSetOutChar function is required
(rather than SOMOutCharRoutine); it is optional on other operating systems.

An application programmer might wish to supply a customized replacement routine
to:

     Direct the output to stderr,

     Record the output in a log file,

     Collect characters and handle them in larger chunks,

     Send the output to a window to display it,

     Place the output in a clipboard, or

     Some combination of these.

 With SOMOutCharRoutine, an application program would use code similar to the
 following to install the replacement routine:

  #include <som.h>
  #pragma linkage(myCharacterOutputRoutine, system)
  /* Define a replacement routine: */
  int SOMLINK myCharacterOutputRoutine (char c)
  {
       (Customized code goes here)
  }

  /* After the next stmt all output */
  /* will be sent to the new routine   */
  SOMOutCharRoutine = myCharacterOutputRoutine;

 With somSetOutChar, an application program would use code similar to the
 following to install the replacement routine:

  #include <som.h>
  static int irOutChar(char c);

  static int irOutChar(char c)
  {
      (Customized code goes here)
  }

  main (...)
  {
      ...
  somSetOutChar((somTD_SOMOutCharRoutine *) irOutChar);
  }


ΓòÉΓòÉΓòÉ 6.10. Customizing Error Handling ΓòÉΓòÉΓòÉ

When an error occurs within any of the SOM-supplied methods or functions, an
error-handling procedure is invoked. The default error-handling procedure
supplied by SOM, pointed to by the global variable SOMError, has the following
signature:

void  (*SOMError) (int errorCode ,  string fileName,  int lineNum );

The default error-handling procedure inspects the  errorCode  argument and
takes appropriate action, depending on the last decimal digit of errorCode (see
"Exceptions and error handling" in Using SOM Classes in Client Programs for a
discussion of error classifications). In the default error handler, fatal
errors terminate the current process. The remaining two arguments (fileName and
lineNum), which indicate the name of the file and the line number within the
file where the error occurred, are used to produce an error message.

An application programmer might wish to replace the default error handler with
a customized error-handling routine to:

     Record errors in a way appropriate to the particular application,

     Inform the user through the application's user interface,

     Attempt application-level recovery by restarting at a known point, or

     Shut down the application.

 An application program would use code similar to the following to install the
 replacement routine:

  #include <som.h>
  /* Define a replacement routine: */
  void myErrorHandler (int errorCode, string fileName, int lineNum)
  {
       (Customized code goes here)
  }
  ...
  /* After the next stmt all errors */
  /* will be handled by the new routine   */
  SOMError = myErrorHandler;

 When any error condition originates within the classes supplied with SOM, SOM
 is left in an internally consistent state. If appropriate, an application
 program can trap errors with a customized error-handling procedure and then
 resume with other processing. Application programmers should be aware,
 however, that all methods within the SOM run-time library behave atomically.
 That is, they either succeed or fail; but if they fail, partial effects are
 undone wherever possible. This is done so that all SOM methods remain usable
 and can be re-executed following an error.


ΓòÉΓòÉΓòÉ 7. Distributed SOM (DSOM) ΓòÉΓòÉΓòÉ

Note:  The SOMobject Base Toolkit provides the capability for implementing
       Workstation Distributed System Object Module (DSOM) (distribution among
       processes on a single machine).

Implementing an application that is distributed across a network of machines
requires Workgroup DSOM, which is not available.

 The following subjects are discussed in this section:

     A Simple DSOM Example
     Basic Client Programming
     Basic Server Programming
     Implementing Classes
     Configuring DSOM Applications
     Running DSOM Applications
     DSOM as a CORBA-compliant Object Request Broker
     Advanced Topics
     Error Reporting and Troubleshooting
     Limitations


ΓòÉΓòÉΓòÉ 7.1. What is Distributed SOM? ΓòÉΓòÉΓòÉ

Whereas the power of SOM technology comes from the fact that SOM insulates the
client of an object from the object's implementation, the power of DSOM lies in
the fact that DSOM insulates the client of an object from the object's
location.

Distributed SOM (or DSOM) provides a framework that allows application programs
to access objects across address spaces. That is, application programs can
access objects in other processes.

This version of DSOM currently supports distribution among processes on the
same machine (referred to as Workstation DSOM).

DSOM runs on the AIX (Release 3.2.5 and above), OS/2 (Release 2.0 and above),
Windows 95, and Windows NT (Version 3.51) operating systems. A Workstation DSOM
application can run on a machine in either environment using core capabilities
of the SOMobjects system. Future releases of DSOM may support large,
enterprise-wide networks.

This version of DSOM can be viewed as a System Object Model extension that
allows a program to invoke methods on SOM objects in other processes. Other
versions of SOM, such as the AIX and OS/2 versions, support Workgroup DSOM.
They can be viewed as a SOM extension also. In addition they can be viewed as
an Object Request Broker (ORB); that is, a standardized "transport" for
distributed object interaction. In this respect, DSOM complies with the Common
Object Request Broker Architecture (CORBA) specification, published by the
Object Management Group (OMG) and x/Open.

This chapter describes DSOM from both perspectives.

Note:  Portions of this discussion of DSOM pertain to Workgroup DSOM only and
       are not supported by this Windows version of the product. References to
       topics such as ORB, CORBA, and selection of servers do not apply to this
       version. These discussions are presented here to provide a more complete
       understanding of DSOM in general.


ΓòÉΓòÉΓòÉ 7.2. DSOM features ΓòÉΓòÉΓòÉ

Here is a quick summary of some of DSOM's more important features:

     Uses the standard SOM Compiler, Interface Repository, language bindings,
      and class libraries.  Thus, DSOM provides a growth path for
      non-distributed SOM applications.

     Allows an application program to access local objects.

     Uses existing interprocess communication (IPC) facilities for Workstation
      communication.

     Provides support for writing multi-threaded servers and event-driven
      programs.

     Provides a default object server program, which can be easily used to
      create SOM objects and make those objects accessible to one or more
      client programs. If the default server program is used, SOM class
      libraries are loaded upon demand, so no server programming or compiling
      is necessary.

     Complies with the CORBA 1.1 specification, which is important for
      application portability.


ΓòÉΓòÉΓòÉ 7.3. When to use DSOM ΓòÉΓòÉΓòÉ

DSOM should be used for those applications that require sharing of objects
among multiple programs. The object actually exists in only one process (this
process is known as the object's server); the other processes (known as
clients) access the object through remote method invocations, made transparent
by DSOM.

DSOM should also be used for applications that require objects to be isolated
from the main program. This is usually done in cases where reliability is a
concern, either to protect the object from failures in other parts of the
application, or to protect the application from an object.


ΓòÉΓòÉΓòÉ 7.4. Chapter Outline ΓòÉΓòÉΓòÉ

Briefly, this section covers the following subjects:

     Tutorial example
     Programming DSOM applications
     Configuring DSOM applications
     Running DSOM applications
     DSOM and CORBA
     Advanced topics
     Error reporting and troubleshooting


ΓòÉΓòÉΓòÉ 7.5. Tutorial example ΓòÉΓòÉΓòÉ

First, a complete example shows how an existing SOM class implementation (a
"stack") can be used with DSOM to create a distributed "Stack" application.
Using the "Stack" example as a backdrop, the basic DSOM interfaces are
introduced.


ΓòÉΓòÉΓòÉ 7.6. Programming DSOM applications ΓòÉΓòÉΓòÉ

All DSOM applications involve three kinds of programming:

     Client programming:  writing code that uses objects;

     Server programming:  writing code that implements and manages objects.

     Implementing classes:  writing code that implements objects.

 Three sections ("Basic Client Programming", "Basic Server Programming", and
 Implementing Classes") describe how to create DSOM applications from these
 three points of view. In turn, the structure and services of the relevant DSOM
 run-time environment are explained.

 Note:  The three sections are presented in the order above to aid in their
 explanation. However, the actual programming tasks are likely to be performed
 in the opposite order.

 Additional examples are provided in these sections to illustrate DSOM
 services.


ΓòÉΓòÉΓòÉ 7.7. Configuring DSOM applications ΓòÉΓòÉΓòÉ

The section "Configuring DSOM Applications" explains what is necessary to set
up a DSOM application, once the application has been built.


ΓòÉΓòÉΓòÉ 7.8. Running DSOM applications ΓòÉΓòÉΓòÉ

The section "Running DSOM Applications" explains what is necessary to run a
DSOM application, once it has been built and configured.


ΓòÉΓòÉΓòÉ 7.9. DSOM and CORBA ΓòÉΓòÉΓòÉ

Those readers interested in using DSOM as a CORBA-compliant ORB should read the
section entitled "DSOM as a CORBA compliant Object Broker." That section
answers the question: How are CORBA concepts implemented in DSOM?


ΓòÉΓòÉΓòÉ 7.10. Advanced topics ΓòÉΓòÉΓòÉ

The section on "Advanced Topics" covers the following:

   1. "Peer versus client/server processes" demonstrates how peer-to-peer
      object interactions are supported in DSOM.

   2. "Dynamic Invocation Interface" details DSOM support for the CORBA dynamic
      invocation interface to dynamically build and invoke methods on remote
      objects.

   3. "Creating user-supplied proxy classes" describes how to override proxy
      generation by the DSOM run time and, instead, install a proxy object
      supplied by the user.

   4. "Customizing the default base proxy class"discusses how the
      SOMDClientProxy class can be subclassed to define a customized base class
      that DSOM will use during dynamic proxy-class generation.


ΓòÉΓòÉΓòÉ 7.11. Error reporting and troubleshooting ΓòÉΓòÉΓòÉ

The section on "Error Reporting and Troubleshooting" discusses facilities to
aid in problem diagnosis.


ΓòÉΓòÉΓòÉ 7.12. A Simple DSOM Example ΓòÉΓòÉΓòÉ

A sample "Stack" application is presented in this section as a tutorial
introduction to DSOM. It demonstrates that, for simple examples like a "Stack",
after very little work, the class can be used to implement distributed objects
that are accessed remotely. The example first presents the "Stack" application
components and the steps that the implementer must perform before the
application can be run, and then describes the run time activity that results
from executing the application. This run-time scenario introduces several of
the key architectural components of the DSOM run-time environment.

The source code for this example is provided with the DSOM samples in the
SOMobjects Developer Toolkit.


ΓòÉΓòÉΓòÉ 7.12.1. The "Stack" interface ΓòÉΓòÉΓòÉ

The example starts with the assumption that the class implementer has
successfully built a SOM class library DLL, called "stack.dll", in the manner
described in Section 5.6, "Creating a SOM Class Library," of Chapter 5,
"Implementing Classes in SOM." The DLL implements the following IDL interface.

#include <somobj.idl>

interface Stack: SOMObject
{
    const long stackSize = 10;
    exception STACK_OVERFLOW{};
    exception STACK_UNDERFLOW{};
    boolean full();
    boolean empty();
    long top() raises(STACK_UNDERFLOW);
    long pop() raises(STACK_UNDERFLOW);
    void push(in long element) raises(STACK_OVERFLOW);

    #ifdef __SOMIDL__
    implementation
    {
      releaseorder: full, empty, top, pop, push;
      somDefaultInit: override;
      long stackTop;                  // top of stack index
      long stackValues[stackSize];    // stack elements
      dllname = "stack.dll";
    };
    #endif
};

This DLL could have been built without the knowledge that it would ever be
accessed remotely (that is, built following the procedures in Chapter 5). Note,
however, that some DLLs may require changes in the way their classes pass
arguments and manage memory, in order to be used by remote clients (see the
topic "Implementation Constraints" in section 6.5, "Implementing Classes").


ΓòÉΓòÉΓòÉ 7.12.2. The "Stack" class implementation ΓòÉΓòÉΓòÉ

#define Stack_Class_Source
#include <stack.ih>

SOM_Scope boolean  SOMLINK full(Stack somSelf, Environment *ev)
{
    StackData *somThis = StackGetData(somSelf);
    StackMethodDebug("Stack","full");

    /* Return TRUE if stack is full. */
    return (_stackTop == stackSize);
}

SOM_Scope boolean  SOMLINK empty(Stack somSelf, Environment *ev)
{
    StackData *somThis = StackGetData(somSelf);
    StackMethodDebug("Stack","empty");

    /* Return TRUE if stack is empty.*/
    return (_stackTop == 0);
}

SOM_Scope long  SOMLINK top(Stack somSelf, Environment *ev)
{
    StackData *somThis = StackGetData(somSelf);
    StackMethodDebug("Stack","top");

    if (_stackTop > 0)
    {
       /* Return top element in stack without removing it from
        * the stack.
        */
       return (_stackValues[_stackTop-1]);
    }
    else
    {
       somSetException(ev, USER_EXCEPTION,
                       ex_STACK_UNDERFLOW, NULL);
       return (-1L);
    }
}

SOM_Scope long  SOMLINK pop(Stack somSelf, Environment *ev)
{
    StackData *somThis = StackGetData(somSelf);
    StackMethodDebug("Stack","pop");

    if (_stackTop > 0)
    {
       /* Return top element in stack and remove it from the
        * stack.
        */
       _stackTop--;
       return (_stackValues[_stackTop]);
    }
    else
    {
       somSetException(ev, USER_EXCEPTION,
                       ex_STACK_UNDERFLOW, NULL);
       return (-1L);
    }
}

SOM_Scope void  SOMLINK push(Stack somSelf,
                             Environment *ev, long el)
{
    StackData *somThis = StackGetData(somSelf);
    StackMethodDebug("Stack","push");

    if (_stackTop < stackSize)
    {
      /* Add element to top of the stack. */
      _stackValues[_stackTop] = el;
      _stackTop++;
    }
    else
    {
       somSetException(ev, USER_EXCEPTION,
                       ex_STACK_OVERFLOW, NULL);
    }
}

SOM_Scope void  SOMLINK somDefaultInit(Stack somSelf,
                                       somInitCtrl* ctrl)
{
    StackData *somThis;
    somInitCtrl globalCtrl;
    somBooleanVector myMask;
    StackMethodDebug("Stack","somDefaultInit");
    Stack_BeginInitializer_somDefaultInit;

    Stack_Init_SOMObject_somDefaultInit(somSelf, ctrl);

    /* stackTop is index into stackValues for next pushed
     * stack element.
     * stackValues[0..(stackSize-1)] holds stack elements.
     */
    _stackTop = 0;
}


ΓòÉΓòÉΓòÉ 7.12.3. Client program using a local stack ΓòÉΓòÉΓòÉ

A simple client program written to use a local "Stack" object is displayed
below. This C program is shown so that the differences between a local and
remote client program can be highlighted.

#include <stack.h>

boolean OperationOK(Environment *ev);

int main(int argc, char *argv[])
{
  Environment ev;
  Stack stk;
  long num = 100;

  SOM_InitEnvironment(&ev);

  /* The StackNewClass invocation is optional and unnecessary
   * in the client program when the class object is created in
   * the SOMInitModule function that is invoked during DLL
   * initialization.
   */
  StackNewClass(Stack_MajorVersion, Stack_MinorVersion);
  stk = StackNew();

  /* Verify successful object creation */
  if ( stk != NULL )
  {
     while ( !_full(stk, &ev) )
     {
        _push(stk, &ev, num);
        somPrintf("Top: %d\n", _top(stk, &ev));
        num += 100;
     }

     /* Test stack overflow exception */
     _push(stk, &ev, num);
     OperationOK(&ev);

     while ( !_empty(stk, &ev) )
     {
        somPrintf("Pop: %d\n", _pop(stk, &ev));
     }

     /* Test stack underflow exception */
     somPrintf("Top Underflow: %d\n", _top(stk, &ev));
     OperationOK(&ev);
     somPrintf("Pop Underflow: %d\n", _pop(stk, &ev));
     OperationOK(&ev);

     _push(stk, &ev, -10000);
     somPrintf("Top: %d\n", _top(stk, &ev));
     somPrintf("Pop: %d\n", _top(stk, &ev));

     _somFree(stk);
  }

  SOM_UninitEnvironment(&ev);

  return(0);
}

boolean OperationOK(Environment *ev)
{
   char *exID;

   switch (ev->_major)
   {
     case SYSTEM_EXCEPTION:
       exID = somExceptionId(ev);
       somPrintf("System exception: %s\n", exID);
       somdExceptionFree(ev);
       return (FALSE);

     case USER_EXCEPTION:
       exID = somExceptionId(ev);
       somPrintf("User exception: %s\n", exID);
       somdExceptionFree(ev);
       return (FALSE);

     case NO_EXCEPTION:
       return (TRUE);

     default:
       somPrintf("Invalid exception type in Environment.\n");
       somdExceptionFree(ev);
       return (FALSE);
   }
}


ΓòÉΓòÉΓòÉ 7.12.4. Client program using a remote stack ΓòÉΓòÉΓòÉ

The preceding program has been rewritten below showing how DSOM can be used to
create and access a "Stack" object somewhere in the system. The exact location
of the object does not matter to the application; it just wants a "Stack"
object. Note that the stack operations of the two programs are identical. The
main differences lie in stack creation and destruction, as highlighted below.
(Also see "Memory management" later for more information on allocating and
freeing memory.)

#include <somd.h>
#include <stack.h>

int main(int argc, char *argv])
{
  Environment ev;
  Stack stk;
  long num = 100;

  SOM_InitEnvironment(&ev);
  SOMD_Init(&ev);

size='auto'.
  /* The StackNewClass invocation is optional and unnecessary
   * in the client program when the class object is created in
   * the SOMInitModule function that is invoked during DLL
   * initialization.
   */
  StackNewClass (Stack_MajorVersion, Stack_MinorVersion);
  stk = _somdNewObject(SOMD_ObjectMgr, &ev, "Stack", "");

  /* Verify successful object creation */
  if ( OperationOK(&ev) )
  {
     while ( !_full(stk, &ev) )
     {
        _push(stk, &ev, num);
        somPrintf("Top: %d\n", _top(stk, &ev));
        num += 100;
     }

     /* Test stack overflow exception */
     _push(stk, &ev, num);
     OperationOK(&ev);

     while ( !_empty(stk, &ev) )
     {
        somPrintf("Pop: %d\n", _pop(stk, &ev));
     }

     /* Test stack underflow exception */
     somPrintf("Top Underflow: %d\n", _top(stk, &ev));
     OperationOK(&ev);
     somPrintf("Pop Underflow: %d\n", _pop(stk, &ev));
     OperationOK(&ev);

     _push(stk, &ev, -10000);
     somPrintf("Top: %d\n", _top(stk, &ev));
     somPrintf("Pop: %d\n", _top(stk, &ev));

     _somdDestroyObject(SOMD_ObjectMgr, &ev, stk);

     if ( OperationOK(&ev) )
     {
        somPrintf("Stack test successfully completed.\n");
     }
  }
  SOMD_Uninit(&ev);
  SOM_UninitEnvironment(&ev);

  return(0);
}

size='auto'.
boolean OperationOK(Environment *ev)
{
   char *exID;

   switch (ev->_major)
   {
     case SYSTEM_EXCEPTION:
       exID = somExceptionId(ev);
       somPrintf("System exception: %s\n", exID);
       somdExceptionFree(ev);
       return (FALSE);

     case USER_EXCEPTION:
       exID = somExceptionId(ev);
       somPrintf("User exception: %s\n", exID);
       somdExceptionFree(ev);
       return (FALSE);

     case NO_EXCEPTION:
       return (TRUE);

     default:
       somPrintf("Invalid exception type in Environment.\n");
       somdExceptionFree(ev);
       return (FALSE);
   }
}

Let's step through the differences.

First, every DSOM program must include the file <somd.h> (when using C ++,
<somd.xh>). This file defines constants, global variables, and run-time
interfaces used by DSOM. Usually, this file is sufficient to establish all
necessary DSOM definitions.

Next, DSOM requires its own initialization call.

        SOMD_Init(&ev);

The call to SOMD_Init initializes the DSOM run-time environment SOMD_Init must
be called before any DSOM run-time calls are made. A side effect of calling
SOMD_Init is that a run-time object, called the DSOM Object Manager, is
created, and a pointer to it is stored in the global variable, SOMD_ObjectMgr,
for programming convenience. The DSOM Object Manager provides basic run-time
support for clients to find, create, destroy, and identify objects. The Object
Manager is discussed in detail in the section entitled "Basic Client
Programming."

Next, the local stack creation statement,

        stk = StackNew();
was replaced by


        stk = _somdNewObject(SOMD_ObjectMgr, &ev, "Stack", "");

The call to somdNewObject asks the DSOM Object Manager (SOMD_ObjectMgr) to
create a "Stack" object, wherever it can find an implementation of "Stack".
(There are other methods with which one can request specific servers.) If no
object could be created, NULL is returned, and an exception is raised.
Otherwise, the object returned is a "Stack" proxy.

A proxy is an object that is a local representative for a remote target object.
A proxy inherits the target object's interface, so it responds to the same
methods. Operations invoked on the proxy are not executed locally, but are
forwarded to the "real" target object for execution. The client program always
has a proxy for each remote target object on which it operates.

From this point on, the client program treats the "Stack" proxy exactly as it
would treat a local "Stack". The "Stack" proxy takes responsibility for
forwarding requests to, and yielding results from, the remote "Stack". For
example,

        _push(stk,&ev,num);

causes a message representing the method call to be sent to the server process
containing the remote object. The DSOM run-time in the server process decodes
the message and invokes the method on the target object. The result (in this
case, just an indication of completion) is then returned to the client process
in a message. The DSOM run time in the client process decodes the result
message and returns any result data to the caller.

At the end of the original client program, the local "Stack" was destroyed by
the statement,

        _somFree(stk);

whereas, in the client program above, the "Stack" proxy and the remote "Stack"
are destroyed by the statement,

        _somdDestroyObject(SOMD_ObjectMgr, &ev, stk);

If the client only wants to release its use of the remote object (freeing the
proxy) without destroying the remote object, it can call the somdReleaseObject
method instead of somdDestroyObject.

Finally, the client must shut down DSOM, so that any operating system resources
acquired by DSOM for communications or process management can be returned:

        SOMD_Uninit(&ev);
This call must be made at the end of every DSOM program.


ΓòÉΓòÉΓòÉ 7.12.4.1. Using specific servers ΓòÉΓòÉΓòÉ

In DSOM, the process that manages a target object is called the object's
server. Servers are implemented as programs that use SOM classes.  Server
implementations are registered with DSOM in an Implementation Repository. The
Implementation Repository is a database queried by clients in order to find
desired servers, and queried by DSOM in order to activate those servers upon
demand.

The example above placed no constraints on the DSOM Object Manager as to where
the remote "Stack" object should be created. The somdNewObject call creates a
remote object of a specified class in an arbitrary server that implements that
class. However, the DSOM Object Manager provides methods for finding specific
servers.

For example, the client program above can be modified slightly to find a
specific server named "StackServer", which has already been registered in
DSOM's Implementation Repository.  (Note that the programmer knew or discovered
that the "StackServer" server implementation supports the "Stack" class.)  The
highlighted lines below show the changes that were made:

#include <somd.h>
#include <stack.h>

int main(int argc, char *argv[]) {
        Stack stk;
        Environment e;
        SOMDServer server;

        SOM_InitEnvironment(&e);
        SOMD_Init(&e);

        server =
          _somdFindServerByName(SOMD_ObjectMgr, &e, "StackServer");
        stk = _somdCreateObj(server, &e, "Stack", "");

        _push(stk,&e,100);
        _push(stk,&e,200);
        _pop(stk,&e);
        if (!_empty(stk,&e)) somPrintf("Top: %d\n", _top(stk,&e));

        _somdDeleteObj(server, &e, stk);
        _somdReleaseObject(SOMD_ObjectMgr, &e, stk);
        _somdReleaseObject(SOMD_ObjectMgr, &e, server);
        SOMD_Uninit(&e);
        SOM_UninitEnvironment(&e);

        return(0);
}

This version of the program replaces the somdNewObject operation with calls to
somdFindServerByName and somdCreateObj. The somdFindServerByName method
consults the Implementation Repository to find the DSOM server implementation
whose name is "StackServer", and creates a server proxy, which provides a
connection to that server. Every DSOM server process has a server object that
defines methods to assist in the creation and management of objects in that
server. Server objects must be instances of SOMDServer or one of its
subclasses. The somdFindServerByName returns a proxy to the SOMDServer object
in the named server.

Once the client has the server proxy, it can create and destroy objects in that
server. The somdCreateObj call creates an object of the class "Stack" in the
server named "StackServer".

To free the remote "Stack" object, the example shows a somdDeleteObj request on
the stack object's server. Next, somdReleaseObject requests are made on the
DSOM Object Manager, to free the stack proxy and the server proxy in the
client. (Note that these three calls are equivalent to the somdDestroyObject
call in the previous example.)


ΓòÉΓòÉΓòÉ 7.12.5. A note on finding existing objects ΓòÉΓòÉΓòÉ

The two examples above show how clients can create a remote, transient object
for their exclusive use. Clients may want to find and use objects that already
exist. In that case, the calls to somdNewObject or somdCreateObj would be
replaced with other "lookup" calls on some directory object that would take an
object name or identifier and return a proxy to the remote object.

The basic mechanisms that DSOM provides for naming and locating objects will be
discussed in section  "Basic Client Programming."


ΓòÉΓòÉΓòÉ 7.12.6. "Stack" server implementation ΓòÉΓòÉΓòÉ

A server consists of three parts. First, a "main" program, when run, provides
an address space for the objects it manages, and one or more process "threads"
that can execute method calls. (Windows 3.x and AIX currently do not have
multi-thread support, while Windows 95, Windows NT, OS/2, and AIX 4.1 do.)
Second, a server object, derived from the SOMDServer class, provides methods
used to manage objects in the server process. Third, one or more class
libraries provide object implementations. Usually these libraries are
constructed as dynamically linked libraries (DLLs), so they can be loaded and
linked by a server program dynamically.

In this simple example, we can use the default DSOM server program, which is
already compiled and linked. The default server behaves as a simple server, in
that it simply receives and executes requests continuously. The default server
creates its server object from the class, SOMDServer. The default server will
load any class libraries it needs upon demand.

The "Stack" class library, "stack.dll", can be used without modification in the
distributed application. This is possible because the "Stack" class is "well
formed"; in other words, there are no methods that implicitly assume the client
and the object are in the same address space.

Thus, by using the default server and the existing class library, a simple
"Stack" server can be provided without any additional programming!

An application may require more functionality in the server program or the
server object than the default implementations provide. A discussion on how to
implement server programs and server objects is found later in this chapter, in
section 6.4, "Basic Server Programming".


ΓòÉΓòÉΓòÉ 7.12.7. Compiling the application ΓòÉΓòÉΓòÉ

DSOM programs and class libraries are compiled and linked like any other SOM
program or library. The header file "somd.h" (or for C++, "somd.xh") should be
included in any source program that uses DSOM services. DSOM run-time calls can
be resolved by linking with the SOMobjects Toolkit library: "somtk.lib" (The
DSOM DLL(s)  - "somd.dll", will be loaded at run time.) For more information,
see "Compiling and linking" in Chapter 3, "Using SOM classes in Client
Programs," and the same topic in Chapter 5, "Implementing Classes in SOM."


ΓòÉΓòÉΓòÉ 7.12.8. Installing the implementation ΓòÉΓòÉΓòÉ

Before the application can be run, certain environment variables must be set
and the stack class and server implementations must be registered in the SOM
Interface Repository and DSOM Implementation Repository.


ΓòÉΓòÉΓòÉ 7.12.8.1. Setting environment variables ΓòÉΓòÉΓòÉ

Several environment variables are used by SOM and DSOM. These variables need to
be set before registering the DSOM application in the Interface and
Implementation Repositories.

For this example, the following environment variables could be set as shown. A
full description of the environment variables and how to set them is given in
section 6.6, "Configuring DSOM."


set USER=pat
set HOSTNAME=localhost
set SOMDDIR=c:\somddir
rem *** The following variables are usually set somenv.bat
rem ***
assuming "c:\som"
rem *** is the value of %SOMBASE% supplied by the user.
set SOMIR=c:\som\etc\som.ir:som.ir
set PATH=.;c:\som\lib;<previous PATH>

USER identifies the user of a DSOM client application. DSOM sends the USER ID
with every remote method call, in case the remote object wishes to perform any
access control checking. This is discussed later in the section "Basic Server
Programming."

HOSTNAME is always set to "localhost" for Workstation SOM.

SOMIR gives a list of files that together constitute the Interface Repository.
The IR is used by DSOM to guide the construction and interpretation of request
messages. For DSOM, it is preferable to use full pathnames in the list of IR
files, since the IR will be shared by several programs that may not all reside
in the same directory.

SOMDDIR gives the name of a directory used to store DSOM configuration files,
including the Implementation Repository.

PATH gives a list of directories where EXEs and DLLs can be found.


ΓòÉΓòÉΓòÉ 7.12.8.2. Registering the class in the Interface Repository ΓòÉΓòÉΓòÉ

Before an object can be accessed remotely by DSOM, it is necessary to register
the class's interface and implementation in the Interface Repository (IR). DSOM
uses the interface information when transforming local method calls on proxies
into request messages transmitted to remote objects.

DSOM servers also consult the IR to find the name of the DLL for a dynamically
loaded class. The DLL name for the "Stack" class must be specified using the
dllname="stack.dll" modifier in the implementation statement of the "Stack"
IDL. The Interface Repository is described in detail in Chapter 7, "The
Interface Repository Framework."

The IDL specification of "Stack" is compiled into the Interface Repository
using the following command:

        sc -u -sir stack.idl

When a class has not been compiled into the Interface Repository, DSOM will
generate a run-time error when an attempt is made to invoke a method from that
class. The error indicates that the method's descriptor was not found in the
IR.


ΓòÉΓòÉΓòÉ 7.12.8.3. Registering the server in the Implementation Repository ΓòÉΓòÉΓòÉ

It is necessary to register a description of a server's implementation in the
Implementation Repository. DSOM uses this information to assist clients in
finding servers, and in activating server processes upon demand.

For this example, where the default server is used, we need only to identify
the server's name, and the class that the server implements. This is
accomplished using the regimpl utility discussed in section 6.6, "Configuring
DSOM Applications". The following commands define a default server, named
"StackServer", which supports the Stack class:

        regimpl -A -i StackServer
        regimpl -a -i StackServer -c Stack


ΓòÉΓòÉΓòÉ 7.12.9. Running the application ΓòÉΓòÉΓòÉ

This section discusses:

     Starting the DSOM daemon
     Running the client

 Starting the DSOM daemon

 Before running a DSOM application, the DSOM daemon, somdd, must be started.

     The daemon can be started manually from the command line, or it could be
      started automatically from a start-up script run at boot time. It may be
      run in the background with the commands start somdd (The somdd program
      requires no parameters. An optional -q parameter can be used to set
      "quiet" mode, to suppress messages.)

      The somdd daemon is responsible for establishing a "binding" (that is, a
      connection) between a client process and a server.  It will activate the
      desired server automatically, if necessary.

 Running the client

 Once the DSOM daemon is running, the application may be started. This is
 accomplished by running the client program. If the StackServer is not running,
 it will be started automatically by the DSOM daemon when the client attempts
 to invoke a method on one of its objects. After the client program ends, the
 server will continue to run, accepting connections from new clients. To
 terminate the server, give its console window focus and press ctrl-c.


ΓòÉΓòÉΓòÉ 7.12.10. "Stack" example run-time scenario ΓòÉΓòÉΓòÉ

The following scenario steps through the actions taken by the DSOM run time in
response to each line of code in the second "Stack" client program presented
above. The illustration following the scenario shows the processes, and the
objects within them, that participate in these actions.

     Initialize an environment for error passing:

            SOM_InitEnvironment(&e);

     Initialize DSOM:

            SOMD_Init(&e);

 This causes the creation of the DSOM Object Manager (with SOMDObjectMgr
 interface). The global variable SOMD_ObjectMgr points to this object.

     Initialize "Stack" class object:

            StackNewClass(Stack_MajorVersion, Stack_MinorVersion);

     Find the "StackServer" implementation and assign its proxy to the
      variable server:

            server = _somdFindServerByName(SOMD_ObjectMgr, &e, "StackServer");

 This causes the creation of the server proxy object in the client process.
 Proxy objects are shown as shaded circles. Note that the "real" server object
 in the server process is not created at this time. In fact, the server process
 has not yet been started.

     Ask the server object to create a "Stack" and assign "Stack" proxy to
      variable stack.

            stk = _somdCreateObj(server, &e, "Stack", "");

 This causes somdd, the DSOM daemon (which is already running) to activate the
 stack server process (by starting the "generic" server program). The stack
 server process, upon activation, creates the "real" SOMDServer object in the
 server process. The SOMDServer object works with the DSOM run time to create a
 local "Stack" object and return a "Stack" proxy to the client. (The details of
 this procedure are deferred until section 6.4, "Basic Server Programming".)

     Ask the "Stack" proxy to push 100 onto the remote stack:

            _push(stk,&e,100);

 This causes a message representing the method call to be marshalled and sent
 to the server process. In the server process, DSOM demarshals the message and,
 with the help of the SOMDServer, locates the target "Stack" object upon which
 it invokes the method ("push"). The result (which is void in this case) is
 then passed back to the client process in a message.

     Invoke more "Stack" operations on the remote stack, via the proxy:

            _push(stk,&e,200);

            _pop(stk,&e);

            if (!_empty(stk,&e)) t = _top(stk,&e);

     Explicitly destroy both the remote stack, the stack proxy, and the server
      proxy:

            _somdDeleteObj(server, &e, stk);

            _somdReleaseObject(SOMD_ObjectMgr, &e, stk);

            _somdReleaseObject(SOMD_ObjectMgr, &e, server);

     Free the error-passing environment:

            SOM_UninitEnvironment(&e);

 This scenario has introduced the key processes in a DSOM application: client,
 server, and somdd. Also introduced are the key objects that comprise the DSOM
 run-time environment: the SOMD_ObjectMgr in the client process and the
 SOMD_ServerObject in the server process.


ΓòÉΓòÉΓòÉ 7.12.11. Summary ΓòÉΓòÉΓòÉ

This example has introduced the key concepts of building, installing, and
running a DSOM application. It has also introduced some of the key components
that comprise the DSOM application run-time environment.

The following sections, "Basic Client Programming," "Basic Server Programming,"
and "Implementing Classes," provide more detail on how to use, manage, and
implement remote objects, respectively.


ΓòÉΓòÉΓòÉ 7.13. Basic Client Programming ΓòÉΓòÉΓòÉ

For the most part, client programming in DSOM is exactly the same as client
programming in SOM, since DSOM transparently hides the fact that an object is
remote when the client accesses the object.

However, a client application writer also needs to know how to create, locate,
use, save, and destroy remote objects. (This is not done using the usual SOM
bindings.) The DSOM run-time environment provides these services to client
programs primarily through the DSOM Object Manager. These run-time services
will be detailed in this section. Examples of how an application developer uses
these services are provided throughout the section.


ΓòÉΓòÉΓòÉ 7.13.1. DSOM Object Manager ΓòÉΓòÉΓòÉ

DSOM defines a DSOM Object Manager, which provides services needed by clients
to create, find and use objects in the DSOM run-time environment.

The DSOM Object Manager is derived from an abstract, generic "object manager"
class, called ObjectMgr. This abstract ObjectMgr class defines a basic set of
methods that support object creation, location (with implicit activation), and
destruction.

As an abstract class, ObjectMgr defines only an interface; there is no
implementation associated with ObjectMgr. Consequently, an application should
not create instances of the ObjectMgr class.

 An abstract Object Manager class was defined under the expectation that
applications will often need simultaneous access to objects implemented and
controlled by a variety of object systems. Such object systems may include
other ORBs (in addition to DSOM), object-oriented databases, and so forth. It
is likely that each object system will provide the same sort of basic services
for object creation, location, and activation, but each using a different
interface.

Thus, the ObjectMgr abstract class defines a simple and "universal" interface
that can be mapped to any object system. The application would only have to
understand a single, common ObjectMgr interface. Under this scheme, specific
object managers are defined by subclassing the ObjectMgr class and overriding
the ObjectMgr methods to map them into the object system-specific programming
interfaces.

DSOM's Object Manager, SOMDObjectMgr, is defined as a specific class of
ObjectMgr. It defines methods for:

     Finding servers that implement particular kinds of objects

     Creating objects in servers

     Obtaining object identifiers (string IDs)

     Finding objects, given their identifiers

     Releasing and destroying objects

 These functions will be discussed in the remainder of this section.

 Note:  The OMG has standardized an "object lifecycle" service, which includes
        support for creating and destroying distributed objects.  The DSOM
        Object Manager may be augmented in the future with an OMG-compliant
        lifecycle service.


ΓòÉΓòÉΓòÉ 7.13.2. Initializing a client program ΓòÉΓòÉΓòÉ

A client application must declare and initialize the DSOM run time before
attempting to create or access a remote object. The SOMD_Init procedure
initializes all of the DSOM run time, including the  SOMDObjectMgr object. The
global variable, SOMD_ObjectMgr is initialized to point to the local DSOM
Object Manager.

A client application must also initialize all application classes used by the
program. For each class, the corresponding <className>NewClass call should be
made.

Note:  In non-distributed SOM programs, the <className>New macro (and the new
       operator provided for each class by the SOM C++ bindings) implicitly
       calls the procedure <className>NewClass when creating a new object. This
       is not currently possible in DSOM because, when creating remote objects,
       DSOM uses a generic method that is not class-specific.

 This was shown in the "Stack" example in section 6.2. In a similar example of
 an application that uses "Car" and "Driver" objects, the initialization code
 might look like this:

  #include <somd.h>    /* needed by all clients */
  #include <Car.h>     /* needed to access remote Car */
  #include <Driver.h>  /* needed to access remote Driver */

  main()
  {
      Environment ev; /* ev used for error passing */
      SOM_InitEnvironment(&ev);

      /* Do DSOM initialization */
      SOMD_Init(&ev);

      /* Initialize application classes */
      CarNewClass(Car_MajorVersion, Car_MinorVersion);
      DriverNewClass(Driver_MajorVersion, Driver_MinorVersion);
      ...
  }

 As shown, client programs should include the "somd.h" file (or, for C++
 programs, the "somd.xh" file) in order to define the DSOM runtime interfaces.

 Note also that, since Environments are used for passing error results between
 a method and its caller, an Environment variable (ev) must be declared and
 initialized for this purpose.

 The calls to "CarNewClass" and "DriverNewClass" are required if the client
 will be creating or accessing Cars and Drivers. The procedures "CarNewClass"
 and "DriverNewClass" create class objects for the classes "Car" and "Driver".
 When a DSOM Object Manager method like somdNewObject is invoked to create a
 "Car", it expects the "Car" class object to exist. If the class does not yet
 exist, the "ClassNotFound" exception will be returned.


ΓòÉΓòÉΓòÉ 7.13.3. Exiting a client program ΓòÉΓòÉΓòÉ

At the end of a client program, the SOMD_Uninit procedure must be called to
free DSOM run-time objects, and to release system resources such as semaphores,
shared memory segments, and so on. SOMD_Uninit should be called even if the
client program terminates unsuccessfully; otherwise, system resources will not
be released.

For example, the exit code in the client program might look like this:

    ...
    SOMD_Uninit(&e);
    SOM_UninitEnvironment(&e);
}

Note also the SOM_UninitEnvironment call, which frees any memory associated
with the specified Environment structure.


ΓòÉΓòÉΓòÉ 7.13.4. Creating remote objects ΓòÉΓòÉΓòÉ

Distributed objects can be created in several different ways in DSOM.

     The client can create an object on any server that implements that class
      of object.

     The client can find a specific server upon which to create an object.

     A server can create an object and register a reference to the object in
      some well-known directory. (An object reference contains information that
      reliably identifies a particular object.)

 The first two cases are discussed immediately below. The last case is
 discussed near the end of this section.


ΓòÉΓòÉΓòÉ 7.13.4.1. Creating an object in an arbitrary server ΓòÉΓòÉΓòÉ

Following is an example of how to create a new remote object in the case where
the client does not care in which server the object is created. In this
situation, the client defers these decisions to the DSOM Object Manager
(SOMD_ObjectMgr) by using the somdNewObject method call, which has this IDL
definition:


  // (from file om.idl)


  SOMObject somdNewObject(in Identifier objclass, in string hints);

  // Returns a new object of the named class.  This is a "basic"
  // creation method, where the decisions about where and how to
  // create the object are mostly left up to the Object Manager.
  // However, the Object Manager may optionally define creation
  // "hints" which the client may specify in this call.

Here is the example of how a remote "Car" would be created using somdNewObject:

#include <somd.h>
#include <Car.h>

main()
{
    Environment ev;
    Car car;

    SOM_InitEnvironment(&ev);
    SOMD_Init(&ev);

    /* create the class object */
    CarNewClass(Car_MajorVersion, Car_MinorVersion);

    /* create a Car object on some server, let the
       Object Manager choose which one */
    car = _somdNewObject(SOMD_ObjectMgr, &ev, "Car", "");
    ...
}

The main argument to the somdNewObject method call is a string specifying the
name of the class of the desired object. The last argument is a string that may
contain "hints" for the Object Manager when choosing a server. In this example,
the client is providing no hints. (Currently, the DSOM Object Manager simply
passes the hints to the server object in a somdCreateObj call.)


ΓòÉΓòÉΓòÉ 7.13.4.2. Proxy objects ΓòÉΓòÉΓòÉ

As far as the client program is concerned, when a remote object is created, a
pointer to the object is returned. However, what is actually returned is a
pointer to a proxy object, which is a local representative for the remote
target object.

Proxies are responsible for ensuring that operations invoked on it get
forwarded to the "real" target object that it represents. The DSOM run time
creates proxy objects automatically, wherever an object is returned as a result
of some remote operation. The client program will always have a proxy for each
remote target object on which it operates. Proxies are described further in the
sections entitled "DSOM as a CORBA-compliant Object Request Broker" and
"Advanced Topics".

In the example above, a pointer to a "Car" proxy is returned and put in the
variable "car". Any subsequent methods invoked on "car" will be forwarded and
executed on the corresponding remote "Car" object.

Proxy objects inherit behavior from the SOMDClientProxy class.


ΓòÉΓòÉΓòÉ 7.13.4.3. Servers and server objects ΓòÉΓòÉΓòÉ

In DSOM, the process that manages a target object is called the object's
server. Servers are implemented as programs that use SOM classes. The example
above placed no constraints on the DSOM Object Manager as to which server
should create the remote "Car" object. However, if the client desires more
control over distribution of objects, the DSOM Object Manager provides methods
for finding specific servers.

Server implementations are registered with DSOM in an Implementation
Repository. Server implementations are described by a unique ID, a unique
(user-friendly) name, the program name that implements the server, the classes
that are implemented by the server, the machine on which the server is located,
whether the server is multi-threaded, and so forth. (See section 6.6 for more
information on registering server implementations.) A client can ask the DSOM
Object Manager to find a particular server:

     By name

     By ID

     By a class it supports

 When a client asks for a "server", it is given (a proxy to) a server object
 that provides interfaces for managing the objects in the server. There is one
 server object per server process. All server objects are instances of the
 SOMDServer class, or its subclasses. The default method provided by SOMDServer
 for creating objects is:

    // (from file somdserv.idl)

    SOMObject somdCreateObj(in Identifier objclass, in string hints);

    // Creates an object of the specified class.  This method
    // may optionally define creation "hints" which the client
    // may specify in this call.  (Hints are ignored by default.)

 Section 6.4 explains how to create application-specific server objects,
 derived from SOMDServer, which override SOMDServer methods and introduce their
 own methods for object management.


ΓòÉΓòÉΓòÉ 7.13.4.4. Creating an object in a specific server ΓòÉΓòÉΓòÉ

The following example demonstrates how a client application creates a new
object in a remote server chosen by the client. The DSOM Object Manager method
somdFindServerByName is used to find and create a proxy to the server object
for the server implementation named "myCarServer". The method somdCreateObj is
then invoked on the server object to create the remote "Car". A proxy to the
remote "Car" is returned. (The "Stack" client presented in the previous section
used the same methods to create a remote "Stack".)

 /* find a specific Car server */
    server =
      _somdFindServerByName(SOMD_ObjectMgr, &ev, "myCarServer");

 /* create a remote Car object on that server */
    car = _somdCreateObj(server, &ev, "Car", "");
    ...
}

Note:  If the specified server does not provide any implementation of the
       desired class, a NULL pointer will be returned and a "ClassNotFound"
       exception will be raised.

 Three other methods can be invoked on the DSOM Object Manager to find server
 implementations: somdFindServer, somdFindServersByClass, and
 somdFindAnyServerByClass. The IDL declarations of these methods follow:


  SOMDServer somdFindServer(in ImplId serverid);

  sequence<SOMDServer> somdFindServersByClass(in Identifier objclass);

  SOMDServer somdFindAnyServerByClass(in Identifier objclass);

 The somdFindServer method is similar to the somdFindServerByName method,
 except that the server's implementation ID (of type ImplId) is used to
 identify the server instead of the server's user-friendly name (or "alias").
 The implementation ID is a unique string generated by the Implementation
 Repository during server registration. (See section 6.6 for more details.)

 The somdFindServersByClass method, given a class name, returns a sequence of
 all servers that support the given class. The client program may then choose
 which server to use, based on the server's name, program, or other
 implementation attributes (e.g., the server is multi-threaded). (See the topic
 below, "Inquiring about a remote object's implementation.")

 Finally, the somdFindAnyServerByClass method simply selects any one of the
 server implementations registered in the Implementation Repository that
 supports the given class, and returns a server proxy for that server.

 Once the server proxy is obtained, methods like somdCreateObj, shown in the
 example above, can be invoked upon it to create new objects.


ΓòÉΓòÉΓòÉ 7.13.4.5. Inquiring about a remote object's implementation ΓòÉΓòÉΓòÉ

A client may wish to inquire about the (server) implementation of a remote
object. All objects in a server, including the "server object", share the same
implementation definition. This is common when using the somdFindServersByClass
call, where a sequence of server proxies is returned, and some choice must be
made about which to use.

When a proxy is obtained by a client, the client can inquire about the
underlying server implementation by obtaining its corresponding
ImplementationDef. An ImplementationDef object  contains a set of attributes
that describe a server implementation. To get the ImplementationDef associated
with a remote object, the get_implementation method (implemented on SOMDObject
and inherited by SOMDClientProxy) can be called.

For example, if a program has a proxy for a remote server object, it can get
the ImplementationDef for the server with method calls similar to the
following:


    ImplementationDef implDef;
    SOMDServer server;

    ...
    implDef = _get_implementation(server, &ev);

Once the ImplementationDef has been obtained, the application can access its
attributes using the _get_impl_xxx methods.

The ImplementationDef class is discussed further in section 6.6, "Configuring
DSOM."


ΓòÉΓòÉΓòÉ 7.13.5. Destroying remote objects ΓòÉΓòÉΓòÉ

There are several ways of destroying objects or their proxies in DSOM, just as
there are several ways to create objects. Remote objects can be asked to
destroy themselves, or, the SOMDObjectMgr and the SOMDServer can participate in
the deletion.


ΓòÉΓòÉΓòÉ 7.13.5.1. Destroying objects via a proxy ΓòÉΓòÉΓòÉ

DSOM provides means for deleting remote objects via their proxies. For example,
if somFree is invoked on a proxy, the somFree call gets forwarded directly to
the target object, just like any other target method call. For example,


_somFree(car);

frees the remote car.  Note that, by default, invoking somFree on the proxy
does not free the proxy, only the remote object.  However, the following call
can be issued as part of a client-program initialization, so that invoking
somFree on a proxy frees both the remote object and the proxy:

   __set_somd21somFree(SOMD_ObjectMgr, ev, TRUE);

All subsequent invocations of somFree on a proxy object will result in both the
remote object and the proxy being freed.

To be explicit about whether the proxy or the remote object is being deleted,
the methods somdTargetFree and somdProxyFree, defined on proxies, can be used:

_somdTargetFree(car, &ev);
frees the remote "Car" (but not the proxy) and

_somdProxyFree(car, &ev);

frees the proxy (but not the remote "Car").

Note:  CORBA specifies a third method for deleting object references. (Proxies
       are a specialized type of object reference.) The method


                      _release(car, &ev);


               deletes the proxy (but not the target object).


ΓòÉΓòÉΓòÉ 7.13.5.2. Destroying objects via the DSOM Object Manager ΓòÉΓòÉΓòÉ

Having created a remote object with somdNewObject or somdCreateObj, the remote
object and its local proxy may be destroyed by invoking the method
somdDestroyObject on the DSOM Object Manager using the proxy as an argument.
For example,


/* create the car */
car = _somdNewObject(SOMD_ObjectMgr, &ev, "Car", "");
...
/* destroy the car (and its proxy) */
_somdDestroyObject(SOMD_ObjectMgr, &ev, car);

If the client does not want to destroy the remote object, but is finished
working with it, the somdReleaseObject method should be used instead, e.g.,

_somdReleaseObject(SOMD_ObjectMgr, &ev, car);

This deletes the local proxy, but not the remote object.

Both somdDestroyObject and somdReleaseObject are defined on the ObjectMgr, so
that the Object Manager is aware of the client's actions, in case it wants to
do any bookkeeping.

The object passed to either the somdDestroyObject method or the
somdReleaseObject method can be either a local SOM object or a DSOM proxy
object.  When a local SOM object is passed, somdDestroyObject has the same
behavior as somFree. If a local SOM object is passed to somdReleaseObject,
however, this has no effect.


ΓòÉΓòÉΓòÉ 7.13.5.3. Destroying objects via a server object ΓòÉΓòÉΓòÉ

The somdDestroyObject method described above sends a request to delete a remote
object to the object's server. It does so to ensure that the server has an
opportunity to participate in, if not perform, the deletion. The method defined
on the SOMDServer class for destroying objects is somdDeleteObj. If the client
has a proxy for the server object, it can also invoke somdDeleteObj directly,
instead of calling somdDestroyObject.

Destroying objects via the server object, rather than asking the object itself
(as in somFree or somdTargetFree), allows the server object do any clean-up
that is needed. For simple applications, this may not be necessary, but for
applications that provide their own application-tailored server objects, it may
be critical.


ΓòÉΓòÉΓòÉ 7.13.6. Creating remote objects using user-defined metaclasses ΓòÉΓòÉΓòÉ

An application may wish to define its own constructor methods for a particular
class, via a user-supplied metaclass. In this case, the somdNewObject method
should not be used, since it simply calls the default constructor method,
somNew, defined by SOMClass.

Instead, the application can obtain a proxy to the actual class object in the
server process. It can do so via the somdGetClassObj method, invoked on the
SOMDServer proxy returned by one of the somdFindServerXxx methods. The
application-defined constructor method can then be invoked on the proxy for the
remote class object.

Note:  The same issues apply to destructor methods. If the application defines
       its own destructor methods, they can be called via the class object
       returned by somdGetClassObj, as opposed to calling somdDestroyObject.

 The following example creates a new object in a remote server using an
 application-defined constructor method, "makeCar", which is assumed to have
 been defined in the metaclass of "Car", named "MetaCar".

  #include <somd.h>
  #include <Car.h>
  main( )
  {
      Environment ev;
      SOMDServer server;
      Car car;
      MetaCar carClass;

      SOM_InitEnvironment(&ev);
      SOMD_Init(&ev);

      /* find a Car server */
      server = _somdFindAnyServerByClass(SOMD_ObjectMgr, &ev, "Car");

      /* get the class object for Car */
      carClass = (MetaCar) _somdGetClassObj(server, &ev, "Car");

      /* create the car object */
      car = _makeCar(carClass, &ev, "Red", "Toyota", "2-door");

      ...
  }


ΓòÉΓòÉΓòÉ 7.13.7. Saving and restoring references to objects ΓòÉΓòÉΓòÉ

A proxy is a kind of "object reference". An object reference contains
information that is used to identify a target object.

To enable clients to save references to remote objects (in a file system, for
example) or exchange references to remote objects (with other application
processes), DSOM must be able to externalize proxies. To "externalize a proxy"
means to create a string ID for a proxy that can be used by any process to
identify the remote target object. DSOM must also support the translation of
string IDs back into proxies.

The DSOM Object Manager defines two methods for converting between proxies and
their string IDs: somdGetIdFromObject and somdGetObjectFromId.

Here is an example client program that creates a remote "Car" object. It
generates a string ID corresponding to the proxy, and saves the string ID to a
file for later use.

#include <stdio.h>
#include <somd.h>
#include <Car.h>
main( )
{
    Environment ev;
    Car car;
    string somdObjectId;
    FILE* file;

    SOM_InitEnvironment(&ev);
    SOMD_Init(&ev);

    /* create a remote Car object */
    car = _somdNewObject(SOMD_ObjectMgr, &ev, "Car", "");

    /* save the reference to the object */
    somdObjectId = _somdGetIdFromObject(SOMD_ObjectMgr, &ev, car);
    file = fopen("/u/joe/mycar", "w");
    fprintf(file, "%s", somdObjectId);
...

Next is an example client program that retrieves the string ID and regenerates
a valid proxy for the original remote "Car" object (assuming the remote "Car"
object can still be found in the server).

...
    Environment ev;
    Car car;
    char buffer[256];
    string somdObjectId;
    FILE* file;

...
    /* restore proxy from its string form */
    file = fopen("/u/joe/mycar", "r");
    somdObjectId = (string) buffer;
    fscanf(file, "%s", somdObjectId);
    car = _somdGetObjectFromId(SOMD_ObjectMgr, &ev, somdObjectId);
...

Once the proxy has been regenerated, methods can be invoked on the proxy and
they will be forwarded to the remote target object, as always.

Note:  The somdGetIdFromObject and somdGetObjectFromId methods directly
       correspond to the CORBA methods ORB_object_to_string and
       ORB_string_to_object, defined on the ORB class.


ΓòÉΓòÉΓòÉ 7.13.7.1. Finding existing objects ΓòÉΓòÉΓòÉ

The SOMDObjectMgr and SOMDServer classes support the methods described above,
which allow clients to create objects in servers. However, it is also likely
that clients will want to find and use objects that have already been created,
usually by the servers that implement them. For example, a print service will
create printer objects, and must then export them to clients. In that case, the
calls to somdNewObject or somdCreateObj would be replaced with other "lookup"
calls on some directory (server) object which would take an object name or
identifier and return a proxy to a corresponding remote object. Likewise, the
server that owns the object would register the exported object in the
directory.

It is important to understand that DSOM does not provide a directory service
such as the one described. But such a directory object could be implemented by
the application, where a table or collection object maps object names to
proxies. The string IDs for the proxies in the directory object could be saved
using a file. A directory server implemented using DSOM could be used to share
the directory among processes.

Upon a lookup call, the directory server could find the corresponding proxy (or
its string ID) in the directory, and return it to the caller.


ΓòÉΓòÉΓòÉ 7.13.8. Finding server objects ΓòÉΓòÉΓòÉ

The DSOM Object Manager can be used to find server object proxies using the
somdFindServerXxx methods. However, it is important to point out that an
application can also augment those services, by managing server proxies itself.
Server proxies can be maintained in an application-specific directory, stored
in a file, or passed from process to process, just as any other proxies.


ΓòÉΓòÉΓòÉ 7.13.9. Invoking methods on remote objects ΓòÉΓòÉΓòÉ

As described earlier, DSOM proxies are local representatives of remote objects,
and as such, they can be treated like the target objects themselves. Method
calls are invoked in exactly the same manner as if the object is local. This is
true both for method calls using the static bindings (as most of our examples
have shown), as well as for dynamic dispatching calls, where SOM facilities
(such as the somDispatch method) are used to construct method calls at run
time.

CORBA 1.1 also defines a dynamic invocation interface that is implemented by
DSOM. It is described later in section 6.9, "Advanced Topics".

The DSOM run time is responsible for transporting any input method argument
values supplied by the caller (defined by legal IDL types) to the target object
in a remote call. Likewise, the DSOM run time transports the return value and
any output argument values back to the caller following the method call.

Note:  DSOM uses the Interface Repository (IR) to discover the "signature" of a
       method (that is, the method's prototype). It is important that the
       contents of the IR match the method bindings used by the application
       program (i.e. the same IDL file is used to update the IR and to generate
       bindings).

 DSOM can make remote invocations only of methods whose parameter types are
 among the following IDL types:  basic types (short, long, unsigned short,
 unsigned long, float, double, char, boolean, octet), enum, struct, union,
 sequence, string, array, any, and object. The members of a struct, union,
 sequence, or array and the value of an any, must also be from the above list
 of supported DSOM types.

 In addition to the preceding types, DSOM also supports method parameters of
 type pointer to one of the above types (for example, long*) Pointers to
 pointers are not supported, however, and pointers embedded within one of the
 above types (for example, a pointer within a struct) are not supported The
 "void *" type is also not supported. Currently, DSOM has the limitation that
 NULL pointer values cannot be returned as inout or out method arguments
 although it is expected that this limitation will be addressed in a future
 release.

 Types declared as SOMFOREIGN types are not currently supported by DSOM.
 Because the SOM somId is declared as a SOMFOREIGN type, this implies that any
 method having a parameter of type somId cannot be invoked remotely using DSOM.
 This restriction includes the  SOM methods: somRespondsTo, somSupportsMethod,
 somGetMethodDescriptor, somGetMethodIndex, and somGetNthMethodInfo.

 When a method parameter is an object type (that is, an instance of SOMObject
 or some class derived from SOMObject), a client program making a remote
 invocation of that method must pass an object reference for that parameter,
 rather than passing a local SOMObject, unless the client program is also DSOM
 server program, in which case DSOM will automatically convert the local object
 into an object reference.

 Methods having the procedure SOM IDL modifier cannot be invoked remotely using
 DSOM.  This is because these "methods" are called directly, rather than via
 the normal method resolution mechanisms on which DSOM relies.


ΓòÉΓòÉΓòÉ 7.13.9.1. Determining memory allocation and ownership ΓòÉΓòÉΓòÉ

When a method is invoked that returns a result of type string, sequence, or
array, DSOM will allocate memory in the client's address space for the result.
Ownership of this memory becomes the responsibility of the client program. When
the client program has finished using it, the client should free the memory
using the ORBfree function, rather than using free or SOMFree (This is because
the memory has been allocated by DSOM using special memory management
techniques; therefore, the client should ask DSOM to also free the memory.)

When invoking a method using DSOM, the client program is  responsible for
providing storage for all in arguments and for all inout/out arguments, with
the following exceptions: DSOM will allocate storage for a string or for the
_buffer field of a sequence when used as an out argument, and will allocate
storage for the_value field of an any when used as an inout or out argument.
This storage becomes the responsibility of the client program and should later
be freed using ORBfree. For a string or sequence used as an inout argument, the
out result is constrained to be no larger than the size of the in argument
allocated by the client.


ΓòÉΓòÉΓòÉ 7.13.10. Passing object references in method calls ΓòÉΓòÉΓòÉ

When pointers to objects are returned as method output values (as in the
previous examples), DSOM automatically converts the object pointers (in the
server) to object proxies in the client.

Likewise, when a client passes object (proxy) pointers as input arguments to a
method, DSOM automatically converts the proxy argument in the client to an
appropriate object reference in the server.

Note:  If the proxy is for an object that is in the same server as the target
       object, DSOM gives the object reference to the server object for
       resolution to a SOM object pointer. Otherwise, DSOM leaves the proxy
       alone, since the proxy must refer to an object in some process other
       than the target's server.


ΓòÉΓòÉΓòÉ 7.13.11. Memory management ΓòÉΓòÉΓòÉ

DSOM programs must manage four different kinds of memory resources: objects,
object references, Environment structures, and blocks of memory. There are
different techniques for allocating and releasing each kind of resource.

Objects and object references

Creating and destroying remote objects was discussed previously in this section
(see "Creating remote objects" and "Destroying remote objects"). Creating and
destroying local objects is described in section 3.2, "Using SOM Classes - the
Basics," in Chapter 3, "Using SOM Classes in Client Programs."  Object
references are typically created automatically by DSOM as needed by the client
program. They are also released in the client program by using either the
release method or the somdProxyFree method. (The two methods are equivalent.)

Environment structures

When a client invokes a method and the method returns an exception in the
Environment structure, it is the client's responsibility to free the exception.
This is done by calling either exception_free or somdExceptionFree on the
Envirnmen structure in which the exception was returned. (The two functions are
equivalent.) A similar function, somExceptionFree, is available for SOM
programmers however DSOM programmers can use somdExceptionFree to free all
exceptions (regardless of whether they were returned from a local or remote
method call).

Blocks of memory

For allocating and releasing blocks of memory within a client program, SOM
provides the SOMMalloc and SOMFree functions (analogous to the C "mallo" and
"free" functions). The "Memory Management" section of Chapter 3 describes these
functions. To release memory allocated by DSOM in response to a remote method
call, however, DSOM client programs should use the ORBfree function

For example, when a method is invoked that returns a result of type string,
sequence, or array, DSOM will allocate memory for the result in the client's
address space. Ownership of this memory becomes the responsibility of the
client program. When finished using this memory, the client program should free
it using the ORBfree function, rather than free or SOMFree. This is because the
memory has been allocated by DSOM using special memory-management techniques;
therefore, the client should ask DSOM to also free the memory. If the storage
is freed using SOMFree rather than ORBfree, then memory leaks will result.

The differences between the SOMFree and ORBfree functions are twofold:

   1. First, SOMFree should only be used to free memory not allocated by
      DSO(for example, memory the client program allocated itself using
      SOMMalloc), while ORBfree should be used to free memory allocated by DSOM
      in response to a remote method call.

   2. Second, SOMFee only frees a single block of memory (in the same way that
      the C "free" function does), while ORBfree will free an entire data
      structure, including any allocated blocks of memory within in. For
      example, if a remote method call returns a sequence of structs, and each
      struct contains a string, ORBfree will free, with a single call, not only
      the sequence's "_buffer" member, but also each struct and all the strings
      within the structs. Freeing a similar data structure using SOMFree would
      require multiple calls (one for each call to SOMMalloc used to build the
      data structure).

 Some programmers may wish to use a single function to free blocks of memory,
 regardless of whether they were allocated locally or by DSOM in response to a
 remote method call. For these programmers, DSOM provides a function,
 SOMD_NoORBfree, which can be called just after calling SOMD_Init in the client
 program. (It requires no arguments and returns no value.) This function
 specifies that the client program will free all memory blocks using SOMFree,
 rather than ORBfree. In response to this call, DSOM will not keep track of the
 memory it allocates for the client. Instead, it assumes that the client
 program will be responsible for walking all data structures returned from
 remote method calls, while calling SOMFree for each block of memory within.


ΓòÉΓòÉΓòÉ 7.13.11.1. Memory management for method parameters ΓòÉΓòÉΓòÉ

For each method, five SOM IDL modifiers are available to specify the method's
memory-management policy  (that is, whether the caller or the object owns the
parameters' memory after the method is invoked). These modifiers are
memory_management, caller_owns_result, caller_owns_parameters,
object_owns_result, and object_owns_parameters. For a complete description of
these modifiers and their meanings, see the section entitled "Implementation
Statements" in Chapter 4, "SOM IDL and the SOM Compiler."

Note that the memory-management policy for a particular parameter applies to
the parameter and all the memory embedded within it (for example, if a struct
is owned by the caller, then so are all the struct's members). Also note that
the "object-owned" memory-management policy, specified by the
object_owns_result and object_owns_parameters modifiers, is not  supported by
DSM for metods invoked using the Dynamic Invocation Interface (DII). (This is
because the "object-owned" policy is not CORBA-compliant, and because it
precludes reusing Request objects to invoke a method multiple times.)


ΓòÉΓòÉΓòÉ 7.13.11.2. The CORBA policy for parameter memory management ΓòÉΓòÉΓòÉ

When a class contains the SOM IDL modifier memory_management = corba, this
signifies that all methods introduced by the class follow the CORBA
specification for parameter memory management, except where a particular method
has an explicit modifier (object_owns_result or object_owns_parameters) that
indicates otherwise. The remainder of this section describes the CORBA
specification for parameter memory management.

Caller frees parameters and return results

The CORBA memory-management policy specifies that the caller of a method is
responsible for freeing all parameters and the return result after the method
call is complete. This applies regardless of whether the parameter was
allocated by the caller or the object (or, in the case of a remote method call,
by DSOM). In other words, the CORBA policy asserts that parameters are
uniformly "caller-owned".

Allocation responsibilities

Whether the parameter or return result should be allocated by the caller or by
the object depends on the type  of the parameter and its mode  ("in", "inout",
"out", or "return").  In general, the caller is responsible for allocating
storage for most parameters and return results. More specifically, CORBA
requires that storage for all "in" arguments, for all "inout" or "out"
arguments, and for all "return" results must be provided by the client program,
with certain exceptions as itemized below.

The object is responsible for allocating storage as follows:

     for strings when used as "out" arguments or as "return" results

     for the "_buffer" field of sequences when used as "out" arguments or as
      "return" results,

     for the "_value" field of anys when used as "inout" or "out" arguments or
      as "return" results,

     for pointer types when used as "inout" or "out" arguments or as "return"
      results,

     for arrays when used as "return" results, and

     for objects when used as "inout" or "out" arguments or as "return"
      results.

 Note:  For "inout" strings and sequences, the "out" result is constrained to
        be no larger than the size of the "in" argument allocated by the
        client.

 Ownership of memory allocated in the above cases becomes the responsibility of
 the client program. For remote method calls, when a remote object allocates
 memory for a parameter or "return" value, DSOM subsequently allocates memory
 in the client's address space for the parameter or result. For a
 parameter/result that is an object (rather than a block of memory) DSOM
 automatically creates an object reference (a proxy object) in the client's
 address space. In each case, the memory or the proxy object becomes the
 responsibility of the client program and should later be freed by the client,
 using ORBfree for blocks of memory or release for proxy objects.


ΓòÉΓòÉΓòÉ 7.13.11.3. The 'somdReleaseResources' method and object-owned parameters ΓòÉΓòÉΓòÉ

As stated earlier, the CORBA policy asserts that method parameters and return
results are uniformly caller-owned. This means the method caller has the
responsibility for freeing memory after invoking a method, regardless of
whether the memory was allocated by the caller or the object.

A class implementor can designate certain method parameters and results as
object-owned, however, by using the object_owns_result and
object_owns_parameters SOM IDL modifiers. These modifiers signify that the
object, rather than the caller, is responsible for freeing the memory
associated with the parameter/result. For "in" parameters, the object can free
the memory any time after receiving it; for "inout" and "out" parameters, and
for return results, the object will free the memory sometime before the object
is destroyed. (See the section entitled "Implementation statements" in Chapter
4, "SOM IDL and the SOM Compiler," for more information on these modifiers.)

When a DSOM client program makes a remote method invocation, via a proxy, and
the method being invoked has an object-owned parameter or return result, then
the client-side memory associated with the parameter/result will be owned by
the caller's proxy, and the server-side memory will be owned by the remote
object. The memory owned by the caller's proxy will be freed when the proxy is
released by the client program. (The time at which the server-side memory will
be freed depends on the implementation of the remote object.)

A DSOM client can also instruct a proxy object to free all memory that it owns
on behalf of the client without releasing the proxy (assuming that the client
program is finished using the object-owned memory), by invoking the
somdReleaseResource method on the proxy object. Calling somdReleaseResources
can prevent unused memory from accumulating in a proxy.

For example, consider a client program repeatedly invoking a remote method
"get_string", which returns a string that is designated (in SOM IDL) as
"object-owned". The proxy on which the method is invoked will store the memory
associated with all the returned strings, even if the strings are not unique,
until the proxy is released. If the client program only uses the last result
returned from "get_string", then unused memory accumulates in the proxy. The
client program can prevent this by invoking somdReleaseResources on the proxy
object periodicall (for example, each time it finishes using the result of the
last "get_string" call).


ΓòÉΓòÉΓòÉ 7.13.12. Writing clients that are also servers ΓòÉΓòÉΓòÉ

In many applications, processes may need to play both client and server roles.
That is, objects in the process may make requests of remote objects on other
servers, but may also implement and export objects, requiring that it be able
to respond to incoming requests. Details of how to write programs in this
peer-to-peer style are explained in section 6.9, "Advanced Topics".


ΓòÉΓòÉΓòÉ 7.13.13. Compiling and linking clients ΓòÉΓòÉΓòÉ

All client programs must include the header file "somd.h" (or for C++,
"somd.xh") in addition to any "<className>.h" (or "<className>.xh") header
files they require from application classes. All DSOM client programs must link
to the SOMobjects Toolkit library: "somtk.lib" For more information, see the
topic "Compiling and linking" in Chapter 3, "Using SOM Classes in Client
Programs."


ΓòÉΓòÉΓòÉ 7.14. Basic Server Programming ΓòÉΓòÉΓòÉ

Server programs execute and manage object implementations. That is, they are
responsible for:

     Notifying the DSOM daemon that they are ready to begin processing
      requests,

     Accepting client requests,

     Loading class library DLLs when required,

     Creating/locating/destroying local objects,

     Demarshalling client requests into method invocations on their local
      objects,

     Marshalling method invocation results into responses to clients, and

     Sending responses back to clients.

 As mentioned previously, DSOM provides a simple, "generic" server program that
 performs all of these tasks. All the server programmer needs to provide are
 the application class library(ies) DLL that the implementer wants to
 distribute. Optionally, the programmer can also supply an application#specific
 server class, derived from SOMDServer. (The SOMDServer class can be used by
 default.) The server program does the rest automatically.

 The "generic" server program is called somdsvr and can be found in
 %SOMBASE%\bin\somdsvr.exe

 Some applications may require additional flexibility or functionality than
 what is provided by the generic server program. In that case,
 application-specific server programs can be developed. This section discusses
 the steps involved in writing such a server program.

 To create a server program, a server writer needs to know what services the
 DSOM run-time environment will provide and how to use those services to
 perform the duties (listed above) of a server. The DSOM run-time environment
 provides several key objects that can be used to perform server tasks. These
 objects and the services they provide will be discussed in this section.
 Examples showing how to use the run-time objects to write a server are also
 shown.


ΓòÉΓòÉΓòÉ 7.14.1. Server run-time objects ΓòÉΓòÉΓòÉ

There are three DSOM run-time objects that are important in a server:

     The server's implementation definition (ImplementationDef),

     The SOM Object Adapter (SOMOA), and

     The application-specific server object (an instance of either SOMDServer
      or a class derived from SOMDServer).


ΓòÉΓòÉΓòÉ 7.14.1.1. Server implementation definition ΓòÉΓòÉΓòÉ

A server's implementation definition must be registered in the Implementation
Repository before a server can be used. When a client attempts to invoke a
method on a remote object, DSOM consults the Implementation Repository to find
the location of the target object's server.

An implementation definition is represented by an object of class
ImplementationDef, whose attributes describe a server's ID, user-assigned
alias, host name, program pathname, the class of its server object, whether or
not it is multi-threaded, and so forth. Implementation IDs uniquely identify
servers within the Implementation Repository, and are used as keys into the
Implementation Repository when retrieving the ImplementationDef for a
particular server.

It is possible to change the implementation characteristics of a server. The
implementation ID identifies a logical server, and the ImplementationDef
describes the current implementation of that logical server.

See the topic "Registering Servers and Classes" in section 6.6 for details on
server registration. Two registration methods are described: "manual," (via the
regimpl, the wregimpl, or the pregimpl utility) and "programmatic,"
ImplRepository methods.

When a server is initialized, it must retrieve a copy of its ImplementationDef,
and keep it in a global variable (SOMD_ImplDefObject). This variable is used by
the DSOM run time. (Client-only programs may leave the SOMD_ImplDefObject
variable set to NULL.)


ΓòÉΓòÉΓòÉ 7.14.1.2. SOM Object Adapter (SOMOA) ΓòÉΓòÉΓòÉ

The SOM Object Adapter (SOMOA) is the main interface between the server
application and the DSOM run time. The SOMOA is responsible for most of the
server duties listed at the beginning of this section. In particular, the SOMOA
object handles all communications an interpretation of inbound requests and
outbound results. When clients send requests to a server, the requests are
received and processed by the SOMOA.

The SOMOA works together with the server object to create and resolve DSOM
references to local objects, and dispatch methods on objects.

There is one SOMOA object per server process. (The SOMOA class is implemented
as a single instance class.)


ΓòÉΓòÉΓòÉ 7.14.1.3. Server object ΓòÉΓòÉΓòÉ

Each server process contains a single server object, which has the following
responsibilities for managing objects in the server:

     Provides an interface to client applications for basic object creation
      and destruction services, as well as any other application-specific
      object-management services that may be required by clients. For example,
      a print server may have a method that returns a list of all printers
      managed by that server. Clients may call this method to find out what
      printers are available.

     Provides an interface to the SOM Object Adapter for support in the
      creation and management of DSOM object references (which are used
      identify an object in the server), and for dispatching requests.

     The server class, SOMDServer, defines the base interface that must be
      supported by any server object. In addition, SOMDServer provides a
      default implementation that is suited to managing transient SOM objects
      in a server. This section will show how an application might override the
      basic SOMDServer methods and introduce new methods in order to tailor the
      server object functionality to a particular application.


ΓòÉΓòÉΓòÉ 7.14.2. Server activation ΓòÉΓòÉΓòÉ

Server programs may be activated either

     Automatically by the DSOM daemon, somdd, or

     Manually via command line invocation, or under application control.

 When a server is activated automatically by somdd, it will be passed a single
 argument (in argv[1]) that is the implementation ID assigned to the server
 implementation when it was registered into the Implementation Repository
 (discussed above and in section 6.6, "Configuring DSOM Applications"). This is
 useful when the server program cannot know until activation which "logical"
 server it is implementing. (This is true for the generic server provided with
 DSOM.) The implementation ID is used by the server to retrieve its
 ImplementationDef from the Implementation Repository.

 A server that is not activated by somdd may obtain its ImplementationDef from
 the Implementation Repository in any manner that is convenient: by ID, by
 alias, and so forth. Moreover, a server may choose to "register itself"
 dynamically, as part of its initialization. To do so, the server would use the
 programmatic interface to the Implementation Repository.

 For example, suppose that the server program "myserver" was designed so that
 it could be activated either automatically or manually. This requires that it
 be written to expect the implementation ID as its first argument, and to use
 that argument to retrieve its ImplementationDef from the Implementation
 Repository. If an application defines a server in the Implementation
 Repository whose implementation ID is

  2bcdc4f2-0f62f780-7f-00-10005aa8afdc
 then "myserver" could be run as that server by invoking the following command:

  myserver 2bcdc4f2-0f62f780-7f-00-10005aa8afdc


ΓòÉΓòÉΓòÉ 7.14.3. Initializing a server program ΓòÉΓòÉΓòÉ

The following subjects are discussed in this section:

     Initializing the DSOM run-time environment
     Initializing the server's ImplementationDef
     Initializing the SOM Object Adapter
     When initialization fails


ΓòÉΓòÉΓòÉ 7.14.3.1. Initializing the DSOM run-time environment ΓòÉΓòÉΓòÉ

The first thing the server program should do is to initialize the DSOM run time
by calling the SOMD_Init function. This causes the various DSOM run-time
objects to be created and initialized, including the Implementation Repository
(accessible via the global variable SOMD_ImplRepObject), which is used in the
next initialization step.


ΓòÉΓòÉΓòÉ 7.14.3.2. Initializing the server's ImplementationDef ΓòÉΓòÉΓòÉ

Next, the server program is responsible for initializing its implementationDef,
referred to by the global variable SOMD_ImplDefObject. It is initialized to
NULL by SOMD_Init. (For client programs it should be left as NULL.) If the
server implementation was registered with the Implementation Repository before
the server program was activated (as will be the case for all servers that are
activated automatically by somdd), then the ImplementationDef can be retrieved
from the Implementation Repository. Otherwise, the server program can register
its implementation with the Implementation Repository dynamically (as shown in
section 6.6, "Configuring DSOM applications").

The server can retrieve its ImplementationDef from the Implementation
Repository by invoking the find_impldef method on SOMD_ImplRepObject.  It
supplies, as a key, the implementation ID of the desired ImplementationDef.

The following code shows how a server program might initialize the DSOM
run-time environment and retrieve its ImplementationDef from the Implementation
Repository.

#include <somd.h> /* needed by all servers */
main(int argc, char **argv)
{
   Environment ev;
   SOM_InitEnvironment(&ev);

/* Initialize the DSOM run-time environment */
   SOMD_Init(&ev);

/* Retrieve its ImplementationDef from the Implementation
   Repository by passing its implementation ID as a key */
   SOMD_ImplDefObject =
      _find_impldef(SOMD_ImplRepObject, &ev, argv[1]);
...
}


ΓòÉΓòÉΓòÉ 7.14.3.3. Initializing the SOM Object Adapter ΓòÉΓòÉΓòÉ

The next step the server must take before it is ready to accept and process
requests from clients is to create a SOMOA object and initialize the global
variable SOMD_SOMOAObject to point to it. This is accomplished by the
assignment:

SOMD_SOMOAObject = SOMOANew();

Note:  The SOMOA object is not created automatically by SOMD_Init because it is
       only required by server processes.

 After the global variables have been initialized, the server can do any
 application-specific initialization required before processing requests from
 clients. Finally, when the server is ready to process requests, it must call
 the impl_is_ready method on the SOMOA:

  _impl_is_ready(SOMD_SOMOAObject, &ev, SOMD_ImplDefObject);

 The SOMOA will then set up a communications port for incoming messages, which
 it registers with the DSOM daemon. Once the DSOM daemon has been notified of
 the server's port, it assists client applications in "binding" (i.e.,
 establishing a connection) to that server.

 The impl_is_ready method also causes the server object, whose class is defined
 in the server's ImplementationDef, to be created. The server object can be
 referenced through the global variable, SOMD_ServerObject.


ΓòÉΓòÉΓòÉ 7.14.3.4. When initialization fails ΓòÉΓòÉΓòÉ

It is possible that a server will encounter some error when initializing
itself. Servers must attempt to notify DSOM that their activation failed, using
the activate_impl_failed method. This method is called as follows:

/* tell the daemon (via SOMOA) that activation failed */
_activate_impl_failed(SOMD_SOMOAObject,&ev, SOMD_ImplDefObject, rc);
Server writers should be aware, however, that until the server's
SOMD_ImpldefObject has been initialized, it is not possible to call the
_activate_impl_failed method on the DSOM daemon.

Note:  A server program should not call activate_impl_failed once it has called
       impl_is_ready.


ΓòÉΓòÉΓòÉ 7.14.4. Processing requests ΓòÉΓòÉΓòÉ

The SOMOA is the object in the DSOM run-time environment that receives client
requests and transforms them into method calls on local server objects. In
order for SOMOA to listen for a request, the server program must invoke one of
two methods on SOMD_SOMOAObject. If the server program wishes to turn control
over to SOMD_SOMOAObject completely (that is, effectively have SOMD_SOMOAObject
go into an infinite request-processing loop), then it invokes the
execute_request_loop method on SOMD_SOMOAObject as follows:

rc = _execute_request_loop(SOMD_SOMOAObject, &ev, SOMD_WAIT);

Note:  This is the way the DSOM provided "generic" server program interacts
       with SOMD_SOMOAObject.

 The execute_request_loop method takes an input parameter of type Flags. The
 value of this parameter should be either SOMD_WAIT or SOMD_NO_WAIT. If
 SOMD_WAIT is passed as argument, execute_request_loop will return only when an
 error occurs. If SOMD_NO_WAIT is passed, it will return when there are no more
 outstanding messages to be processed. SOMD_NO_WAIT is usually used when the
 server is being used with the event manager. See "Peer vs. client-server
 processes" in section 6.9, "Advanced Topics," for more details.

 If the server wishes to incorporate additional processing between request
 executions, it can invoke the execute_next_request method to receive and
 execute requests one at a time:

  for(;;) {
     rc = _execute_next_request(SOMD_SOMOAObject, &ev, SOMD_NO_WAIT);
     /* perform app-specific code between messages here, e.g., */
        if (!rc) numMessagesProcessed++;
  }

 Just like execute_request_loop, execute_next_request has a Flags argument that
 can take one of two values: SOMD_WAIT or SOMD_NO_WAIT. If execute_next_request
 is invoked with the SOMD_NO_WAIT flag and no message is available, the method
 returns immediately with a return code of SOMDERROR_NoMessages. If a request
 is present, it will execute it. Thus, it is possible to "poll" for incoming
 requests using the SOMD_NO_WAIT flag.


ΓòÉΓòÉΓòÉ 7.14.5. Exiting a server program ΓòÉΓòÉΓòÉ

When a server program exits, it should notify the DSOM run time that it is no
longer accepting requests. This should be done whether the program exits
normally, or as the result of an error. If this is not done, somdd will
continue to think that the server program is active, allowing clients to
attempt to connect to it, as well as preventing a new copy of that server from
being activated.

To notify DSOM when the server program is exiting, the deactivate_impl method
defined on SOMOA should be called. For example,

 /* tell DSOM (via SOMOA) that server is now terminating */
    _deactivate_impl(SOMD_SOMOAObject, &ev, SOMD_ImplDefObject);

Note:  For robustness, it would be worthwhile to add appropriate "exit
       handlers" or "signal handlers" to your application servers that call the
       deactivate_impl method upon abnormal program termination. This ensures
       the the DSOM daemon is made aware of the server's termination, so that
       client connections are no longer allowed.

 Finally, at the end of a server program, the SOMD_Uninit procedure must be
 called to free DSOM run-time objects, and to release semaphores, shared memory
 segments, and any other system resources.

 For example, the exit code in the server program might look like this:

      ...
      SOMD_Uninit(&e);
      SOM_UninitEnvironment(&e);
  }

 Observe also the SOM_UninitEnvironment call, which frees any memory associated
 with the specified Environment structure.

 When a Windows DSOM server application receives a WM_QUIT message while
 processing incoming requests, the execute_request_loop method will return
 SOMDERROR_WMQUIT.  When the server receives SOMDERROR_WMQUIT, it should
 perform its usual clean up and termination procedures.


ΓòÉΓòÉΓòÉ 7.14.6. Managing objects in the server ΓòÉΓòÉΓòÉ

The following subjects are discussed in this section:

     Object references, ReferenceData, and the ReferenceData table
     Simple SOM object references
     SOMDServer (default server-object class)
     Creation and destruction of SOM objects
     Mapping objects to object references
     Hints on the use of create vs. create_constant
     Mapping object references to objects
     Dispatching a method


ΓòÉΓòÉΓòÉ 7.14.6.1. Object references, ReferenceData, and the ReferenceData table ΓòÉΓòÉΓòÉ

One of SOMOA's responsibilities is to support the creation of object references
(SOMDObjects). Recall from the "Stack" example discussion (in Section 6.2) that
an object reference  is an exportable "handle" to an object and that proxies
are examples of object references. The SOMOA interface supports three
operations for creating object references: create, create_constant, and
create_SOM_ref.

The create and create_constant methods allow a serve to associate
application-specific data about an object with an object reference for that
object. This data, called reference data, is represented in a sequence of up to
1024 bytes of information about the object. This sequence, defined by the type
ReferenceData, may contain the object's location, state, or any other
characteristics meaningful to the application. Usually, ReferenceData is used
by a server process to locate or activate an object in the server.
ReferenceData, and hence the methods create and create_constant are usually
only used in connection with persistent objects (objects whose lifetimes exceed
that of the process that created them).

The create method differs from the create_constant method in the following way:
ReferenceData associated with an object reference constructed by
create_constant is immutable whereas the the ReferenceData associated with an
object reference created by create can be changed (via the change_id method).
References created with create_constant return true when the method is_constant
is invoked on them.

The create method stores the ReferenceData in a ReferenceData table associated
with the server, while create_constant maintains the ReferenceData as a
constant part of the object reference. The ReferenceData associated with an
object reference (whether it was constructed using create or create_constant
can be retrieved via the SOMOA method get_id.

The IDL SOMOA interface declarations of create, create_constant, get_id, and
change_id, and the SOMDObject interface declaration of is_constant are
presented below.

/* From the SOMOA interface */

   sequence <octet,1024> Referencedata;
   SOMDObject create(in ReferenceData id, in InterfaceDef intf,
                     in ImplementationDef impl);

   SOMDObject create_constant(in ReferenceData id,
                              in InterfaceDef intf,
                              in ImplementationDef impl);

   ReferenceData get_id(in SOMDObject objref);

   void change_id(in SOMDObject objref, in ReferenceData id);

/* From the SOMDObject interface */

   boolean is_constant();

An example of how ReferenceData can be used by an application follows the
description of SOMDServer objects in the next section


ΓòÉΓòÉΓòÉ 7.14.6.2. Simple SOM object references ΓòÉΓòÉΓòÉ

In order to efficiently support the generation and interpretation of references
to SOM objects, the SOMOA defines another method called create_SOM_ref.

The create_SOM_ref method creates a simple DSOM reference (SOMDObject) for a
local SOM object. The reference is "special" in that, unlike create and
create_constant there is no user-supplied ReferenceData associated with the
object and because the reference is only valid while the SOM object exists in
memory. The SOMObject to which it refers can be retrieved via the
get_SOM_object method. The is_SOM_ref method can be used to tell if the
reference was created using create_SOM_ref or not. The IDL declarations for
create_SOM_ref, get_SOM_object, and is_SOM_ref are displayed below:

/* from SOMOA's interface */

   SOMDObject create_SOM_ref(in SOMObject somobj,
                             in ImplementationDef impl);

   SOMObject get_SOM_object(in SOMDObject somref);

/* from SOMDObject's interface */

   boolean is_SOM_ref();


ΓòÉΓòÉΓòÉ 7.14.6.3. SOMDServer (default server-object class) ΓòÉΓòÉΓòÉ

Every server has a server object that implements three kinds of activities:

     Creation and destruction of SOM objects

     Mapping between SOMObjects and SOMDObjects, and

     Dispatching methods on SOM objects

 Additional, application-specific server methods (for initialization, server
 control, etc.) can be defined in a subclass of the SOMDServer class. The class
 of the server object to be used with a server is contained in the server's
 ImplementationDef.

 Following are the IDL declarations of the SOMDServer operations:

  // methods called by a client

     SOMObject somdCreateObj(in Identifier objclass, in string hints)

     void somdDeleteObj(in SOMObject somobj);

     SOMClass somdGetClassObj(in Identifier objclass);

  // methods called by SOMOA

     SOMDObject somdRefFromSOMObj(in SOMObject somobj);

     SOMObject somdSOMObjFromRef(in SOMDObject objref);

     void somdDispatchMethod(in SOMObject somobj,
                             out somToken retValue,
                             in somId methodId,
                             in va_list ap);


ΓòÉΓòÉΓòÉ 7.14.6.4. Creation and destruction of SOM objects ΓòÉΓòÉΓòÉ

The SOMDServer class defines methods for the basic creation of SOM objects in
the server process (somdCreateObj), and for finding the SOM class object for a
specified class (somdGetClassObj). With somdGetClassObj, a client can get a
proxy to a class object on the server, so that methods introduced in the
class's metaclass (for example, class-specific constructors, etc.) may be
invoked directly on the class object. Examples of client use of these two
methods were presented earlier in Sections 6.2 and 6.3.

With somdDeleteObj, the client can involve the server object in object
destruction. (The methods somdTargetFree and somFree are defined on the object
themselves and do not involve the server object.) Involving the server object
in object creation and destruction can be important for applications that need
more control over how objects are created and destroyed, or if the application
needs to keep track of an object's creation and destruction.


ΓòÉΓòÉΓòÉ 7.14.6.5. Mapping objects to object references ΓòÉΓòÉΓòÉ

SOMDServer also defines methods that implement mappings between SOMObjects and
SOMDObjects (object references) and a method for dispatching method calls on
SOM objects. These methods are used by the SOM Object Adapter (SOMOA) when
converting remote requests into method calls and results into responses.

Recall from the topic "Proxy objects" in Section 6.3, "Basic Client
Programming", that servers return proxies to remote objects as method results,
not the remote objects themselves. Recall also that class libraries need not be
designed to be distributed (that is, the code that implements the classes need
not be aware of the existence of proxy objects at all). Thus, it is up to the
DSOM run-time environment to ensure that proxies, rather than remote objects,
are returned to clients. The SOMD_SOMOAObject and SOMD_ServerObject work
together to perform this service. Whenever a result from a remote method call
includes a SOMObject, the SOMD_SOMOAObject invokes the somdRefFromSOMObj method
on SOMD_ServerObject, asking it to create a SOMDObject from the SOMObject.

The default implementation (i.e., SOMDServer's implementation) for
somdRefFromSOMObj uses the create_SOM_ref method to return a "simple" reference
for the SOMObject. Application-specific server objects (instances of a subclass
of SOMDServer) may elect to use create or create_constant to construct the
object reference if the application requires Reference Data to be stored.


ΓòÉΓòÉΓòÉ 7.14.6.6. Hints on the use of create vs. create_constant ΓòÉΓòÉΓòÉ

Enough context now exists so that the following question may be answered: "If
object references constructed with create support changeable ReferenceData, but
object references constructed with create_constant do not, why would I ever
want to use create_constant?"

Invocations of create add entries to a table called the ReferenceData Table.
The ReferenceData Table is persistent; that is, ReferenceData saved in it
persists between server activations. Two calls to create with the same
arguments do not return the same SOMDObject (per CORBA 1.1 specifications) That
is, if create is called twice with the same arguments, two entries in the
ReferenceData Table will be created. If a server using create wishes to avoid
cluttering up the ReferenceData Table with multiple references to the same
object, it must maintain a table of its own to keep track of the references it
has created to avoid calling create twice with the same arguments.

The create_constant method stores the ReferenceData as part of the SOMDObject's
state; that is, it does not add entries to the ReferenceData Table. The
create_constant method, then, might be used by a server that does not want to
maintain a table of references nor pay the penalty of cluttering up the
ReferenceData Table with multiple entries.


ΓòÉΓòÉΓòÉ 7.14.6.7. Mapping object references to objects ΓòÉΓòÉΓòÉ

The somdSOMObjFromRef method maps SOMDObjects to SOMObjects. This method is
invoked by  SOMOA on the server object, for each object reference found as a
parameter in a request.

The default implementation for somdSOMObjFromRef returns the address of the
SOMObject for which the specified object reference was create (using the
somdRefFromSOMObj method). If the object reference was not created by the same
server process, then an exception (BadObjref) is raised. The default
implementation does not, however, verify that the original object (for which
the object reference was created) still exists. If the original object has been
deleted (for example, by another client program), then the address returned
will not represent a valid object, and any methods invoked on that object
pointer will result in server failure. Note: The default implementation of
somdSOMObjFromRef does not check that the original object address is still
valid because the check is very expensive and seriously degrades server
performance.

To have a server verify that all results from somdSOMObjFromRef represent valid
objects, server programmers can subclass SOMDServer and override the
somdSOMObjFromRef method to perform a validity check on the result (using the
somIsObj function). For example, a subclass "MySOMDServer" of SOMDServer could
implement the somdSOMObjFromRef method as follows:

SOM_Scope SOMObject SOMLINK somdSOMObjFromRef(MySOMDServer somSelf,
                                              Environment * ev,
                                              SOMDObject objref)
{
    SOMObject obj;
    StExcep_INV_OBJREF *ex;

    /* MySOMDServerData *somThis = MySOMDServerGetData(somSelf); */
    MySOMDServerMethodDebug(*MySOMDServer*, *somdSOMObjFromRef");

    obj = MySOMDServer_parent_SOMDServer_somdSOMObjFromRef(somSelf,
                                                    ev, objref);
    if (somIsObj(obj))
        return (obj);
    else {
        ex = (StExcep_INV_OBJREF *)
              SOMMalloc(sizeof(StExcep_INV_OBJREF));
        ex->minor = SOMDERROR_BadObjref;
        ex->completed = NO;
        somSetException(ev, USER_EXCEPTION,
                        ex_StExcep_INV_OBJREF, ex);
        return (NULL);
    }
}


ΓòÉΓòÉΓòÉ 7.14.6.8. Dispatching a method ΓòÉΓòÉΓòÉ

After SOMOA (with the help of the local server object) has resolved all the
SOMDObjects present in a request, it is ready to invoke the specified method on
the target. Rather than invoking somDispatch directly  on the target, it calls
the somdDispatchMethod method on the server object. The parameters to
somdDispatchMethod are the same as the parameters for SOMObject::somDispatch
(see the SOM Programming Reference for a complete description).

The default implementation for somdDispatchMethod in SOMServer simply invokes
SOMObject::somDispatch on the specified target object with the supplied
arguments. The reason for this indirection through the server object is to give
the server object a chance to intercept method calls coming into the server
process, if desired.


ΓòÉΓòÉΓòÉ 7.14.7. Identifying the source of a request ΓòÉΓòÉΓòÉ

CORBA 1.1 specifies that a Basic Object Adapter should provide a facility for
identifying the principal (or user) on whose behalf a request is being
performed. The get_principal method, defined by BOA and impleented by SOMOA
returns a Principal object, which identifies the caller of a particular method.
From this information, an application can perform access control checking.

In CORBA 1.1, the interface to Principal is not defined, and is left up to the
ORB implementation. In the current release of DSOM, a Principal object is
defined to have two attributes:

 userName (string)   Identifies the name of the user who invoked a request.

 hostName (string)   Identifies the name of the host from which the request
                     originated.

 Currently, the value of the UserName attribute is obtained from the USER
 environment variable in the calling process. Likewise, the hostName attribute
 is obtained from the HOSTNAME environment variable. This facility is intended
 to provide basic information about the source of a request, and currently, is
 not based on any specific authentication (i.e., "login") scheme. More rigorous
 authentication and security mechanisms will be considered for future DSOM
 implementations.

 The IDL prototype for the get_principal method, defined on BOA (SOMOA) is as
 follows:

  Principal  get_principal (in SOMDObject obj,
                             in Environment *req_ev);

 This call will typically be made either by the target object or by the server
 object, when a method call is received. The get_principal method uses the
 Environment structure associated with the request, and an object reference for
 the target object, to produce a Principal object that define the request
 initiator.

 Note:  CORBA 1.1 defines a "tk_Principal" TypeCode which is used to identify
        the type of Principal object arguments in requests, in case special
        handling is needed when building the request. Currently, DSOM does not
        provide any special handling of objects of type "tk_Principal"; they
        are treated like any other object.


ΓòÉΓòÉΓòÉ 7.14.8. Compiling and linking servers ΓòÉΓòÉΓòÉ

The server program must include the "somd.h" header file. Server programs must
link to the SOMobjects Toolkit library: "somtk.lib"

For more information, see the topic "Compiling and linking" in Chapter 5,
"Implementing Classes in SOM."


ΓòÉΓòÉΓòÉ 7.15. Implementing Classes ΓòÉΓòÉΓòÉ

DSOM has been designed to work with a wide range of object implementations,
including SOM class libraries as well as non-SOM object implementations. This
section describes the necessary steps in using SOM classes or non-SOM object
implementations with DSOM.


ΓòÉΓòÉΓòÉ 7.15.1. Using SOM class libraries ΓòÉΓòÉΓòÉ

It is quite easy to use SOM classes in multi-process DSOM-based applications as
exemplified by the sample DSOM application presented in section 6.2, "A Simple
DSOM Example". In fact, in many cases, existing SOM class libraries may be used
in DSOM applications without requiring any special coding or recoding for
distribution. This is possible through the use of DSOM's generic server
program, which uses SOM and the SOM Object Adapter (SOMOA) to load SOM class
libraries on demand, whenever an object of a particular class is created or
activated.

The topic "Registering servers and classes" in section 6.6 "Configuring DSOM
Applications" discusses how to register a server implementation consisting of a
DSOM generic server process and one or more SOM class libraries.


ΓòÉΓòÉΓòÉ 7.15.1.1. Role of DSOM generic server program ΓòÉΓòÉΓòÉ

The generic server program provides basic server functionality: it continuously
receives and executes requests (via an invocation of the SOMOA's
execute_request_loop method), until the server is stopped. Some requests result
in the creation of SOM objects; the generic server program will find and load
the DLL for the object's class automatically, if it has not already been
loaded.

When generic server program functionality is not sufficient for the particular
application, application-specific server programs can be developed. For
example, some applications may want to interact with a user or I/O device
between requests. The previous section, entitled "Basic Server Programming,"
discussed the steps involved in writing a server program.


ΓòÉΓòÉΓòÉ 7.15.1.2. Role of SOM Object Adapter ΓòÉΓòÉΓòÉ

The SOM Object Adapter is DSOM's standard object adapter. It provides basic
support for receiving and dispatching requests on objects. As an added feature,
the SOMOA and the server process's server object collaborate t automate the
task of converting SOM object pointers into DSOM object references, and vice
versa. That is, whenever an object pointer is passed as an argument to a
method, the SOMOA and the server object convert the pointer to a DSOM bject
reference (since a pointer to an object is meaningless outside the object's
address space).


ΓòÉΓòÉΓòÉ 7.15.1.3. Role of SOMDServer ΓòÉΓòÉΓòÉ

The server process's server object (whose default class is SOMDServer) is
responsible for creating/destroying objects on the server via somdCreateObj,
somdGetClassObj, and somdDeleteObj, for mapping between object references
(SOMDObjects) and SOMObjects via somdRefFromSOMObj and somdSOMObjFromRef, and
for dispatching remote requests to server process objects via
somdDispatchMethod. These last three methods are invoked on the server object
by the SOMOA when objects are to be returned to clients, when incoming requests
contain object references, and when the method is ready to be dispatched,
respectively. By partitioning out these mapping and dispatching functions into
the server object, the application can more easily customize them, without
having to build object adapter subclasses.

SOMDServer can be subclassed by applications that want to manage object
location, object activation, and method dispatching. An example of such an
application (which provides a server class implementation for persistent SOM
objects) is shown in section 6.4, "Basic Server Programming."

These features of SOMOA and SOMDServer make it possible to take existing OM
classes, which have been written for a single-address space environment, and
use them unchanged in a DSOM application. More information on the SOMOA and
server objects can be found in the "Basic Server Programming" section.


ΓòÉΓòÉΓòÉ 7.15.1.4. Implementation constraints ΓòÉΓòÉΓòÉ

The generic server program (somdsvr), the SOMOA, and the SOMDServer make it
easy to use SOM classes with DSOM. However, if there are any parts of the class
implementation that were written expecting a single-process environment, the
class may have to be modified to behave properly in a client/server
environment. Some common implementation practices to avoid are listed below

     Printing to standard output. Any text printed by a method will appear at
      the server, as opposed to the client. In fact, the server may not be
      attached to a text display device or window, so the text may be lost
      completely. It is preferred that any textual output generated by a method
      be returned as an output string.

      Note:  Passing textual output between the client program and the called
             method via an "inout string" parameter is strongly discouraged. As
             discussed in the CORBA 1.1 specification (page 94), the size of
             the output string is constrained by the size of the input string.
             If there was no input string value, the size of the output string
             would be constrained to 0 bytes. Instead, it is preferred that
             textual data be returned either as an output string (DSOM provides
             the storage), or by passing a character array buffer (client
             provides the storage).

     Creating and deleting objects. Methods that create or delete objects may
      have to be modified if the created objects are intended to be remote. The
      calls to create local objects are different than the calls to create
      remote objects.

     Using pointers to client-allocated memory in instance variables. Consider
      the following example: A class has a method that accepts a pointer to a
      data value created by the client (e.g., a string or a struct), and simply
      stores the pointer in an instance variable or attribute. However, in
      DSOM, the called method is passed a pointer to a copy of the value (in
      the request message body), but the copy is freed at the end of the
      request. If the data value is meant to persist between requests, the
      object is responsible for making its own copy of it. (The implementation
      of the "_set_printerName" method in the topic "Wrapping a printer API"
      later in this section is an example of a method performing such a copy.)

     Using "procedure" methods. Methods having the procedure SOM IDL modifier
      cannot be invoked remotely using DSOM.  This is because these "methods"
      are called directly, rather than by the normal method resolution
      mechanisms on which DSOM relies.

 In addition to those coding practices which simply do not "port" to a
 distributed environment, there are a few other restrictions that are imposed
 by DSOM's (current) implementation.

     Using parameter types not supported by DSOM. DSOM can make remote
      invocations only of methods whose parameter types are among the following
      IDL types:  basic types short, long, unsigned short, unsigned long,float,
      double, char, boolean, octet), enum struct, union, sequence, string,
      array, any, and object (an interface name, designating a pointer to an
      object that supports that interface).  The members of a struct, union,
      sequence, or array, and the value of any any, must also be from the above
      list of supported DSOM types.

      In addition to the above types, DSOM also supports method parameters of
      type pointer to one of the above types (for example,  long*).  Pointers
      to pointers are not supported, however, and pointers embedded within one
      of the above types (for example, a pointer within a struct) are not
      supported.  The "void*" type is also not supported.  Currently, DSOM has
      the limitation that NULL pointer values cannot be returned as inout or
      out method arguments, although it is expected that this limitation will
      be addressed in the future release.

      Types declared as SOMFOREIGN types are not currently supported by DSOM.

     Packing of structures used as method arguments. If a compiler option is
      used to pack or optimize storage of structs (including reordering of
      struct members) or unions, it is important to indicate the exact
      alignment of the structures using alignment modifiers expressed in the
      implementation section of the IDL file. This information must then be
      updated in the Interface Repository.

 Some applications may need to associate specific identification information
 with an object, to support application-specific object location or activation.
 In that case, an application server should create object references
 explicitly, using the create or create_constant method in SOMOA. A logical
 place to put these calls is in a subclass of SOMDServer, as it is the server
 object that is responsible for producing/activating objects from object
 references.


ΓòÉΓòÉΓòÉ 7.15.2. Using other object implementations ΓòÉΓòÉΓòÉ

As an Object Request Broker, DSOM must support a wide range of object
implementations, including non-SOM implementations. For example, in a print
spooler application, the implementation of a print queue object may be provided
by the operating system, where the methods on the print queue are executable
programs or system commands. As another example, consider an application that
uses a large, existing class library that is not implemented using SOM.

In each of these examples, the application must participate  in object
identification, activation, initialization, and request dispatching. Each
server supplies a server object (derived from SOMDServer) that works in
conjunction with the SOMOA for this purpose.


ΓòÉΓòÉΓòÉ 7.15.2.1. Wrapping a printer API ΓòÉΓòÉΓòÉ

Presented below is a simple example showing how an existing API could be
"wrapped" as SOM objects. The API is admittedly trivial, but it is hoped that
readers understand this simple example well enough to create more sophisticated
applications of their own. This example is given for OS/2, rather than for
Windows 95 /NT. Again, it is intented to serve as a simple conceptual framework
that

 will serve as a basis for developing more sophisticated "wrappers".
The "API" wrapped in this example is comprised of two system calls.


The first one asks for a file to be printed on a specific printer:

print /D:<printerName> <filename>

The second one asks for the file currently being printed on device
<printerName> to be cancelled.

print /D:<printerName> /C

Two IDL interfaces are declared in the module "PrinterModule": "Printer" and
"PrinterServer". The "Printer" interface wraps the two system calls. The
"PrinterServer" interface describes a subclass of SOMDServer.
"PrinterModule::PrinterServer" will be the class of the server object in the
print-server application.

#include <somdserv.idl>

module PrinterModule {
  interface Printer : SOMObject {
    attribute string printerName;
    void print(in string fname);
    void cancel();
    #ifdef __SOMIDL__
    implementation {
      printerName: noset;   // memory to be allocated
      };
    #endif
  };

  interface PrinterServer :SOMDServer{
    #ifdef __SOMIDL__
    implementation {
      somdCreateObj: override;
      somdRefFromSOMObj: override;
      somdSOMObjFromRef: override;
      };
    #endif
  };

};

Note that the "Printer" interface defines one attribute, "printerName", that
will be used to identify the printer. It will be set when a "Printer" is
created. Printer's two operations, "print" and "cancel", correspond to the two
system commands the interface is encapsulating. The "PrinterServer" interface
does not introduce any new attributes or operations. It does specify that three
of SOMDServer's methods will have their implementations overridden.

The next three method procedures show how the "Printer" interface is
implemented for the "_set_printerName", "print", and "cancel" methods. Recall
(from the earlier topic "Implementation constraints") that "_set" methods for
attributes must be explicitly implemented in order to allocate their memory, if
data values need to persist between DSOM requests.

SOM_Scope void  SOMLINK PrinterModule_Printer_set_printerName(
 PrinterModule_Printer somSelf, Environment *ev, string printerName)
{
    PrinterModule_PrinterData *somThis =
      PrinterModule_PrinterGetData(somSelf);

    if (_printerName) SOMFree(_printerName);
    _printerName = (string)SOMMalloc(strlen(printerName) + 1);
    strcpy(_printerName, printerName);
}


SOM_Scope void  SOMLINK PrinterModule_Printerprint(
 PrinterModule_Printer somSelf, Environment *ev, string fname)
{
    long rc;
    PrinterModule_PrinterData *somThis =
      PrinterModule_PrinterGetData(somSelf);
    string printCommand = (string)
      SOMMalloc(strlen(_printerName) + strlen(fname) + 10 + 1);

    sprintf(printCommand,"print /D:%s %s",_printerName,fname);
    rc = system(printCommand);
    if (rc) raiseException(ev,rc);
}


SOM_Scope void  SOMLINK PrinterModule_Printercancel(
 PrinterModule_Printer somSelf, Environment *ev)
{
    long rc;
    PrinterModule_PrinterData *somThis =
      PrinterModule_PrinterGetData(somSelf);
    string printCommand =
      (string) SOMMalloc(strlen(_printerName) + 12 + 1);

    sprintf(printCommand,"print /D:%s /C",_printerName);
    rc = system(printCommand);
    if (rc) raiseExeception(ev,rc);
}

Note:  The implementation of the "raiseException" procedure shown in the
       example above must be provided by the application. However, it is not
       shown in this example.

 The three method procedures that implement the "PrinterServer" interface's
 three overridden methods of SOMServer are very similar to the method
 procedures of the "MyPServer" server-object class presented in the previous
 section (6.4), and therefore have not been shown here.


ΓòÉΓòÉΓòÉ 7.15.3. Parameter memory management ΓòÉΓòÉΓòÉ

There are five SOM IDL modifiers available for specifying the memory-management
policy for the parameters of a method (regardless of whether the caller or the
object owns the parameters' memory after the method is invoked). These
modifiers are:

  memory_management,  caller_owns_result,  caller_owns_parameters,

    object_owns_result, and  object_owns_parameters.

See the section entitled "Implementation Statements" in Chapter 4, "SOM IDL and
the SOM Compiler," for a complete description of these modifiers and their
meanings. Note that the memory-management policy for a particular parameter
applies to the parameter and all the memory embedded within it (for example, if
a struct is owned by the caller, then so are all the struct's members).

When a class contains the memory_management = corba SOM IDL modifier, this
signifies that all methods introduced by the class follow the CORBA
specification for parameter memory management, except where a particular method
has an explicit modifier  (object_owns_result or object_owns_parameters) that
indicates otherwise. For a description of the CORBA specification, see the
earlier subtopic entitled "The CORBA policy for parameter memory management"
(under the topic "Memory Management" in Section 6.3 of this chapter).


ΓòÉΓòÉΓòÉ 7.15.4. Building and registering class libraries ΓòÉΓòÉΓòÉ

The generic server uses SOM's run-time facilities to load class libraries
dynamically. Thus, dynamically linked libraries (DLLs) should be created for
the classes, just as they would be for non-distributed SOM-based applications.
For more information, see the topic Creating a SOM class library in Chapter 5
"Implementing classes in SOM."

During the development of the DLL, it is important to remember the following
steps:

     Export a routine called SOMInitModule in the DLL, which will be called by
      SOM to initialize the class objects implemented in that library.
      SOMInitModule should contain a <className>NewClass call for each class in
      the DLL.

     For each class in the DLL, specify the DLL name in the class's IDL file.
      The DLL name is specified using the dllname=<name> modifier in the
      implementation statement of the interface definition. If not specified,
      the DLL filename is assumed to be the same as the class name.

     For each class in the DLL, compile the IDL description of the class into
      the Interface Repository. This is accomplished by invoking the following
      command syntax:


                    sc -sir -u stack.idl

      Note:  If the classes are not compiled into the Interface Repository,
             DSOM will generate a run-time error (30056:
             SOMDERROR_BadDescriptor) when an attempt is made to lookup the
             signature of a method in the class (for example, on a method
             call).

     Put the DLL in one of the directories or listed in PATH for Windows. AIX,
      and Windows.)


ΓòÉΓòÉΓòÉ 7.16. Configuring DSOM Applications ΓòÉΓòÉΓòÉ

The following subjects are discussed in this section:

     Preparing the environment
     Registering class interfaces
     Registering servers and classes
     The 'regimpl', registration utility
     Programmatic interface to the Implementation Repository


ΓòÉΓòÉΓòÉ 7.16.1. Preparing the environment ΓòÉΓòÉΓòÉ

Some environment variables must be defined before running DSOM.

 HOSTNAME=<name>                   Each machine that is running DSOM must have
                                   its HOSTNAME variable set. For Workstation
                                   DSOM; <name> must be set to "localhost".

 USER=<name>                       USER specifies the name of the DSOM user
                                   running a client program.

 SOMIR=<file(s)>                   SOMIR specifies a list of files (separated
                                   by a semicolon ) which together make up the
                                   Interface Repository. See Chapter 7, "The
                                   Interface Repository Framework," for more
                                   information on how to set this variable.

                                   Note:  For DSOM, it is preferable to use
                                          full pathnames in the list of IR
                                          files, since the IR will be shared by
                                          several programs that may not all be
                                          started in the same directory.

 SOMDDIR=<directory>               SOMDDIR specifies the directory where
                                   various DSOM files should be located,
                                   including the Implementation Repository
                                   files. See the later section in this chapter
                                   entitled "Registering servers and classes"
                                   for more information.

                                   Note:  If this value is not set, DSOM will
                                          attempt to use a default directory:
                                          and %SOMBASE%\ETC\DSOM

 SOMDPORT=<integer>                In DSOM, servers, clients and DSOM daemons
                                   communicate with each other using a
                                   "sockets" abstraction. In particular DSOM
                                   clients establish connections to DSOM
                                   servers by communicating with the DSOM
                                   daemon, somdd, running on each server
                                   machine. The daemon is designed to listen
                                   for client requests on a well-known port.

                                   Normally, somdd will look in the
                                   %ETC%\SERVICES file for its well-known port
                                   number. However, if the user has set the
                                   SOMDPORT environment variable, the value of
                                   SOMDPORT will be used and the "services"
                                   file will not be consulted. The user should
                                   pick a 16-bit integer that is not likely to
                                   be in use by another application (check the
                                   "services" file for ports reserved for use
                                   on your machine). Typically, values below
                                   1024 are reserved and should not  be used.

                                   Note:  If there is no "services" file and
                                          the SOMDPORT environment variable is
                                          not set, DSOM will use a default port
                                          number (currently 9393).

 SOMDTIMEOUT=<integer>             SOMDTIMEOUT specifies how long a receiver
                                   should wait for a message. The value should
                                   be expressed in seconds. The default value
                                   is 600 seconds (10 minutes).

 SOMDDEBUG=<integer>               SOMDDEBUG may optionally be set to enable
                                   DSOM run-time error messages. If set to 0,
                                   error reporting is disabled. If set to 1,
                                   error reporting is enabled. Error reports
                                   may be directed to the file named by
                                   SOMDMESSAGELOG, if set.

 SOMDMESSAGELOG=<file>             SOMDMESSAGELOG may optionally be set to the
                                   name of a file where DSOM run-time error
                                   messages are recorded. If not set, error
                                   messages will be reported on the standard
                                   output device.

 SOMDNUMTHREADS=<integer>          SOMDNUMTHREADS may optionally be set to the
                                   maximum number of requests threads created
                                   per server.  If SOMDNUMTHREADS is not set,
                                   then a separate thread will be created for
                                   each request.


ΓòÉΓòÉΓòÉ 7.16.2. Registering class interfaces ΓòÉΓòÉΓòÉ

DSOM relies heavily on the Interface Repository for information on method
signatures (that is, a description of the method's parameters and return
value). It is important to compile the IDL for all application classes into the
IR before running the application.

For each class in the DLL, compile the IDL description of the class into the
Interface Repository. This is accomplished by invoking the following command
syntax:

    sc -sir -u stack.idl
    .*(on AIX or OS/2)

If the default SOM IR (supplied with the SOMobjects Toolkit and Run times) is
not used by the application, the user's IR must include the interface
definitions for:

     the Sockets class

     the server class (derived from SOMDServer), and

     the definitions of the standard DSOM exceptions (found in file
      "stexcep.idl") that may be returned by a method call.


ΓòÉΓòÉΓòÉ 7.16.3. Registering servers and classes ΓòÉΓòÉΓòÉ

Implementation definitions:

The Implementation Repository holds ImplementationDef objects The
ImplementationDef class defines attributes necessary for the SOMOA to find and
activate the implementation of an object. Details of the ImplementationDef
object are not currently defined in the CORBA 1.1 specification; the attributes
that have been defined are required by DSOM.

Listed below is each available attribute, with its corresponding type in
parentheses, followed by a description of its purpose:

 impl_id (string)              Contains the DSOM-generated identifier for a
                               server implementation.

 impl_alias (string)           Contains the "alias" (user-friendly name) for a
                               server implementation.

 impl_program (string)         Contains the name of the program or command file
                               which will be executed when a process for this
                               server is started automatically by somdd. If the
                               full pathname is not specified, the directories
                               specified in the  PATH environment variable will
                               be searched for the named program or command
                               file.

                               Optionally, the server program can be run under
                               control of a "shell" or debugger, by specifying
                               the shell or debugger name first, followed by
                               the name of the server program. (A space
                               separates the two program names.)  For example,

                                                                                idbg myprogram.exe

                               will start the program "myprogram" under control
                               of "idbg".

                               Servers that are started automatically by somdd
                               will always be passed their impl_id as the first
                               parameter, in order to retrieve their
                               ImplementationDef (if desired).

 impl_flags (Flags)            Contains a bit-vector of flags used to identify
                               server options (for example, the
                               IMPLDEF_MULTI_THREAD flag indicates
                               multi-threading). See the impldef.idl file for
                               the complete set of valid ImplementationDef
                               flags. Unused flag bits are reserved for future
                               use by IBM.

 impl_server_class (string)    Contains the name of the SOMDServer class or
                               subclass created by the server process.

 impl_refdata_file (string)    Contains the full pathname of the file used to
                               store ReferenceData for the server.

 impl_refdata_bkup (string)    Contains the full pathname of the backup mirror
                               file used to store ReferenceData for the server.
                               This file can be used to restore a copy of the
                               primary file in case it becomes corrupted. (It
                               would be a good idea to keep the primary and
                               backup files in different disk volumes.)

 impl_hostname (string)        Contains the hostname of the machine where the
                               server is located.


ΓòÉΓòÉΓòÉ 7.16.4. Implementation registration utilities ΓòÉΓòÉΓòÉ

Before an implementation (a server program and class libraries) can be used by
client applications, it must be registered with DSOM by running the
implementation registration utility, regimpl The  regimpl  utility can also be
executed from the DOS command line;  this facility is available primarily for
use in batch files. During execution of regimpl, DSOM updates it database to
include the new server implementation and the associated classes. This enables
DSOM to find and, if necessary, to activate the server so that clients can
invoke methods on it.

Typically, DSOM users employ the generic SOM-object server program, described
below. A discussion on how to write a specific (non-generic) server program is
found in the earlier section, "Basic Server Programming."


ΓòÉΓòÉΓòÉ 7.16.4.1. Registration steps Using 'regimpl' ΓòÉΓòÉΓòÉ

Registering a server implementation and its classes requires the steps
described in the following paragraphs.

First, make sure the SOMDDIR environment variable is defined to the name of the
Implementation Repository directory, as discussed in the section "Preparing the
Environment."

Then, to run the regimpl utility, at the system prompt enter:

> regimpl

This brings up the DSOM Implementation Registration Utility menu, shown below.
To begin registering the new implementation, select "1.Add" from the
IMPLEMENTATION OPERATIONS section; that is, at the "Enter operation:" prompt,
enter "1" (as shown in bold):

DSOM IMPLEMENTATION REGISTRATION UTILITY
(C) Copyright IBM Corp. 1992,1993.  All rights reserved.

Implementation data being loaded from: /u/xyz/dsomRepos/

[ IMPLEMENTATION OPERATIONS ]
 1.Add  2.Delete  3.Change
 4.Show one  5.Show all  6.List aliases
[ CLASS OPERATIONS ]
 7.Add  8.Delete  9.Delete from all  10.List classes
[ SAVE & EXIT OPERATIONS ]
 11.Save data  12.Exit
Enter operation: 1

The regimpl utility then issues several prompts for information about the
server implementation (typical responses are shown in bold as an example).

Implementation alias.  Enter a "shorthand" name for conveniently referencing
the registered server implementation while using regimpl:

Enter an alias for new implementation:  myServer

Program name.  Enter the name of the program that will execute as the server.
This may be the name of one of the DSOM generic servers (discussed under the
later topic "Running DSOM Servers") or a user-defined name for one of these
servers. If the program is located in PATH, only the program name needs to be
specified. Otherwise, the pathname must be specified.

Enter server program name:(default: somdsvr)  <return>

Multi-threading.  Specify whether or not the server expects the SOM Object
Adapter (SOMOA) to run each method in a separate thread or not. Notes: You must
ensure that methods executed by the server are "thread safe".

Allow multiple threads in the server? [y/n]
(default: n) : n

Server class.  Enter the name of the SOMDServer class or subclass that will
manage the objects in the server.

Enter server class (default: SOMDServer) : <return>

Reference data file name.  Enter the full pathname of the file used to store
ReferenceData associated with object references created by this server. Note: A
file name is required only  if the server is using the create method to
generate object references.

Enter object reference file name (optional) :  <return>
Backup reference data file name.  Enter the full pathname of the backup file
used to mirror the primary ReferenceData file for this server. Note: a file
name is required only if (1) a primary reference data file has been specified,
and (2) the application desires an online backup to be maintained. This file
can be used to restore a copy of the primary file should it become corrupted.

Enter object reference backup file name (optional) : <return>

Host machine name.  This is the name of the machine on which the server program
code is stored. The same name should be indicated in the HOSTNAME environment
variable. (If "localhost" is entered, the contents of the HOSTNAME environment
variable will be used.

Enter host machine name:(default: localhost)  <return>

The regimpl system next displays a summary of the information defined thus far,
and asks for confirmation before adding it. Enter "y" to save the
implementation information in the Implementation Repository.


================================================================
Implementation id.........: 2befc82b-13a11e00-7f-00-10005ac9272a
Implementation alias......: myServer
Program name..............: somdsvr
Multithreaded.............: No
Server class..............:SOMDServer
Object reference file.....:
Object reference backup...:
Host Name.................: localhost

The above implementation is about to be added. Add? [y/n] y
Implementation 'myServer' successfully added
Add class.  Once the server implementation is added, the complete menu
reappears. The next series of prompts and entries will identify the classes
associated with this server. To begin, from the CLASS OPERATIONS section,
select "7.Add":


[ IMPLEMENTATION OPERATIONS ]
 1.Add  2.Delete  3.Change
 4.Show one  5.Show all  6.List aliases
[ CLASS OPERATIONS ]
 7.Add  8.Delete  9.Delete from all  10.List classes
[ SAVE & EXIT OPERATIONS ]
 11.Save data  12.Exit

Enter operation: 7
Class name.  Enter the name of a class associated with the implementation
alias.

Enter name of class:  class1

Implementation alias.  Enter the alias for the server that implements the new
class (this should be the same alias as given above).

Enter alias of implementation that implements class:  myServer

Class 'class1' now associated with implementation 'myServer'

The top-level menu will then reappear. Repeat the previous three steps until
all classes have been associated with the server.

Then, from the SAVE & EXIT OPERATIONS section, select "11.Save data" to
complete the registration. Finally, select "12.Exit" to exit the regimpl
utility.

[ IMPLEMENTATION OPERATIONS ]
 1.Add  2.Delete  3.Change
 4.Show one  5.Show all  6.List aliases
[ CLASS OPERATIONS ]
 7.Add  8.Delete  9.Delete from all  10.List classes
[ SAVE & EXIT OPERATIONS ]
 11.Save data  12.Exit

Enter operation:  11

Enter operation:  12


ΓòÉΓòÉΓòÉ 7.16.4.2. Command line interface to 'regimpl' ΓòÉΓòÉΓòÉ

The regimpl utility also has a command line interface. The command flags
correspond to the interactive commands described above. The syntax of the
regimpl commands follow.

Note:  The regimpl command and any optional regimpl command flags can be
       entered at a system prompt, and the command will execute as described
       below.

 To enter interactive mode:

    regimpl

 To add an implementation:

    regimpl -A -i <str> [-p <str>] [-v <str>] [-f <str>] [-b <str>]
                          [-h <str>] [-m {on|off}] [-z <str>] [-n {on|off}]

 To update an implementation:

    regimpl -U -i <str> [-p <str>] [-v<str>] [-f <str>]  [-b <str>]
                           [-h <str>] [-m {on|off}] [-n {on|off}]

 To delete one or more implementations:

    regimpl -D -i <str> [-i ...]

 To list all, or selected, implementations:

    regimpl -L  [-i <str> [-i ...]]

 To list all implementation aliases:

    regimpl -S

 To add class associations to one or more implementations:

    regimpl -a -c <str> [-c ...] -i <str> [-i ...rbrk.

 To delete class associations from all, or selected, implementations:

   regimpl -d -c <str> [-c ...][-i <str> [-...]]

 To list classes associated with all, or selected, implementation:

    regimpl -l [-i <str> [-i ...]]

 The following parameters are used in the commands described above:


  -i <str>    = Implementation alias name(maximum of 16 -i names)
  -p <str>    = Server program name  (default: somdsvr)
  -v <str>    = Server-class name  (default: SOMDServer)
  -f <str>    = Reference data file name  (optional)
  -b <str>    = Reference data backup file name  (optional)
  -h <str>    = Host machine name  (default: localhost)
  -m {on|off} = Enable multi-threaded server  (optional)
  -z <str>    = Implementation ID (optional)
  -c <str>    = Class name(maximum of 16 -c names)
  -n {on|off} = Designate the server as nonstoppable (optional),
                meaning that the server cannot be stopped using
                the SOMDServerMgr interfaces or the "dsom" utility.


ΓòÉΓòÉΓòÉ 7.16.5. Programmatic interface to the Implementation Repository ΓòÉΓòÉΓòÉ

The Implementation Repository can be accessed and updated dynamically using the
programmatic interface provided by the ImplRepository class (defined in
"implrep.idl"). The global variable SOMD_ImplRepObject is initialized by
SOMD_Init to point to the ImplRepositor object. The following methods are
defined on it:

            void  add_impldef (in ImplementationDef impldef)

      Adds an implementation definition to the Implementation Repository.
      (Note: The value of the "impl_id" attribute is ignored. A unique ImplId
      will be generated for the newly added ImplementationDef.)

            void  delete_impldef (in ImplId implid);

      Deletes an implementation definition from the Implementation Repository.

            void update_impldef(in ImplementationDef impldef);

      Updates the implementation definition (defined by the "impl_id" of the
      supplied implementationDef) in the Implementation Repository.

            ImplementationDef find_impldef(in ImplId implid);

      Returns a server implementation definition given its ID.

            ImplementationDef find_impldef_by_alias(in string alias_name);

      Returns a server implementation definition, given its user-friendly
      alias.

            sequence<ImplementationDef>  find_impldef_by_class
                                                        (in string classname)

      Returns a sequence of ImplementationDefs for those servers that have an
      association with the specified class. Typically, a server is associated
      with the classes it knows how to implement, by registering its known
      classes via the add_class_to_impldef method.

            ORBStatus  find_all_impldefs  (out sequence<ImplementationDef> outimpldefs);

      Retrieves all ImplementationDef objects in the Implementation Repository.

 The following methods maintain an association between server implementations
 and the names of the classes they implement. These methods effectively
 maintain a mapping of <className, Implid>.

            void add_class_to_impldef (in ImplId implid,
                                        in string classname);

      Associates a class, identified by name, with a server, identified by its
      ImplId. This type of association is used to lookup server implementations
      via the find_impldef_by_class method.

            void  remove_class_from_impldef (
                                         in ImplId implid,
                                         in string classname);

      Removes the association of a particular class with a server.

            void remove_class_from_all (

                                       in string classname);

      Removes the association of a particular class from all server
      implementations in the Implementation Repository.

            sequence<string>  find_classes_by_impldef
                                          (in ImplId implid);

      Returns a sequence of class names associated with a server.

 With the ImplRepository programmatic interface, it is possible for an
 application to define additional server implementations at run time.


ΓòÉΓòÉΓòÉ 7.17. Running DSOM Applications ΓòÉΓòÉΓòÉ

Prior to starting the DSOM processes, the DSOM executables should be installed
and the DSOM environment variables should be set appropriately, as discussed in
the earlier section, "Configuring DSOM Applications."


ΓòÉΓòÉΓòÉ 7.17.1. Running the DSOM daemon (somdd) ΓòÉΓòÉΓòÉ

To run a DSOM application, the DSOM daemon, somdd, must be started: The daemon
can be started manually from the command line, or could be started
automatically from a start-up script run at boot time. It may be run in the
background with the commands start somdd. (The somdd command has the following
syntax:

somdd [-q]

where the optional -q flag signifies "quiet" mode.  By default, somdd will
produce a" ready" message when the DSOM daemon is ready to process requests,
and it will produce diagnostic messages as errors are encountered if the
SOMDDEBUG environment variable is set to 1. In quiet mode, however, the "ready"
message will not appear, and diagnostic messages will not appear even if
SOMDEBUG is set. Alternatively, if the SOMDMESSAGELOG environment variable is
set, diagnostic error messages will be sent directly to the specified message
log file, regardless of whether the -q flag is specified.

 The somdd daemon is responsible for "binding" a client process to a server
 process and will activate the desired server if necessary. The binding
 procedure is such that the client will consult the Implementation Repository
 to find out which machine contains a desired server, and will then contact the
 DSOM daemon on the server's machine to retrieve the server's communications
 address (a port). Servers are activated dynamically as separate processes.


ΓòÉΓòÉΓòÉ 7.17.2. Running DSOM servers ΓòÉΓòÉΓòÉ

Once the somdd daemon is running, application programs can be started. If the
application uses the generic SOM server, somdsvr, it can be started either from
the command line or automatically upon demand. When starting somdsvr from the
command line, the server's implementation ID or alias must be supplied as an
argument. The command syntax for starting a generic SOM server is:

   somdsvr  [ impl_id  | -a alias ]

For example, the command

>  somdsvr  2ad2688fb-00389c00-7f-00-10005ac900d8

would start a somdsvr for an implementation with the specified ID. Likewise,
the command

>  somdsvr  -a  myServer

would start a somdsvr that represents an implementation of "myServer".


ΓòÉΓòÉΓòÉ 7.18. DSOM as a CORBA-compliant Object Request Broker ΓòÉΓòÉΓòÉ

The Object Management Group (OMG) consortium defines the notion of an Object
Request Broker (ORB) that supports access to remote objects in a distributed
environment. Thus, Distributed SOM is an ORB. SOM and DSOM together comply with
the OMG's specification of the Common Object Request Broker Architecture
(CORBA).

Since the interfaces of SOM and DSOM are largely determined by the CORBA
specification, the CORBA components and interfaces are highlighted in this
section.

The CORBA specification defines the components and interfaces that must be
present in an ORB, including the:

     Interface Definition Language (IDL) for defining classes (discussed in
      Chapter 4, "SOM IDL and the SOM Compiler").

     C usage bindings (procedure-call formats) for invoking methods on remote
      objects,

     Dynamic Invocation Interface and an Interface Repository, which support
      the construction of requests (method calls) at run time (for example, for
      interactive desktop applications), and

     Object Request Broker run-time programming interfaces.

 SOM and DSOM were developed to comply with these specifications (with only
 minor extensions to take advantage of SOM services). Although the capabilities
 of SOM are integral to the implementation of DSOM, the application programmer
 need not be aware of SOM as the implementation technology for the ORB.

 This section assumes some familiarity with The Common Object Request Broker:
 Architecture and Specification, Revision 1.1 (also referred to as "CORBA
 1.1"). The specification is published jointly by the Object Management Group
 and x/Open. The mapping of some CORBA 1.1 terms and concepts to DSOM terms and
 concepts is described in the remainder of this section.


ΓòÉΓòÉΓòÉ 7.18.1. Mapping OMG CORBA terminology onto DSOM ΓòÉΓòÉΓòÉ

This section discusses how various CORBA concepts and terms are defined in
terms of DSOM's implementation of the CORBA 1.1 standard.


ΓòÉΓòÉΓòÉ 7.18.1.1. Object Request Broker run-time interfaces ΓòÉΓòÉΓòÉ

In the previous sections, the SOMDObjectMgr and SOMDServer classes were
introduced. These are classes defined by DSOM to provide basic support in
managing objects in a distributed application. These classes are built upon
Object Request Broker interfaces defined  by  CORBA for building and
dispatching requests on objects. The ORB interfaces, SOMDObjectMgr and
SOMDServer, together provide the support for implementing distributed
applications in DSOM.

CORBA 1.1 defines the interfaces to the ORB components in IDL. In DSOM, the ORB
components are implemented as SOM classes whose interfaces are expressed using
the same CORBA 1.1 IDL. Thus, an application can make calls to the DSOM run
time using the SOM language bindings of its choice.

Interfaces for the following ORB run-time components are defined in CORBA 1.1,
and are implemented in DSOM. They are introduced briefly here, and discussed in
more detail throughout this chapter. (See the SOM Programming Reference for the
complete interface definitions.)

 Object               The Object interface defines operations on an "object
                      reference", which is the information needed to specify an
                      object within the ORB.

                      In DSOM, the class SOMDObject implements the CORBA 1.1
                      Object interface. (The"SOMD" prefix was added to
                      distinguish this class from SOMObject.) The subclass
                      SOMDClientProxy extends SOMDObject with support for proxy
                      objects.

 ORB                  (Object Request Broker) The ORB interface defines utility
                      routines for building requests and saving references to
                      distributed objects. The global variable SOMD_ORBObject
                      is initialized by SOMD_Init and provides the reference to
                      the ORB object.

 ImplementationDef    An ImplementationDef object is used to describe an
                      object's implementation. Typically, the ImplementationDef
                      describes the program that implements an object's server,
                      how the program is activated, and so on.

                      (CORBA 1.1 introduces ImplementationDef as the name of
                      the interface, but leaves the remainder of the IDL
                      specification to the particular ORB. DSOM defines an
                      interface for ImplementationDef.)

                      ImplementationDef objects are stored in the
                      Implementation Repository (defined in DSOM by the
                      ImplRepository class).

 InterfaceDef         An InterfaceDef object is used to describe an IDL
                      interface in a manner that can be queried and manipulated
                      at run time when building requests dynamically, for
                      example.

                      InterfaceDef objects are stored in the Interface
                      Repository (described fully in Chapter 7, "The Interface
                      Repository Framework").

 Request              A Request object represents a specific request on an
                      object, constructed at run-time. The Request object
                      contains the target object reference, operation (method)
                      name, a list of input and output arguments. A Request can
                      be invoked synchronously (wait for the response),
                      asynchronously (initiate the call, and later, get the
                      response), or as a "oneway" call (no response expected).

 NVList               An NVList is a list of NamedValue structures, used
                      primarily in building Request objects. A NamedValue
                      structure consists of a name, typed value, and some flags
                      indicating how to interpret the value, how to
                      allocate/free the value's memory, and so on.

 Context              A Context object contains a list of "properties" that
                      represent information about an application process's
                      environment. Each Context property consists of a
                      <name,string_value> pair,and is used by application
                      programs or methods much like the "environment variables"
                      commonly found in operating systems like AIX and OS/2 and
                      Windows. IDL method interfaces can explicitly list which
                      properties are queried by a method, and the ORB will pass
                      those property values to a remote target object when
                      making a request.

 Principal            A Principal object identifies the principal ("user") on
                      whose behalf a request is being performed.

                      (CORBA 1.1 introduces the name of the interface,
                      Principal, but leaves the remainder of the IDL
                      specification to the particular ORB. DSOM defines an
                      interface for Principal.)

 BOA                  (Basic Object Adapter) An Object Adapter provides the
                      primary interface between an implementation and the ORB
                      "core". An ORB may have a number of Object Adapters, with
                      interfaces that are appropriate for specific kinds of
                      objects.

 The Basic Object Adapter is intended to be a general-purpose Object Adapter
 available on all CORBA-compliant Object Request Brokers. The BOA interface
 provides support for generation of object references, identification of the
 principal making a call, activation and deactivation of objects and
 implementations, and method invocation on objects.

 In DSOM, BOA is defined as an abstract class. The SOMOA (SOM Object Adapter)
 class, derived from BOA, is DSOM's primary Object Adapter implementation. The
 SOMOA interface extends the BOA interface with several of its own methods that
 are not defined by CORBA 1.1.


ΓòÉΓòÉΓòÉ 7.18.1.2. Object references and proxy objects ΓòÉΓòÉΓòÉ

CORBA 1.1 defines the notion of an object reference, which is the information
needed to specify an object in the ORB. An object is defined by its
ImplementationDef, InterfaceDef, and application-specific "reference data" used
to identify or describe the object. An object reference is used as a handle to
a remote object in method calls. When a server wants to export a reference to
an object it implements, it supplies the object's ImplementationDef,
InterfaceDef, and reference data to the Object Adapter, which returns the
reference.

The structure of an object reference is opaque to the application, leaving its
representation up to the ORB.

In DSOM, an object reference is represented as an object that can simply be
used to identify the object on that server. The DSOM class that implements
simple object references is called SOMDObject (corresponding to Object in CORBA
1.1.) However, in a client's address space, DSOM represents the remote object
with a proxy object in order to allow the client to invoke methods on the
target object as if it were local. When an object reference is passed from a
server to a client, DSOM dynamically and automatically creates a proxy in the
client for the remote object. Proxies are specialized forms of SOMDObject;
accordingly, the base proxy class in DSOM SOMDClientProxy, is derived from
SOMDObject.

In order to create a proxy object, DSOM must first build a proxy class. It does
so automatically using SOM facilities for building classes at run time. The
proxy class is constructed using multiple inheritance: the proxy object
functionality is inherited from SOMDClientProxy, while just the interface of
the target class is inherited.

In the newly derived proxy class, DSOM overrides each method inherited from the
target class with a "remote dispatch" method that forwards an invocation
request to the remote object. Consequently, the proxy object provides location
transparency, and the client code invokes operations (methods) on the remote
object using the same language bindings as if it were a local target object.

For example, recall the "Stack" class used in the tutorial example given
earlier. When a server returns a reference to a remote "Stack" object to the
client, DSOM builds a "Stack_ _Proxy" class (note two underscores in the name),
derived from SOMDClientProxy and "Stack", and creates a proxy object from that
class.  When the client invokes the "push" method on the proxy,


_push(stk, &ev, 100);

the method is redispatched using the remote-dispatch method of the
SOMDClientProxy class, and the method is forwarded to the target object.

CORBA defines several special operations on object references that operate on
the local references (proxies) themselves, rather than on the remote objects.
These operations are defined by the classes SOMOA (SOM Object Adapter),
SOMDObject (which is DSOM's implementation of CORBA's Object "pseudo-class" and
ORB (Object Request Broker class). Some of these operations are listed below,
expressed in terms of their IDL definitions.

SOMOA methods (inherited from BOA):


            sequence <octet,1024> ReferenceData;
            SOMDObject  create (in ReferenceData id, in InterfaceDef intf
                                in ImplementationDef impl);

 Creates and returns an object reference.

 SOMDObject methods:

            SOMDObject  duplicate ( );

      Creates and returns a duplicate object reference.

            void  release ( );

      Destroys an object reference.

            boolean  is_nil ( );

      Tests to see if the object reference is NULL.

 ORB methods:

            string  object_to_string ( SOMDObject obj );

      Converts an object reference to a (storable) string form.

            SOMDObject  string_to_object ( string str );

      Converts a string form back to the original object reference.


ΓòÉΓòÉΓòÉ 7.18.1.3. Creation of remote objects ΓòÉΓòÉΓòÉ

The OMG has standardized an "object lifecycle service," built on top of the
ORB, for creating and destroying remote objects.  Currently, DSOM provides its
own interface for creating and destroying objects (see "Basic Client
Programming"), but a future release may provide an OMG-compliant lifecycle
service as well.


ΓòÉΓòÉΓòÉ 7.18.1.4. Interface definition language ΓòÉΓòÉΓòÉ

The CORBA specification defines an Interface Definition Language, IDL, for
defining object interfaces. The SOM Compiler compiles standard IDL interface
specifications, but it also allows the class implementer to include
implementation information that will be used in the implementation bindings for
a particular language.

Note:  Before IDL, SOM (version 1.0) had its own Object Interface Definition
       Language (OIDL). SOM classes specified using OIDL must be converted to
       IDL before they can be used with DSOM. The SOMobjects Developer Toolkit
       provides a migration tool for this purpose.


ΓòÉΓòÉΓòÉ 7.18.1.5. C language mapping ΓòÉΓòÉΓòÉ

The CORBA specification defines the mapping of method interface definitions to
C language procedure prototypes, hence SOM defines the same mapping. This
mapping requires passing a reference to the target object and a reference to an
implementation-specific Environment structure as the first and second
parameters, respectively, in any method call.

The Environment structure is primarily used for passing error information from
a method back to its caller. See also the topic "Exceptions and Error Handling"
in Chapter 3, "Using SOM Classes in Client Programs," for a description of how
to "get" and "set" error information in the Environment structure.


ΓòÉΓòÉΓòÉ 7.18.1.6. Dynamic Invocation Interface (DII) ΓòÉΓòÉΓòÉ

The CORBA specification defines a Dynamic Invocation Interface (DII) that can
be used to dynamically build requests on remote objects. This interface is
described in section 6 (page 105) of the CORBA 1.1 document, and is implemented
in DSOM. The DSOM implementation of the DII is described later in this chapter,
in the topic entitled "Dynamic Invocation Interface" under Section 6.9
"Advanced topics." Note that, in DSOM, somDispatch is overridden so that method
invocations on proxy objects are forwarded to the remote target object. SOM
applications can use the SOM somDispatch method for dynamic method calls
whether the object is local or remote.


ΓòÉΓòÉΓòÉ 7.18.1.7. Implementations and servers ΓòÉΓòÉΓòÉ

The CORBA specification defines the term implementation as the code that
implements an object. The implementation usually consists of a program and
class libraries.

Servers are processes that execute object implementations. CORBA 1.1 defines
four activation policies for server implementations: shared, unshared,
server-per-method, and persistent, as follows.

     A shared server implements multiple objects (of arbitrary classes the
      same time, and allows multiple methods to be invoked at the same time.

     An unshared  server, conversely, implements only a single object and
      handles one request at a time.

     The server-per-method  policy requires a separate process to be created
      for each request on an object and, usually, a separate program implements
      each method.

      Under the shared, unshared, and server-per-method activation policies,
      servers are activated automatically (on demand).

     A persistent  server, by contrast, is a shared server that is activated
      "by hand" (for example, from the command shell or from a startup script),
      vs. being activated automatically when the first method is dispatched to
      it.

 The term "persistent server" refers to the relative lifetime of the server: it
 is "always running" when DSOM is running. (CORBA implies that persistent
 servers are usually started at ORB boot time.) It should not be assumed,
 however, that a "persistent" server necessarily implements persistent objects
 (that persist between ORB reboots).

 In DSOM, specific process models are implemented by the server program. That
 is, DSOM simply starts a specified program when a client attempts to connect
 to a server. The four CORBA activation policies, or any other policies, can be
 implemented by the application as necessary. For example,

     an object that requires a server-per-method implementation could itself
      spawn a process at the beginning of each method execution. Alternatively,
      the server object in the "main" server can spawn a process before each
      method dispatch.

     a dedicated server could be registered for each object that requires an
      unshared server implementation (separate process). This may be done
      dynamically (see the topic "Programmatic interface to the Implementation
      Repository" earlier in this chapter).

 An ImplementationDef object, as defined by the CORBA specification, describes
 the characteristics of a particular implementation. In DSOM, an
 ImplementationDef identifies an implementation's unique ID, the program name,
 its location, and so forth. The ImplementationDef objects are stored in an
 Implementation Repository, which is represented in DSOM by an ImplRepository
 object.

 A CORBA-compliant ORB must provide the mechanisms for a server program to
 register itself with the ORB. To "register itself with the ORB" simply means
 to tell the ORB enough information about the server process so that the ORB
 will be able to locate, activate, deactivate, and dispatch methods to the
 server process. DSOM supports these mechanisms, so that server programs
 written in arbitrary languages can be used with DSOM. (See also the next
 topic, "Object Adapters.")

 In addition to the generic registration mechanisms provided by all
 CORBA-compliant ORBs, DSOM provides extra support for using SOM-class DLLs.
 DSOM provides a generic server  program that automatically registers itself
 with DSOM, loads SOM-class DLLs on demand, and dispatches incoming requests on
 SOM objects. Thus, by using the generic server program (when appropriate), a
 user may be able to avoid writing any server program code.


ΓòÉΓòÉΓòÉ 7.18.1.8. Object Adapters ΓòÉΓòÉΓòÉ

An Object Adapter (OA) provides the mechanisms that a server process uses to
interact with DSOM, and vice versa. That is, an Object Adapter is responsible
for server activation and deactivation, dispatching methods, activation and
deactivation of individual objects, and providing the interface for
authentication of the principal making a call.

DSOM defines a Basic Object Adapter (BOA) interface, described in the CORBA
specification, as an abstract class (a class having no implementation, only an
interface specification). The BOA interface represents generic Object Adapter
methods that a server written in an arbitrary language can use to register
itself and its objects with the ORB. Because it is an abstract class having no
implementation, however, the BOA class should not be directly instantiated.

DSOM provides a SOM Object Adapter, SOMOA, derived from the BOA interface, that
uses SOM Compiler and run-time support to accomplish dispatching of methods
(that is, accepting messages, turning them into method invocations, and routing
the invocations to the target object in the server process). SOMOA can be used
to dispatch methods on either SOM or non-SOM object implementations, as
described in the sections "Implementing Classes" and "Basic Server
Programming." It is possible to use non-SOM based implementations with SOMOA,
and often there is no additional programming required to use implementations
(class libraries) already developed using SOM.

The SOMOA works in conjunction with the application-defined server object to
map between objects and object references, and to dispatch methods on objects.
By partitioning out these mapping and dispatching functions into the server
object, the application can more easily customize them, without having to build
object adapter subclasses.

SOMOA introduces two methods that handle execution of requests received by the
server:

      execute_request_loop

      execute_next_request

 Typically, execute_request_loop is used to receive and execute requests,
 continuously, in the server's main thread. The execute_next_request method
 allows a single request to be executed. Both methods have a non-blocking
 option: when there are no messages pending, the method call will return
 instead of wait.

 If the server implementation has been registered as "multi-threaded" (via an
 IMPLDEF_MULTI_THREAD flag in the ImplementationDef), SOMOA will automatically
 run each request in a separate thread. If the "multi-thread" flag is not set,
 the server implementation can still choose to manage its own threads.

 The generic server program provided by DSOM (described in the preceding topic)
 uses execute_request_loop to receive and execute requests on SOM objects.


ΓòÉΓòÉΓòÉ 7.18.1.9. Extensions and limitations ΓòÉΓòÉΓòÉ

The DSOM implementation has the following extensions and limitations in its
implementation of the CORBA specification:

     As just described, the current release of DSOM supports a simple server
      activation policy, which is equivalent to the "shared" and "persistent"
      policies defined by CORBA. DSOM does not explicitly support the
      "unshared" or "server-per-method" server activation policies. Policies
      other than the basic activation scheme must be implemented by the
      application.

     DSOM provides null implementations for the object_is_ready or
      deactivate_obj methods, defined by the BOA interface for the unshared
      server activation policy.

     DSOM does not support the change_implementation method, defined by the
      BOAinterface to allow an application to change the implementation
      definition associated with an object. In DSOM, the ImplementationDef
      identifies the server which implements an object.  In these terms,
      changing an object's ImplementationDef would result in a change in the
      object's server ID. Any existing object references that have the old
      server ID would be rendered invalid.

      It is possible, however, to change the program which implements an
      object's server, or change the class library which implements an object's
      class. To modify the program associated with an ImplementationDef, use
      the update_impldef method defined on ImplRepository. To change the
      implementation of an object's class, replace the corresponding class
      library with a new (upward-compatible) one.

     The OUT_list_MEMORY, IN_COPY_VALUE, and DEPENDENT_LIST flags, used with
      the Dynamic Invocation Interface, are not yet supported.

     The SOM Object Adapter (SOMOA) provides a method (change_id) to update
      the ReferenceData  associated with an object reference created by the
      create call This is useful if the information which describes the object
      must be changed without invalidating copies of the existing object
      reference. CORBA defines no such method;  change_id is an extension to
      the standard BOA methods.

     The SOMOA provides some specialized object reference types which, in
      certain situations, are more efficient or easier-to-use than standard
      object references.

     DSOM supports the SOM extension to IDL that allows method parameters that
      are pointers. Structure, sequence, and array parameters may only contain
      pointers to objects (not arbitrary types).

     The Context::get_values method currently does not support the
      CTX_RESTRICT_SCOPE flag.


ΓòÉΓòÉΓòÉ 7.19. Advanced Topics ΓòÉΓòÉΓòÉ

The following subjects are discussed in this section:

     Peer vs. client/server processes
     Dynamic Invocation Interface
     Creating user-supplied proxies
     Customizing the default base proxy class


ΓòÉΓòÉΓòÉ 7.19.1. Peer vs. client/server processes ΓòÉΓòÉΓòÉ

The client/server model of distributed computing is appropriate when it is
convenient (or necessary) to centralize the implementation and management of a
set of shared objects in one or more servers. However, some applications
require more flexibility in the distribution of objects among processes.
Specifically, it is often useful to allow processes to manage and export some
of their objects, as well as access remote objects owned by other processes. In
these cases, the application processes do not adhere to a strict client/server
relationship; instead, they cooperate as "peers", behaving both as clients and
as servers.

Peer applications must be written to respond to incoming asynchronous requests,
in addition to performing their normal processing. In a multi-threaded system
(like NT), this is best accomplished by dedicating a separate process t  hread
that handles DSOM communications and dispatching. In systems that do not
currently support multi-threading (like AIX), peer applications must be
structured as event-driven programs.


ΓòÉΓòÉΓòÉ 7.19.1.1. Multi-threaded DSOM programs ΓòÉΓòÉΓòÉ

In a system that supports multi-threading, like NT, the easiest way to write a
peer DSOM program is to dedicate a separate thread to perform the usual
"server" processing. This body of this thread would contain the same code as
the simple servers described in section 6.4, "Basic Server Programming."

DSOM_thread(void *params)
{
   Environment ev;
   SOM_InitEnvironment(&ev);

/* Initialize the DSOM run-time environment */
   SOMD_Init(&ev);

/* Retrieve its ImplementationDef from the Implementation
   Repository by passing its implementation ID as a key */
   SOMD_ImplDefObject =
      _find_impldef(SOMD_ImplRepObject, &ev, *(ImplId *)params);

/* Create SOM Object Adapter and begin executing requests */
   SOMD_SOMOAObject = SOMOANew();
   _impl_is_ready(SOMD_SOMOAObject, &ev, SOMD_ImplDefObject);
   _execute_request_loop(SOMD_SOMOAObject, &ev, SOMD_WAIT);
}

Note:  The DSOM run time is "thread safe"; that is, DSOM protects its own  data
       structures and objects from race conditions and update conflicts.
       However, it is the application's responsibility to implement its own
       concurrency control for concurrent thread access to local shared
       application objects.


ΓòÉΓòÉΓòÉ 7.19.1.2. Event-driven DSOM programs using EMan ΓòÉΓòÉΓòÉ

EMan (see Chapter 9 on "The Event Management Framework") is not a replacement
for threads, but it supports processing of asynchronous requests. EMan allows a
program to handle events from multiple input sources; but the handlers run on a
single thread, under control of EMan's main loop.

DSOM provides a runtime function, SOMD_RegisterCallback, which is used by DSOM
to associate user-supplied event handlers with DSOM's communications sockets
and message queues with EMan. Example code is shown below.

DSOM server programs which use EMan must be very careful not to get into
deadlock situations. This is quite easy to do with DSOM, since method calls are
synchronous. If two cooperating processes simultaneously make calls on each
other, a deadlock could result. Likewise, if a method call on remote object B
from A requires a method call back to A, a deadlock cycle will exist. (Of
course, the number of processes and objects which create the cyclic dependency
could be greater than two.)

The application developer must be careful to avoid situations where cooperating
processes are likely to make calls upon each other, creating a cyclic
dependency. Some applications may find it appropriate to use oneway messages to
avoid deadlock cycles, since oneway messages do not cause a process to block.
It may also be possible for an application to defer the actual processing of a
method that may "call back" an originating process, by scheduling work using
EMan client events.


ΓòÉΓòÉΓòÉ 7.19.2. Dynamic Invocation Interface ΓòÉΓòÉΓòÉ

DSOM supports the CORBA dynamic invocation interface (DII), which clients can
use to dynamically build and invoke requests on objects. This section describes
how to use the DSOM DII. Currently, DSOM supports dynamic request invocation
only on objects outside the address space of the request initiator, via
proxies. The somDispatch method (non-CORBA) can be used to invoke methods
dynamically on either local or remote objects, however.

To invoke a request on an object using the DII, the client must explicitly
construct and initiate the request. A request is comprised of an object
reference, an operation, a list of arguments for the operation, and a return
value from the operation. A key to proper construction of the request is the
correct usage of the NamedValue structure and the NVList object. The return
value for an operation is supplied to the request in the form of a NamedValue
structure. In addition, it is usually most convenient to supply the arguments
for a request in the form of an NVList object, which is an ordered set of
NamedValues. This section begins with a description of NamedValues and NVLists
and then details the procedure for building and initiating requests.


ΓòÉΓòÉΓòÉ 7.19.2.1. The NamedValue structure ΓòÉΓòÉΓòÉ

The NamedValue structure is defined in C as:

typedef unsigned long Flags;

struct NamedValue {
        Identifier  name;         // argument name
        any         argument;     // argument
        long        len;          // length/count of arg value
        Flags       arg_modes;    // argument mode flags
};

where: name is an Identifier string as defined in the CORBA specification, and
argument is an any structure with the following declaration:


        struct any {
                TypeCode        _type;
                void*           _value;
        };

_type  is a TypeCode, which has an opaque representation with operations
defined on it to allow access to its constituent parts. Essentially the
Typecode is composed of a field specifying the CORBA type represented and
possibly additional fields needed to fully describe the type. See Chapter 7 of
this manual for a complete explanation of TypeCodes.

_value  is a pointer to the value of the any structure. Important: The contents
of "_value" should always be a pointer to the value, regardless of whether the
value is a primitive, a structure, or is itself a pointer (as in the case of
object references, strings and arrays). For object references, strings and
arrays, _value should contain pointer to the pointer that references the value.
For example:


        string          testString;
        any             testAny;

        testAny._value = &testString;

len is the number of bytes that the argument value occupies. The following
table gives the length of data values for the C language bindings. The value of
len must be consistent with the TypeCode.


Data type               Length
short                   sizeof(short)
unsigned short          sizeof(unsigned short)
long                    sizeof(long)
unsigned long           sizeof(unsigned long)
float                   sizeof(float)
double                  sizeof(double)
char                    sizeof(char)
boolean                 sizeof(boolean)
octet                   sizeof(octet)
string                  strlen(string) - does not include '\0' byte
enum E{}                sizeof(unsigned long)
union U                 sizeof(U)
struct S{}              sizeof(S)
Object                  1
array N of type T1      Length(T1)*N
sequence V of type T2   Length(T2)*V - V is the actual # of elements

The arg_modes field is a bitmask (unsigned long) and may contain the following
flag values:

ARG_IN              the associated value is an input-only argument
ARG_OUT             the associated value is an output-only argument
ARG_INOUT           the associated argument is an in/out argument

These flag values identify the parameter passing mode for the arguments.
Additional flag values have specific meanings for Request and NVList methods
and are listed with their associated methods.


ΓòÉΓòÉΓòÉ 7.19.2.2. The NVList class ΓòÉΓòÉΓòÉ

An NVList contains an ordered set of NamedValues. The CORBA specification
defines several operations that the NVList supports. The IDL prototypes for
these methods are as follows:


// get the number of elements in the NVList
ORBStatus get_count(
                out  long count );

// add an element to an NVList
ORBStatus add_item(
                in  Identifier      item_name,
                in  TypeCode    item_type,
                in  void* value,
                in  Flags item_flags );

// frees the NVList and any associated memory
ORBStatus free();


// frees dynamically allocated memory associated with the list
ORBStatus free_memory();

In DSOM, the NVList is a full-fledged object with methods for getting and
setting elements:


//set the contents of an element in an NVList
ORBStatus set_item(
                in  long            item_number, /* element # to set */
                in  Identifier      item_name,
                in  TypeCode        item_type,
                in  void*           item_value,
                in  long            value_len,
                in  Flags           item_flags );


// get the contents of an element in an NVList
ORBStatus get_item(
                in  long            item_number, /* element # to get */
                out Identifier      item_name,
                out TypeCode        item_type,
                out void*           item_value,
                out long            value_len,
                out Flags           item_flags );


ΓòÉΓòÉΓòÉ 7.19.2.3. Creating argument lists ΓòÉΓòÉΓòÉ

A very important use of the NVList is to pass the argument list for an
operation when creating a request. CORBA 1.1 specifies two methods, defined in
the ORB class, to build an argument list: create_list and
create_operation_list. The IDL prototypes for these methods are as follows:

ORBStatus create_list(
                        in  long        count,        /* # of items */
                        out NVList      new_list );

ORBStatus create_operation_list(
                        in  OperationDef oper,
                        out NVList       new_list );

The create_list method returns an NVList with the specified number of elements.
Each of the elements is empty. It is the client's responsibility to fill the
elements in the list with the correct information using the set_item method.
Elements in the NVList must contain the arguments in the same order as they
were defined for the operation. Elements are numbered from 0 to count-1.

The create_operation_list method returns an NVList initialized with the
argument descriptions for a given operation (specified by the OperationDef).
The arguments are returned in the same order as they were defined for the
operation. The client only needs to fill in the item_value and value_len in the
elemens of the NVList.

In addition to these CORBA-defined methods, DSOM provides a third version,
defined in the SOMDObject class. The IDL prototype for this method is as
follows:


ORBStatus create_request_args(
                        in Identifier operation,
                        out NVList arg_list,
                        out NamedValue result );

Like create_operation_list, the create_request_args method creates the
appropriate NVList for the specified operation.  In addition,
create_request_args initializes the NamedValue that will hold the result with
the expected return type. The create_request_args method is defined as a
companion to the create_request method, and has the advantage that the
InterfaceDef for the operation does not have to be retrieved from the Interface
Repository.

Note:  The create_request_args method is not  defined in CORBA 1.1. Hence, the
       create_operation_list method, defined on the ORB class, should be used
       instead when writing portable CORBA-compliant programs.


ΓòÉΓòÉΓòÉ 7.19.2.4. Building a Request ΓòÉΓòÉΓòÉ

There are two ways to build a Request object. Both begin by calling the
create_request method defined by the SOMDObject class. The IDL prototype for
create_request is as follows:

ORBStatus create_request(
                in  Context            ctx,
                in  Identifier         operation,
                in  NVList             arg_list,
                inout NamedValue       result,
                out Request            request,
                in  Flags              req_flags );

The arg_list can be constructed using the procedures described above and is
passed to the Request object in the create_request call. Alternatively,
arg_list can be specified as NULL and repetitive calls to add_arg  can be used
to specify the argument list. The add_arg method, defined by the Request class,
has the following IDL prototype:

ORBStatus add_arg(
                in  Identifier  name,
                in  TypeCode    arg_type,
                in  void*       value,
                in  long        len,
                in  Flags       arg_flags );


ΓòÉΓòÉΓòÉ 7.19.2.5. Initiating a Request ΓòÉΓòÉΓòÉ

There are two ways to initiate a request, using either the invoke or send
method defined by the Request class. The IDL prototypes for these two
operations ar as follows:


ORBStatus invoke(
                in Flags        invoke_flags );

ORBStatus send(
                in Flags        send_flags );

The invoke method calls the ORB, which handles the remote method invocation and
returns the result. This method will block while awaiting return of the result.

The send method calls the ORB but does not wait for the operation to complete
before returning. To determine when the operation is complete, the client must
call the get_response method (also defined by the Request class), which has
this IDL prototype:


ORBStatus get_response(
                in Flags        response_flags );

The following flag is defined for get_response:


RESP_NO_WAIT       Means that the caller does not want to wait for a

                 response.

get_response determines whether a request has competed. If the RESP_NO_WAIT
flag is set, get_response returns immediately even if the request is still in
progress. If RESP_NO_WAIT is not set, get_response waits until the request is
done before returning.


ΓòÉΓòÉΓòÉ 7.19.2.6. Example code ΓòÉΓòÉΓòÉ

Following is an incomplete example showing how to use the DII to invoke a
request having the method procedure prototype shown here:


string _testMethod( testObject              obj,
                    Environment             *ev,
                    long                    input_value,
);

main()
{
   ORBStatus rc;
   Environment ev;
   SOMDObject obj;
   NVList arglist;
   NamedValue result;
   Context ctx;
   Request reqObj;
   OperationDef opdef;
   Description desc;
   OperationDescription opdesc;
   static long input_value = 999;

   SOM_InitEnvironment(&ev);
   SOMD_Init(&ev);

/* create the argument list */
/* get the operation description from the interface repository */
   opdef = _lookup_id(SOM_InterfaceRepository, *ev,
                  "testObject::testMethod");
   desc = _describe(opdef, &ev);
   opdesc = (OperationDescription *) desc.value._value;

/* fill in the TypeCode field for the result */
   result.argument._type = opdesc->result;

/* Initialize the argument list */
   rc = _create_operation_list(SOMD_ORBObject, &ev, opdef,
                               &arglist);

/* get default context */
   rc = _get_default_context(SOMD_ORBObject, &ev, &ctx);

/* put value and length into the NVList */
   _get_item(arglist, &ev, 0, &name, &tc, &dummy, &dummylen,
             &flags);

   _set_item(arglist, &ev, 0, name, tc, &input_value,
             sizeof(input_value),flags);
    ...
/* create the request - assume the object reference came from
   somewhere -- from a file or returned by a previous request*/
   rc = _create_request(obj, &ev, ctx,
                        "testMethod", arglist, &result, &reqObj,
                        (Flags)0);

/* invoke request */
   rc = invoke(reqObj, &ev, (Flags)0);

/* print result */
   printf("result: %s\n",*(string*)(result.argument._value));
   return(0);
}


ΓòÉΓòÉΓòÉ 7.19.3. Creating user-supplied proxies ΓòÉΓòÉΓòÉ

DSOM uses a proxy object in the client's address space to represent the remote
object. As mentioned earlier in this chapter, the proxy object encapsulates the
operations necessary to forward and invoke methods on the remote object and
return the results. By default, proxy generation is done automatically by the
DSOM run time. However, if desired, the programmer can cause a user-supplied
proxy class to be loaded instead of letting the run time dynamically generate a
default proxy class. User-supplied proxies can be useful in specialized
circumstances when local processing or data caching is desired.

To build a user-supplied proxy class, it is necessary to understand a bit about
how dynamic proxy classes are constructed by the DSOM run time. The DSOM run
time constructs a proxy class by creating an instance of a class that inherits
the interface and implementation of SOMDClientProxy, and the interface (but not
the implementation) of the target class. The methods in the interface of the
target object are all overridden to call the somDispatch method (For more
details, see "Object references and proxy objects" in section 6.8.)

Every SOM object contains the somDispatch method, inherited from SOMObject.
This method is used to dynamically dispatch a method on an object, and can be
overridden with application-specific dispatching mechanisms. In
SOMDClientProxy, the somDispatch method is overridden to forward method calls
to the corresponding remote target object.

So, in effect, when a method is called on a default proxy object created by the
DSOM run time, it redispatches the method to the remote object using DSOM's
version of somDispatch.

Below is a simple example of a user-supplied proxy class. In this particular
example, the proxy object maintains a local, unshared copy of an attribute
("attribute_long") defined in the remote object ("Foo"), while forwarding
method invocations ("method1") on to the remote object. The result is that,
when multiple clients are talking to the same remote "Foo" object, each client
has a local copy of the attribute but all clients share the "Foo" object's
implementation of "method1".

Note:  It is important to understand that simply setting the attribute in one
       client's proxy does not affect the value of the attribute in other
       proxies. Maintaining consistency of the cached data values, if desired,
       is the responsibility of the user-supplied proxy class.

 Following is the IDL file for the "Foo" class:


  // foo.idl


  #include <somdtype.idl>
  #include <somobj.idl>


  interface Foo : SOMObject
  {
          string  method1(out string a, inout long b,
                          in ReferenceData c);
          attribute long attribute_long;


    implementation
    {
       releaseorder: method1, _set_attribute_long,
                     _get_attribute_long;
       dllname="foo.dll";
       somDefaultInit: override;
    };
  };

 The user-supplied proxy class is created by using multiple inheritance between
 SOMDClientProxy and the target object (in this case "Foo"). Thus, the IDL file
 for the user-supplied proxy class "Foo__Proxy" (note the two underscores) is
 as follows:


  // fooproxy.idl


  #include <somdcprx.idl>
  #include <foo.idl>


  interface Foo__Proxy : SOMDClientProxy, Foo
  {
    implementation
    {
       dllname="fooproxy.dll";
       method1: override;
    };
  };

 When a dynamic proxy class is created by the DSOM run time, the methods
 inherited from the target class are automatically overridden to use
 somDispatch. When you build a user-supplied proxy, you need to do this
 explicitly. This is why "method1" is overridden in the implementation section
 of the "fooproxy.idl" file.

 The implementation of "method1", which was added to the template produced by
 the SOM Compiler, simply calls the somDispatch method on "somSelf" Because
 "Foo__Proxy" has inherited the implementation of SOMDClientProxy, calling
 somDispatch within "method1" sends the method to the remote "Foo" object.

  /* fooproxy.c */

  #include <somdtype.h>
  #include <fooproxy.ih>

  SOM_Scope string  SOMLINK method1(Foo__Proxy somSelf,
                                    Environment *ev,
                                    string* a, long* b,
                                    ReferenceData* c)
  {
      string ret_str;
      somId  methodId;


  /*  Foo__ProxyData *somThis = Foo__ProxyGetData(somSelf); */
      Foo__ProxyMethodDebug("Foo__Proxy","method1");


      /* redispatch method, remotely */
      methodId = somIdFromString("method1");
      _somDispatch(somSelf, (void**)&ret_str,
                   methodId, somSelf, ev, a, b, c);
      SOMFree(methodId);


      return ret_str;
  }

 In summary, to build a user-supplied proxy class:

     Create the .idl file with the proxy class inheriting from both
      SOMDClientProxy and from the target class. Important: The user-supplied
      proxy class must be named "<targetClassName>_ _Proxy" (with two
      underscores in the name) and SOMDClientProxy must be the first class in
      the list of parent classes; for example,


                    interface Foo_ _Proxy : SOMDClientProxy, Foo

      Putting SOMDClientProxy first ensures that its version of somDispatch
      will be used to dispatch remote method calls.

      In the implementation section of the .idl file, override all methods that
      are to be invoked on the target class. Do not override methods that are
      to be invoked on the local proxy.

     Compile the .idl file. Be sure the Interface Repository gets updated with
      the .idl file. In the implementation file, for each overridden method,
      call somDispatch with the method name and parameters passed into the
      overridden method. If the proxy class provides an implementation for the
      somInit or somDefaultInit method, then it is important to ensure that
      calling that method more than once on the same proxy object has no
      negative effect.

     Build the DLL and place it in the PATH. Before creating the default
      proxy, the DSOM run time checks the PATH for a DLL containing the class
      named "   <targetClassName>_ _Proxy" If such a DLL is found, DSOM loads
      it instead of dynamically generating a proxy class.


ΓòÉΓòÉΓòÉ 7.19.4. Customizing the default base proxy class ΓòÉΓòÉΓòÉ

Continuing the example from the previous topic, imagine that an application
derives 100 subclasses from the "Foo" class. If the application wishes to cache
the "Foo::attribute_long" attribute in the proxies for all remote Foo-based
objects, the application could supply 100 user-supplied proxy classes,
developed in the manner described above. However, this would become a very
tedious and repetitive task!

Alternatively, it is possible to provide a customized base proxy class for use
in the dynamic generation of DSOM proxy classes. This allows an application to
provide a customized base proxy class, from which other dynamic DSOM proxy
classes can be derived. This is particularly useful in situations where an
application would like to enhance many or all dynamically generated proxy
classes with a common feature.

As described in the previous topic, proxy classes are derived from the
SOMDClientProxy class by default. It is the SOMDClientProxy class that
overrides somDispatch in order to forward method calls to remote objects.

The SOMDClientProxy class can be customized by deriving a subclass in the usual
way (being careful not to replace somDispatch or other methods that are
fundamental to implementing the proxy's behavior). To extend the above example
further, the application might define a base proxy class called "MyClientProxy"
that defines a long attribute called "attribute_long," which will be inherited
by Foo-based proxy classes.

The SOM IDL modifier baseproxyclass can be used to specify which base proxy
class DSOM should use during dynamic proxy-class generation. To continue the
example, if the class "MyClientProxy" were used to construct the proxy class
for a class "XYZ," then the baseproxyclass modifier would be specified s
follows:

// xyz.idl

#include <somdtype.idl>
#include <foo.idl>

interface XYZ : Foo
{
  ...
  implementation
  {
     ...
     baseproxyclass = MyClientProxy;
  };
};

It should be noted that:

     Base proxy classes must be derived from SOMDClientProxy.

     If a class "XYZ" specifies a custom base-proxy class, as in the above
      example, subclasses of "XYZ" do not  inherit the value of the
      baseproxyclass modifier. If needed, the  baseproxyclass modifier must be
      specified explicitly in each class.


ΓòÉΓòÉΓòÉ 7.20. Error Reporting and Troubleshooting ΓòÉΓòÉΓòÉ

When the DSOM run-time environment encounters an error during execution of a
method or procedure, a SYSTEM_EXCEPTION will be raised. The standard system
exceptions are discussed in the topic "Exceptions and Error Handling" in
Chapter 3 "Using SOM Classes in Client Programs." The "minor" field of the
returned exception value will contain a DSOM error code. The DSOM error codes
are listed below.

Although a returned exception value can indicate that a DSOM run-time error
occurred, it may be difficult for the user or application to determine what
caused the error. Consequently, DSOM has been enabled to report run-time error
information, for interpretation by IBM support personnel. These error messages
take the following form:


DSOM <type> error:  <code> [<name>] at  <file>:<line>

where the arguments are as follows:


type    SYSTEM_EXCEPTION type,
code    DSOM error code,
name symbol for DSOM error code (from "somderr.h"),
file    source-file name where the error occurred, and
line    line number where the error occurred.

For example,


DSOM NO_MEMORY error: 30001 [SOMDERROR_NoMemory] at somdobj.c:250

indicates that a "NO_MEMORY" error (error code 30001) occurred in file
"somdobj.c" at line 250. This information is not usually meaningful to the
user; it provides IBM support personnel with a starting point for problem
analysis. There will often be a sequence of error messages; together they
indicate the context in which the error occurred. It is important to give all
reported messages to IBM support personnel for analysis.

There is an environment variable, SOMDDEBUG, which is used to activate error
reporting. There is a corresponding global variable that can be set by an
application program; it is declared as:

extern long SOMD_DebugFlag;

Error reporting is normally disabled. To enable error reporting, the
environment variable SOMDDEBUG should be set to a value greater than 0. To
disable error reporting, SOMDDEBUG should be set to a value less than or equal
to 0.

By default, error messages will display on the standard output device. Error
messages can also be redirected to a log file. For this, the environment
variable SOMDMESSAGELOG should be set to the pathname of the log file. The
SOMD_ Init procedure opens the file named in SOMDMESSAGELOG (if any), during
process initialization.


ΓòÉΓòÉΓòÉ 7.20.1. Error codes ΓòÉΓòÉΓòÉ

The error codes that may be encountered when using DSOM are listed in Appendix
A, "SOMobjects Error Codes," which contains the codes for the entire SOMobjects
Toolkit.


ΓòÉΓòÉΓòÉ 7.20.2. Troubleshooting hints ΓòÉΓòÉΓòÉ

The following hints may prove helpful as you develop and test your DSOM
application.


ΓòÉΓòÉΓòÉ 7.20.2.1. Checking the DSOM setup ΓòÉΓòÉΓòÉ

This checklist will help you make certain that the DSOM environment is set up
correctly.

   1. For all application classes, IDL must be compiled into the Interface
      Repository.  You can verify that a class exists in the Interface
      Repository by executing "irdump<class>" See "Registering class
      interfaces" for more information.

   2. An implementation (a server program and class libraries) must be
      registered with DSOM by running the regimpl utility. See "Registering
      servers and classes" for more information.

   3. Verify that all class libraries and networking libraries are in
      directories specified in PATH.


ΓòÉΓòÉΓòÉ 7.20.2.2. Analyzing problem conditions ΓòÉΓòÉΓòÉ

The DSOM error codes mentioned below can be obtained directly by the
application from the "minor" field of the exception data returned in a system
exception, or from an error report message when SOMDDEBUG is set to a positive
integer value (see the previous topic, "Error reporting").

Symptom:  When running regimpl, a "PERSIST_STORE" or "NO_IMPLEMENT" exception
is returned. The DSOM error code is SOMDERROR_IO or SOMDERROR_NoImplDatabase.

     This may indicate that the Implementation Repository files are not found
      or cannot be accessed. Verify that SOMDDIR is set correctly, to a
      directory that has granted read/write permission to the DSOM user. (It is
      best if the directory name is fully qualified.) If the SOMDDIR variable
      is not set, verify that the default directory %SOMBASE%\etc\dsom) has
      been set up with the correct permissions. Ensure that the the files
      contained in the directory all have read/write permission granted to the
      DSOM user.

 Symptom:  When starting somdd, an "INITIALIZE" exception is returned with
 error code SOMDAlreadyRunning.

     This indicates that there is already an instance of somdd running.  If
      the current instance of somdd does not seem to be responding properly,
      delete all instances of somdd and restart a new copy of somdd.

 Symptom:  When starting up a server program, an exception is returned with a
 DSOM error code of SOMDERROR_ServerAlreadyExists.

     This may indicate that a server process that is already running has
      already registered itself with the DSOM daemon, somdd, using the
      implementation ID of the desired server program.

 Symptom:  A remote method invocation fails and an "INTF_REPOS" exception is
 returned. The DSOM error code is SOMDERROR_BadDescriptor or
 SOMDERROR_ClassNotlnIR.

     This may indicate that the interface describing the method or the method
      itself cannot be found in the Interface Repository. Verify that SOMIR is
      set correctly, and that the IR contains all interfaces used by your
      application.

      If the default SOM IR (supplied with the SOMobjects Toolkit and Runtimes)
      is not used by the application, the user's IR must include the interface
      definitions for the Sockets class, server class (derived from
      SOMDServer), and the definitions of the standard DSOM exceptions (found
      in file "stexcep.idl") that may be returned by a method call.

 Symptom:  A SOMDERROR_ClassNotFound error is returned by a client either when
 creating a remote object using somdNewObject, or when finding a server object
 using somdFindAnyServerByClass. (The methods are defined on the SOMDObjectMgr
 class.)

     This occurs when the class name specified in calls to somdNewObject or
      somdFindAnyServerByClass cannot be found in the Implementation
      Repository. Make sure that the class name has been associated with at
      least one of the server implementations.

 Symptom:  A SOMDERROR_ClassNotlnlR error is returned by a server when creating
 a new object using somdNewObject, somdCreateObj, or somdGetClassObj.

     This error may result if the DLL for the class cannot be found. Verify
      hat:

           -the interface of the object can be found in the IR;

           -the class name is spelled correctly and is appropriately scoped
           (for example, the "Printer" class in the "PrintServer" module must
           have the identifier "PrintServer:: Printer").

     This error can also result when the shared library is statically linked
      to the server program, but the <className>NewClass procedures have not
      been called to initialize the classes.

 Symptom: When invoking a method returns a proxy for a remote object in the
 client, a SOMDERROR_NoParentClass error occurs.

     This error may result when the class libraries used to build the proxy
      class are statically linked to the program, but the <className>NewClass
      procedure have not been called to initialize the classes.

 Symptom:  Following a method call, the SOM run-time error message, "A target
 object failed basic validity checks during method resolution" is displayed.

     Usually this means that the method call was invoked using a bad object
      pointer, or the object has been corrupted.

 Symptom:  A remote object has an attribute or instance variable that is, or
 contains, a pointer to a value in memory (for example, a string, a sequence,
 an "any"). The attribute or instance variable value is set by the client with
 one method call. When the attribute or instance variable is queried in a
 subsequent method call, the value referenced by the pointer is "garbage".

     This may occur because DSOM makes a copy  of argument values in a client
      call, for use in the remote call. The argument values are valid for the
      duration of that call. When the remote call is completed, the copies of
      the argument values are freed.

      In a DSOM application, a class should not assume ownership of memory
      passed to it in a method parameter unless the IDL description of the
      method includes the SOM IDL modifier object_owns_parameters.  Otherwise,
      if a parameter value is meant to persist between method invocations, then
      the object is responsible for making a copy of the parameter value.

 Symptom:  A method defines a (char *) parameter that is used to pass a string
 input value to an object. The object attempts to print the string value, but
 it appears to be "garbage".

     DSOM will support method arguments that are of type "pointer-to-X"
      (pointer types are a SOM extension), by dereference the pointer in the
      call, and copying the base value. The pointer-to-value is reconstructed
      on the server before the actual method call is made.

      While (char *) is commonly used to refer to NULL-terminated strings in C
      programs, (char *) could also be a pointer to a single character or to an
      array of characters. Thus, DSOM interprets the argument type literally as
      a pointer-to-one-character.

      To correctly pass strings or array arguments, the appropriate CORBA type
      should be used (for example, "string" or "char foo[4 ]").

 Symptom:  A segmentation violation occurs when passing an "any" argument to a
 method call, where the "any" value is a string, array, or object reference.
 Note: The NamedValues used in DII calls use "any" fields for the argument
 values.

     This error may occur because the  "_value"  field of the "any" structure
      does not contain the address of a pointer to the target string, array, or
      object reference, as it should. (A common mistake is to set the "_value"
      field to the address of the string, array, or object reference itself.)

 Symptom:  When a server program or a server object makes a call to get_id or
 to get_SOM_object on a SOMDObject, a "BAD_OPERATION" exception is returned
 with an error code of SOMDERROR_WrongRefType.

     This error may occur when the operation get_id is called on a SOMDObject
      that does not have any user-supplied ReferenceData (that is, the
      SOMDObject is a proxy, is nil, or is a simple "SOM ref" created by
      create_SOM_ref). Likewise, this error may occur when the operation
      get_SOM_object is called on a SOMDObject that was not created by the
      create_SOM_ref method.

 Symptom:  A segmentation fault occurs when a SOMD_Uninit call is executed.

     This error could occur if the application has already freed any of the
      DSOM run-time objects that were allocated by the SOMD_Init call,
      including SOMD_ObjectMgr, SOMD_ImplRepObject and SOMD_ORBObject.

 Symptom:  Unexplained program crashes.

     Verify that all DSOM environment variables are set, as described in the
      earlier section "Configuring DSOM Applications". Verify that all class
      libraries are in directories specified in in PATH. Verify that the
      contents of the Interface Repository, specified by SOMIR, are correct.
      Verify that the contents of the Implementation Repository, specified by
      SOMDDIR, are correct. Verify that somdd is running. Set SOMDDEBUG to 1 to
      obtain additional DSOM error messages.

 Symptom:  When starting somdd, an "INITIALIZE" error is returned with error
 code SOMDERROR_NoSocketsClass.

     If SOMSOCKETS is set, verify that the IR contains the Sockets interface
      definition.


ΓòÉΓòÉΓòÉ 7.21. Limitations ΓòÉΓòÉΓòÉ

The following list indicates known limitations of Distributed SOM at the time
of this release.

   1. Currently, objects cannot be moved from one server to another without
      changing the object references (i.e., deleting the object, and creating
      it anew in another server). This yields all copies of the previous
      reference invalid.

   2. The change_implementation method is not supported. This method, defined
      by the BOA interface, is intended to allow an application to change the
      implementation definition associated with an object. However, in DSOM,
      changing the server implementation definition may render existing object
      references (which contain the old server ID) invalid.

   3. Currently, DSOM has a single server activation policy, which corresponds
      to CORBA's "shared" activation policy for dynamic activation, and
      "persistent" activation policy for manual activation. Other activation
      policies, such as "server-per-method" and "unshared" are not directly
      supported, and must be implemented by the application.

      Since the unshared server policy is not directly supported, the
      object_is_ready and deactivate_obj methods, defined in the BOA interface
      have null implementations.

   4. If a server program terminates without calling deactivate_impl,
      subsequent attempts to start that server may fail. The DSOM daemon
      believes the server is still running until it is told it has stopped.
      Attempts to start a server that is believed to be exist results in an
      error (SOMDERROR_ServerAlreadyExists).

   5. Currently, file names used in ImplementationDefs are limited to 255
      bytes. Implementations aliases used in ImplementationDefs are limited to
      50 bytes. Class names used in the Implementation Repository are limited
      to 50 bytes. Hostnames are limited to 32 bytes.

   6. The OUT_LIST_MEMORY, IN_COPY_VALUE, and DEPENDENT_LIST flags, used with
      Dynamic Invocation Interface, are not yet supported.

   7. The Context::get_values method currently does not support the flag
      CTX_RESTRICT_SCOPE.

 Other important notes concerning DSOM are documented in the "README" file in
 the SOMBASE root directory ($SOMBASE on AIX, and %SOMBASE% on OS/2 or
 Windows).


ΓòÉΓòÉΓòÉ 8. The SOM Interface Repository Framework ΓòÉΓòÉΓòÉ

This section covers the following subjects:

     Introduction
     Using the SOM Compiler to Build an Interface Repository
     Managing Interface Repository files
     Programming with the Interface Repository Objects


ΓòÉΓòÉΓòÉ 8.1. Introduction ΓòÉΓòÉΓòÉ

The SOM Interface Repository (IR) is a database that the SOM Compiler
optionally creates and maintains from the information supplied in IDL source
files. The Interface Repository contains persistent objects that correspond to
the major elements in IDL descriptions. The SOM Interface Repository Framework
is a set of classes that provide methods whereby executing programs can access
these objects to discover everything known about the programming interfaces of
SOM classes.

The programming interfaces used to interact with Interface Repository objects,
as well as the format and contents of the information they return, are
architected and defined as part of the Object Management Group's CORBA
standard. The classes composing the SOM Interface Repository Framework
implement the programming interface to the CORBA Interface Repository.
Accordingly, the SOM Interface Repository Framework supports all of the
interfaces described in The Common Object Request Broker: Architecture and
Specification (OMG Document Number 91.12.1, Revision 1.1, chapter 7).

As an extension to the CORBA standard, the SOM Interface Repository Framework
permits storage in the Interface Repository of arbitrary information in the
form of SOM IDL modifiers. That is, within the SOM-unique implementation
section of an IDL source file or through the use of the #pragma modifier
statement, user-defined modifiers can be associated with any element of an IDL
specification. (See the section entitled "SOM Interface Definition Language" in
Chapter 4, "SOM IDL and the SOM Compiler.") When the SOM Compile creates the
Interface Repository from an IDL specification, these potentially arbitrary
modifiers are stored in the IR and can then be accessed via the methods
provided by the Interface Repository Framework.

This chapter describes, first, how to build and manage interface repositories,
and second, the programming interfaces embodied in the SOM Interface Repository
Framework.


ΓòÉΓòÉΓòÉ 8.2. Using the SOM Compiler to Build an Interface Repository ΓòÉΓòÉΓòÉ

The SOMobjects Toolkit includes an Interface Repository emitter that is invoked
whenever the SOM Compiler is run using an sc command with the -u option (which
"updates" the interface repository). The IR emitter can be used to create or
update an Interface Repository file. The IR emitter expects that an environment
variable, SOMIR, was first set to designate a file name for the Interface
Repository. For example, to compile an IDL source file named "newcls.idl" and
create an Interface Repository named "newcls.ir", use a command sequence
similar to the following:

set SOMIR=c:\myfiles\newcls.ir
sc -u newcls

If the SOMIR environment variable is not set, the Interface Repository emitter
creates a file named "som.ir" in the current directory.

The sc or command runs the Interface Repository emitter plus any other emitters
indicated by the environment variable SMEMIT (described in the topic "Running
the SOM Compiler" in Chapter 4, "Implementing SOM Classes"). To run the
Interface Repository emitter by itself, issue the sc command with the -s option
(which overrides SMEMIT) set to "ir". For example:

sc -u -sir newcls
or equivalently,

sc -usir newcls

The Interface Repository emitter uses the SOMIR environment variable to locate
the designated IR file. If the file does not exist, the IR emitter creates it.
If the named interface repository already exists, the IR emitter checks all of
the "type" information in the IDL source file being compiled for internal
consistency, and then changes the contents of the interface repository file to
agree with with the new IDL definition. For this reason, the use of the -u
compiler flag requires that all of the types mentioned in the IDL source file
must be fully defined within the scope of the compilation. Warning messages
from the SOM Compiler about undefined types result in actual error messages
when using the -u flag.

The additional type checking and file updating activity implied by the -u flag
increases the time it takes to run the SOM Compiler. Thus, when developing an
IDL class description from scratch, where iterative changes are to be expected,
it may be preferable not to use the -u compiler option until the class
definition has stabilized.


ΓòÉΓòÉΓòÉ 8.3. Managing Interface Repository files ΓòÉΓòÉΓòÉ

Just as the number of interface definitions contained in a single IDL source
file is optional, similarly, the number of IDL files compiled into one
interface repository file is also at the programmer's discretion. Commonly,
however, all interfaces needed for a single project or class framework are kept
in one interface repository.


ΓòÉΓòÉΓòÉ 8.3.1. The SOM  IR file "som.ir" ΓòÉΓòÉΓòÉ

The SOMobjects Toolkit includes an Interface Repository file ("som.ir") that
contains objects describing all of the types, classes, and methods provided by
the various frameworks of the SOMobjects Toolkit. Since all new classes will
ultimately be derived from these predefined SOM classes, some of this
information also needs to be included in a programmer's own interface
repository files.

For example, suppose a new class, called "MyClass", is derived from SOMObject.
When the SOM Compiler builds an Interface Repository for "MyClass", that IR
will also include all of the information associated with the SOMObject class.
This happens because the SOMObject class definition is inherited by each new
class; thus, all of the SOMObject methods and typedefs are implicitly contained
in the new class as well.

Eventually, the process of deriving new classes from existing ones would lead
to a great deal of duplication of information in separate interface repository
files. This would be inefficient, wasteful of space, and extremely difficult to
manage. For example, to make an evolutionary change to some class interface, a
programmer would need to know about and subsequently update all of the
interface repository files where information about that interface occurred.

One way to avoid this dilemma would be to keep all interface definitions in a
single interface repository (such as "som.ir"). This is not recommended,
however. A single interface repository would soon grow to be unwieldy in size
and become a source of frequent access contention. Everyone involved in
developing class definitions would need update access to this one file, and
simultaneous uses might result in longer compile times.


ΓòÉΓòÉΓòÉ 8.3.2. Managing IRs via the SOMIR environment variable ΓòÉΓòÉΓòÉ

The SOMobjects Toolkit offers a more flexible approach to managing interface
repositories. The SOMIR environment variable can reference an ordered list of
separate IR files, which process from left to right. Taken as a whole, however,
this gives the appearance of a single, logical interface repository. A
programmer accessing the contents of "the interface repository" through the SOM
Interface Repository framework would not be aware of the division of
information across separate files. It would seem as though all of the objects
resided in a single interface repository file.

A typical way to utilize this capability is as follows:

     The first (leftmost) Interface Repository in the SOMIR list would be
      "som.ir". This file contains the basic interfaces and types needed in all
      SOM classes.

     The second file in the list might contain interface definitions that are
      used globally across a particular enterprise.

     A third interface repository file would contain definitions that are
      unique to a particular department, and so on.

     The final interface repository in the list should be set aside to hold
      the interfaces needed for the project currently under development.

 Developers working on different projects would each set their SOMIR
 environment variables to hold slightly different lists. For the most part, the
 leftmost portions of these lists would be the same, but the rightmost
 interface repositories would differ. When any given developer is ready to
 share his/her interface definitions with other people outside of the immediate
 work group, that person's interface repository can be promoted to inclusion in
 the master list.

 With this arrangement of IR files, the more stable repositories are found at
 the left end of the list. For example, a developer should never need to make
 any significant changes to "som.ir", because these interfaces are defined by
 IBM and would only change with a new release of the SOMobjects Toolkit.

 The Interface Repository Framework only permits updates in the rightmost file
 of the SOMIR interface repository list. That is, when the SOM Compiler -u flag
 is used to update the Interface Repository, only the final file on the IR list
 will be affected. The information in all preceding interface repository files
 is treated as "read only".  Therefore, to change the definition of an
 interface in one of the more global interface repository files, a developer
 must overtly construct a special SOMIR list that omits all subsequent (that
 is, further to the right) interface repository files, or else petition the
 owner of that interface to make the change.

 It is important the the rightmost filename in the SOMIR interface repository
 list not appear elsewhere in the list.  For Example, the following setting for
 SOMIR:

       %SOMBASE5\ETC\SOM.IR;SOM.IR;C:\IR\COMPANY.IR;SOM.IR

 would cause problems when attempting to update the SOM.IR file, because SOM.IR
 appears twice in the list.

 Here is an example that illustrates the use of multiple IR files with the
 SOMIR environment variable. In this example, the SOMBASE environment variable
 represents the directory in which the SOMobjects Toolkit files have been
 installed. Only the "myown.ir" interface repository file will be updated with
 the interfaces found in files "myclass1.idl", "myclass2.idl", and
 "myclass3.idl".

  set BASE_IRLIST=%SOMBASE%\IR\SOM.IR;C:\IR\COMPANY.IR;C:\IR\DEPT10.IR
  set SOMIR=%BASE_IRLIST%;D:\MYOWN.IR
  set SMINCLUDE=.;%SOMBASE%\INCLUDE;C:\COMPANY\INCLUDE;C:\DEPT10\INCLUDE
  sc -usir myclass1
  sc -usir myclass2
  sc -usir myclass3


ΓòÉΓòÉΓòÉ 8.3.3. Placing `private' information in the Interface Repository ΓòÉΓòÉΓòÉ

When the SOM Compiler updates the Interface Repository in response to the -u
flag, it uses all of the information available from the IDL source file.
However, if the __PRIVATE__ preprocessor variable is used to designate certain
portions of the IDL file as private, the preprocessor actually removes that
information before the SOM Compiler sees it. Consequently, private information
will not appear in the Interface Repository unless the -p compiler option is
also used in conjunction with -u. For example:

sc -up myclass1

This command will place all of the information in the "myclass1.idl" file,
including the private portions, in the Interface Repository.

If you are using tools that understand SOM and rely on the Interface Repository
to describe the types and instance data in your classes, you may need to
include the private sections from your IDL source files when building the
Interface Repository.


ΓòÉΓòÉΓòÉ 8.4. Programming with the Interface Repository Objects ΓòÉΓòÉΓòÉ

The SOM Interface Repository Framework provides an object-oriented programming
interface to the IDL information processed by the SOM Compiler. Unlike many
frameworks that require you to inherit their behavior in order to use it, the
Interface Repository Framework is useful in its own right as a set of
predefined objects that you can access to obtain information. Of course, if you
need to subclass a class to modify its behavior, you can certainly do so; but
typically this is not necessary.

The SOM Interface Repository contains the fully-analyzed (compiled) contents of
all information in an IDL source file. This information takes the the form of
persistent objects that can be accessed from a running program. There are ten
classes of objects in the Interface Repository that correspond directly to the
major elements in IDL source files; in addition, one instance of another class
exists outside of the IR itself, as follows:

 Contained                All objects in the Interface Repository are instances
                          of classes derived from this class and exhibit the
                          common behavior defined in this interface.

 Container                Some objects in the Interface Repository hold (or
                          contain) other objects. (For example, a module
                          [ModuleDef] can contain an interface [InterfaceDef].)
                          All Interface Repository objects that hold other
                          objects are instances of classes derived from this
                          class and exhibit the common behavior defined by this
                          class.

 ModuleDef                An instance of this class exists for each module
                          defined in an IDL source file. ModuleDefs are
                          Containers, and they can  hold ConstantDefs,
                          TypeDefs,  ExceptionDefs, InterfaceDefs, and other
                          ModuleDefs.

 InterfaceDef             An instance of this class exists for each interface
                          named in an IDL source file. (One InterfaceDef
                          corresponds to one SOM class.)  InterfaceDefs are
                          Containers, and they can hold ConstantDefs, TypeDefs,
                          ExceptionDefs, AttributeDefs, and OperationDefs.

 AttributeDef             An instance of this class exists for each attribute
                          defined in an IDL source file. AttributeDefs are
                          found only inside of (contained by) InterfaceDefs.

 OperationDef             An instance of this class exists for each operation
                          (method, _set_method, and _get_method) defined in an
                          IDL source file. OperationDefs are Containers that
                          can hold ParameterDefs. OperationDefs are found only
                          inside of (contained by) InterfaceDefs.

 ParameterDef             An instance of this class exists for each parameter
                          of each operation (method) defined in an IDL source
                          file. ParameterDefs are found only inside of
                          (contained by) OperationDefs.

 TypeDef                  An instance of this class exists for each typedef,
                          struct, union, or enum defined in an IDL source file.
                          TypeDefs may be found inside of (contained by) any
                          Interface Repository Container except an
                          OperationDef.

 ConstantDef              An instance of this class exists for each constant
                          defined in an IDL source file. ConstantDefs may be
                          found inside (contained by) of any Interface
                          Repository Container except an OperationDef.

 ExceptionDef             An instance of this class exists for each exception
                          defined in an IDL source file. ExceptionDefs may be
                          found inside of (contained by) any Interface
                          Repository Container except an OperationDef.

 Repository               One instance of this class exists for the entire SOM
                          Interface Repository, to hold IDL elements that are
                          global in scope. The instance of this class does not,
                          however, reside within the IR itself.


ΓòÉΓòÉΓòÉ 8.4.1. Methods introduced by Interface Repository classes ΓòÉΓòÉΓòÉ

The Interface Repository classes introduce nine new methods, which are briefly
described below. Many of the classes simply override methods to customize them
for the corresponding IDL element; this is particularly true for classes
representing IDL elements that are only contained within another syntactic
element. Full descriptions of each method are found in the SOM Programming
Reference.

     Contained class methods (all IR objects are instances of this class and
      exhibit this behavior):

      describe                      Returns a structure of type Description
                                    containing all information defined in the
                                    IDL specification of the syntactic element
                                    corresponding to the target Contained
                                    object. For example, for a target
                                    InterfaceDef object, the describe method
                                    returns information about the IDL interface
                                    declaration. The Description structure
                                    contains a "name" field with an identifier
                                    that categorizes the description (such as,
                                    "InterfaceDescription") and a "value" field
                                    holding an "any" structure that points to
                                    another structure containing the IDL
                                    information for that particular element (in
                                    this example, the interface's IDL
                                    specifications).

      within                        Returns a sequence designating the
                                    object(s) of the IR within which the target
                                    Contained object is contained. For example,
                                    for a target TypeDef object, it might be
                                    contained within any other IR object(s)
                                    except an OperationDef object.

     Container class methods (some IR objects contain other objects and
      exhibit this behavior):

      contents                      Returns a sequence of pointers to the
                                    object(s) of the IR that the target
                                    Container object contains. (For example,
                                    for a target InterfaceDef object, the
                                    contents method returns a pointer to each
                                    IR object that corresponds to a part of the
                                    IDL interface declaration.) The method
                                    provides options for excluding inherited
                                    objects or for limiting the search to only
                                    a specified kind of object (such as
                                    AttributeDefs).

      describe_contents             Combines the describe and contents methods;
                                    returns a sequence of ContainerDescription
                                    structures, one for each object contained
                                    by the target Container object. Each
                                    structure has a pointer to the related
                                    object, as well as "name" and "value"
                                    fields resulting from the describe method.

      lookup_name                   Returns a sequence of pointers to objects
                                    of a given name contained within a
                                    specified Container object, or within
                                    (sub)objects contained in the specified
                                    Container object.

     ModuleDef class methods:

                                    Override describe and within.

     InterfaceDef class methods:

      describe_interface            Returns a description of all methods and
                                    attributes of a given interface definition
                                    object that are held in the Interface
                                    Repository.

                                    Also overrides describe and within.

     AttributeDef class method:

                                    Overrides describe.

     OperationDef class method:

                                    Overrides describe.

     ParameterDef class method:

                                    Overrides describe.

     TypeDef  class method:

                                    Overrides describe.

     ConstantDef class method:

                                    Overrides describe.

     ExceptionDef class method:

                                    Overrides describe.

     Repository class methods:

      lookup_id                     Returns the Contained object that has a
                                    specified RepositoryId.

      lookup_modifier               Returns the string value held by a SOM or
                                    user-defined modifier, given the name and
                                    type of the modifier, and the name of the
                                    object that contains the modifier.

      release_cache                 Releases, from the internal object cache,
                                    the storage used by all currently
                                    unreferenced Interface Repository objects.


ΓòÉΓòÉΓòÉ 8.4.2. Accessing objects in the Interface Repository ΓòÉΓòÉΓòÉ

As mentioned above, one instance of the Repository class exists for the entire
SOM Interface Repository. This object does not, itself, reside in the Interface
Repository (hence it does not exhibit any of the behavior defined by the
Contained class). It is, however, a Container, and it holds all ConstantDefs,
TypeDefs, ExceptionDefs, InterfaceDefs, and ModuleDefs that are global in scope
(that is, not contained inside of any other Containers).

When any method provided by the Repository class is used to locate other
objects in the Interface Repository, those objects are automatically
instantiated and activated. Consequently, when the program is finished using an
object from the Interface Repository, the client code should release the object
using the somFree method.

All objects contained in the Interface Repository have both a "name" and a
"Repository ID" associated with them. The name is not guaranteed to be unique,
but it does uniquely identify an object within the context of the object that
contains it. The Repository ID of each object is guaranteed to uniquely
identify that object, regardless of its context.

For example, two TypeDef objects may have the same name, provided they occur in
separate name scopes (ModuleDefs or InterfaceDefs). In this case, asking the
Interface Repository to locate the TypeDef object based on its name would
result in both TypeDef objects being returned. On the other hand, if the name
is looked up from a particular ModuleDef or InterfaceDef object, only the
TypeDef object within the scope of that ModuleDef or InterfaceDef would be
returned. By contrast, once the Repository ID of an object is known, that
object can always be directly obtained from the Repository object via its
Repository ID.

C or C++ programmers can obtain an instance of the Repository class using the
RepositoryNew macro. Programmers using other languages (and C/C++ programmers
without static linkage to the Repository class) should invoke the method
somGetInterfaceRepository on the SOMClassMgrObject. For example,

For C or C++ (static linkage):

#include <repostry.h>
Repository repo;

repo = RepositoryNew();

From other languages (and for dynamic linkage in C/C++):

   1. Use the somEnvironmentNew function to obtain a pointer to the
      SOMClassMgrObject, as described in Chapter 3, "Using SOM Classes."

   2. Use the somResolve or somResolveByName function to obtain a pointer to
      the somGetInterfaceRepository method procedure.

   3. Invoke the method procedure on the SOMClassMgrObject, with no additional
      arguments, to obtain a pointer to the Repository object.

 After obtaining a pointer to the Repository object, use the methods it
 inherits from Container or its own lookup_id method to instantiate objects in
 the Interface Repository. As an example, the contents method shown in the C
 fragment below activates every object with global scope in the Interface
 Repository and returns a sequence containing a pointer to every global object:

  #include <containd.h>      /* Behavior common to all
  IR objects */
  Environment *ev;
  int i;
  sequence(Contained) everyGlobalObject;

  ev = SOM_CreateLocalEnvironment(); /* Get an environment to use */
  printf ("Every global object in the Interface Repository:\n");

  everyGlobalObject = Container_contents (repo, ev, "all", TRUE);

  for (i=0; i < everyGlobalObject._length; i++) {
      Contained aContained;

      aContained = (Contained) everyGlobalObject._buffer[i];
      printf ("Name: %s, Id: %s\n",
          Contained__get_name (aContained, ev),
          Contained__get_id (aContained, ev));
      SOMObject_somFree (aContained);
  }

 Taking this example one step further, here is a complete program that accesses
 every object in the entire Interface Repository. It, too, uses the contents
 method, but this time recursively calls the contents method until every object
 in every container has been found:

  #include <stdio.h>
  #include <containd.h>
  #include <repostry.h>

  void showContainer (Container c, int *next);

  main ()
  {
      int count = 0;
      Repository repo;

      repo = RepositoryNew ();
      printf ("Every object in the Interface Repository:\n\n");
      showContainer ((Container) repo, &count);
      SOMObject_somFree (repo);
      printf ("%d objects found\n", count);
      exit (0);
  }

  void showContainer (Container c, int *next)
  {
      Environment *ev;
      int i;
      sequence(Contained) everyObject;

      ev = SOM_CreateLocalEnvironment (); /* Get an environment */
      everyObject = Container_contents (c, ev, "all", TRUE);

      for (i=0; i<everyObject._length; i++) {
          Contained aContained;

          (*next)++;
          aContained = (Contained) everyObject._buffer[i];
          printf ("%6d. Type: %-12s id: %s\n", *next,
              SOMObject_somGetClassName (aContained),
              Contained__get_id (aContained, ev));
          if (SOMObject_somIsA (aContained, _Container))
              showContainer ((Container) aContained, next);
          SOMObject_somFree (aContained);
      }
  }

 Once an object has been retrieved, the methods and attributes appropriate for
 that particular object can then be used to access the information contained in
 the object. The methods supported by each class of object in the Interface
 Repository, as well as the classes themselves, are documented in the SOM
 Programming Reference.


ΓòÉΓòÉΓòÉ 8.4.3. A word about memory management ΓòÉΓòÉΓòÉ

Several conventions are built into the SOM Interface Repository with regard to
memory management. You will need to understand these conventions to know when
it is safe and appropriate to free memory references and also when it is your
responsibility to do so.

All methods that access attributes (such as, the _get_<attribute> methods)
always return either simple values or direct references to data within the
target object. This is necessary because these methods are heavily used and
must be fast and efficient. Consequently, you should never free any of the
memory references obtained through attributes. This memory will be released
automatically when the object that contains it is freed.

For all methods that give out object references (there are five: within,
contents, lookup_name, lookup_id,  and describe_contents), when finished with
the object, you are expected to release the object reference by invoking the
somFree method. (This is illustrated in the sample program that accesses all
Interface Repository objects.) Do not release the object reference until you
have either copied or finished using all of the information obtained from the
object.

The describe methods (describe, describe_contents, and describe_interface)
return structures and sequences that contain information. The actual structures
returned by these methods are passed by value (and hence should only be freed
if you have allocated the memory used to receive them). However, you may be
required to free some of the information contained in the returned structures
when you are finished. Consult the specific method in the SOM Programming
Reference for more details about what to free.

During execution of the describe and lookup methods, sometimes intermediate
objects are activated automatically. These objects are kept in an internal
cache of objects that are in use, but for which no explicit object references
have been returned as results. Consequently, there is no way to identify or
free these objects individually. However, whenever your program is finished
using all of the information obtained thus far from the Interface Repository,
invoking the release_cache method causes the Interface Repository to purge its
internal cache of these implicitly referenced objects. This cache will
replenish itself automatically if the need to do so subsequently arises.


ΓòÉΓòÉΓòÉ 8.4.4. Using TypeCode pseudo-objects ΓòÉΓòÉΓòÉ

Much of the detailed information contained in Interface Repository objects is
represented in the form of TypeCodes. TypeCodes are complex data structures
whose actual representation is hidden.  A TypeCode is an architected way of
describing in complete detail everything that is known about a particular data
type in the IDL language, regardless of whether it is a (built-in) basic type
or a  (user-defined) aggregate  type.

Conceptually, every TypeCode contains a "kind" field (which classifies it), and
one or more parameters that carry descriptive information appropriate for that
particular category of TypeCode. For example, if the data type is long, its
TypeCode would contain a "kind" field with the value tk_long. No additional
parameters are needed to completely describe this particular data type, since
long is a basic type in the IDL language.

By contrast, if the TypeCode describes an IDL struct, its "kind" field would
contain the value tk_struct, and it would possess the following parameters: a
string giving the name of the struct, and two additional parameters for each
member of the struct: a string giving the member name and another (inner)
TypeCode representing the member's type. This example illustrates the fact that
TypeCodes can be nested and arbitrarily complex, as appropriate to express the
type of data they describe. Thus, a structure that has N members will have a
TypeCode of tk_struct with 2N+1 parameters (a name and TypeCode parameter for
each member, plus a name for the struct itself).

A tk_union TypeCode representing a union with N members has 3N+2 parameters:
the type name of the union, the switch TypeCode, and a label value, member name
and associated TypeCode for each member. (The label values all have the same
type as the switch, except that the default member, if present, has a label
value of zero octet.)

A tk_enum TypeCode  (which represents an enum) has  N+1 parameters: the name of
the enum followed by a string for each enumeration identifier. A tk_string
TypeCode has a single parameter: the maximum string length, as an integer. (A
maximum length of zero signifies an unbounded string.)

A tk_sequence TypeCode has two parameters: a TypeCode for the sequence
elements, and the maximum size, as an integer.  (Again, zero signifies
unbounded.)

A tk_array TypeCode has two parameters: a TypeCode for the array elements, and
the array length, as an integer. (Arrays must be bounded.)

The tk_objref TypeCode represents an object reference; its parameter is a
repository ID that identifies its interface.

See the TypeCode_kind function of the Interface Repository Framework in the SOM
Programming Reference.

TypeCodes are not actually "objects" in the formal sense. TypeCodes are
referred to in the CORBA standard as pseudo-objects and described as "opaque".
This means that, in reality, TypeCodes are special data structures whose
precise definition is not fully exposed. Their implementation can vary from one
platform to another, but all implementations must exhibit a minimal set of
architected behavior. SOM TypeCodes support the architected behavior and have
additional capability as well (for example, they can be copied and freed).

Although TypeCodes are not objects, the programming interfaces that support
them adhere to the same conventions used for IDL method invocations in SOM.
That is, the first argument is always a TypeCode pseudo-object, and the second
argument is a pointer to an Environment structure. Similarly, the names of the
TypeCode functions are constructed like SOM's C-language method-invocation
macros (all functions that operate on TypeCodes are named
TypeCode_<function-name>). Because of this ostensible similarity to an IDL
class, the TypeCode programming interfaces can be conveniently defined in IDL
as shown below.

See the TypeCode_kind function of the Interface Repository Framework in the SOM
Programming Reference.

interface TypeCode {

enum TCKind {
    tk_null, tk_void,
    tk_short, tk_long, tk_ushort, tk_ulong,
    tk_float, tk_double, tk_boolean, tk_char,
    tk_octet, tk_any, tk_TypeCode, tk_Principal, tk_objref,
    tk_struct, tk_union, tk_enum, tk_string,
    tk_sequence, tk_array,

    // The remaining enumerators are SOM-unique extensions
    // to the CORBA standard.
    //
    tk_pointer, tk_self, tk_foreign
};

exception Bounds {};
// This exception is returned if an attempt is made
// by the parameter() operation (described below) to
// access more parameters than exist in the receiving
// TypeCode.

boolean equal (in TypeCode tc);
// Compares the argument with the receiver and returns TRUE
// if both TypeCodes are equivalent.  This is NOT a test for
// identity.

TCKind kind ();
// Returns the type of the receiver as a TCKind.

long param_count ();
// Returns the number of parameters that make up the
// receiving TypeCode.

any parameter (in long index) raises (Bounds);
// Returns the indexed parameter from the receiving TypeCode.
// Parameters are indexed from 0 to param_count()-1.
//
//  The remaining operations are SOM-unique extensions.
//

short alignment ();
// This operation returns the alignment required for an instance
// of the type described by the receiving TypeCode.

TypeCode copy (in TypeCode tc);
// This operation returns a copy of the receiving TypeCode.

void free (in TypeCode tc);
// This operation frees the memory associated with the
// receiving TypeCode. Subsequently, no further use can be
// made of the receiver, which, in effect, ceases to exist.

void print (in TypeCode tc);
// This operation writes a readable representation of the
// receiving TypeCode to stdout.  Useful for examining
// TypeCodes when debugging.

void setAlignment (in short align);
// This operation overrides the required alignment for an
// instance of the type described by the receiving TypeCode.

long size (in TypeCode tc);
// This operation returns the size of an instance of the
// type represented by the receiving TypeCode.
};

A detailed description of the programming interfaces for TypeCodes is given in
the SOM Programming Reference.


ΓòÉΓòÉΓòÉ 8.4.4.1. Providing `alignment' information ΓòÉΓòÉΓòÉ

In addition to the parameters in the TypeCodes that describe each type, a
SOM-unique extension to the TypeCode functionality allows each TypeCode to
carry alignment information as a "hidden" parameter. Use the TypeCode_alignment
function to access the alignment value. The alignment value is a short integer
that should evenly divide any memory address where an instance of the type will
occur.

If no alignment information is provided in your IDL source files, all TypeCodes
carry default alignment information. The default alignment for a type is the
natural boundary for the type, based on the natural boundary for the basic
types of which it may be composed. This information can vary from one hardware
platform to another. The  TypeCode will contain the default alignment
information appropriate to the platform where it was defined.

To provide alignment information for the types and instances of types in your
IDL source file, use the "align=N" modifier, where N is your specified
alignment. Use standard modifier syntax of the SOM Compiler to attach the
alignment information to a particular element in the IDL source file. In the
following example,  align=1 (that is, unaligned or no alignment) is attached to
the struct "abc" and to one particular instance of struct "def" (the instance
data item "y").

interface i {
    struct abc {
        long a;
        char b;
        long c;
    };
    struct def {
        char l;
        long m;
    };

    void foo ();

    implementation {

    //# instance data
        abc x;
        def y;
        def z;

    //# alignment modifiers
        abc: align=1;
        y: align=1;
    };
};

Be aware that assigning the required alignment information to a type does not
guarantee that instances of that type will actually be aligned as indicated. To
ensure that, you must find a way to instruct your compiler to provide the
desired alignment. In practice, this can be difficult except in simple cases.
Most compilers can be instructed to treat all data as aligned (that is, default
alignment) or as unaligned, by using a compile-time option or #pragma. The more
important consideration is to make certain that the TypeCodes going into the
Interface Repository actually reflect the alignment that your compiler
provides. This way, when programs (such as the DSOM Framework) need to
interpret the layout of data during their execution, they will be able to
accurately map your data structures. This happens automatically when using the
normal default alignment.

If you wish to use unaligned instance data when implementing a class, place an
"unattached" align=1 modifier in the implementation section. An unattached
align=N modifier is presumed to pertain to the class's instance data structure,
and will by implication be attached to all of the instance data items.

When designing your own public types, be aware that the best practice of all
(and the one that offers the best opportunity for language neutrality) is to
lay out your types carefully so that it will make no difference whether they
are compiled as aligned or unaligned!


ΓòÉΓòÉΓòÉ 8.4.4.2. Using the `tk_foreign' TypeCode ΓòÉΓòÉΓòÉ

TypeCodes can be used to partially describe types that cannot be described in
IDL (for example, a FILE type in C, or a specific class type in C++). The
SOM-unique extension tk_foreign is used for this purpose. A  tk_foreign
TypeCode contains three parameters:

   1. The name of the type,
   2. An implementation context string, and
   3. A length.

 The implementation context string can be used to carry an arbitrarily long
 description that identifies the context where the foreign type can be used and
 understood. If the length of the type is also known, it can be provided with
 the length parameter. If the length is not known or is not constant, it should
 be specified as zero. If the length is not specified, it will default to the
 size of a pointer. A tk_foreign TypeCode  can also have alignment information
 specified, just like any other TypeCode.

 Using the following steps causes the SOM Compiler to create a foreign TypeCode
 in the Interface Repository:

   1. Define the foreign type as a typedef SOMFOREIGN in the IDL source file.

   2. Use the #pragma modifier statement to supply the additional information
      for the TypeCode as modifiers. The implementation context information is
      supplied using the "impctx" modifier.

   3. Compile the IDL file using the -u option to place the information in the
      Interface Repository.

 For example:

  typedef SOMFOREIGN Point;
  #pragma modifier Point: impctx="C++ Point class",length=12,align=4;

 If a foreign type is used to define instance data, structs, unions,
 attributes, or methods in an IDL source file, it is your responsibility to
 ensure that the implementation and/or usage bindings contain an appropriate
 definition of the type that will satisfy your compiler. You can use the
 passthru statement in your IDL file to supply this definition. However, it is
 not recommended that you expose foreign data in attributes, methods, or any of
 the public types, if this can be avoided, because there is no guarantee that
 appropriate usage binding information can be provided for all languages. If
 you know that all users of the class will be using the same implementation
 language that your class uses, you may be able to disregard this
 recommendation.


ΓòÉΓòÉΓòÉ 8.4.4.3. TypeCode constants ΓòÉΓòÉΓòÉ

TypeCodes are actually available in two forms: In addition to the TypeCode
information provided by the methods of the Interface Repository, TypeCode
constants can be generated by the SOM Compiler in your C or C++ usage bindings
upon request. A TypeCode constant contains the same information found in the
corresponding IR TypeCode, but has the advantage that it can be used as a
literal in a C or C++ program anywhere a normal TypeCode would be acceptable.

TypeCode constants have the form TC_< typename>, where <typename> is the name
of a type (that is, a typedef, union, struct, or enum) that you have defined in
an IDL source file. In addition, all IDL basic types and certain types dictated
by the OMG CORBA standard come with pre-defined TypeCode constants (such as
TC_long, TC_short, TC_char, and so forth). A full list of the pre-defined
TypeCode constants can be found in the file "somtcnst.h". You must explicitly
include this file in your source program to use the pre-defined TypeCode
constants.

Since the generation of TypeCode constants can increase the time required by
the SOM Compiler to process your IDL files, you must explicitly request the
production of TypeCode constants if you need them. To do so, use the
"tcconsts" modifier with the -m option of the sc or somc command. For example,
the command

sc -sh -mtcconsts myclass.idl

will cause the SOM Compiler to generate a "myclass.h" file that contains
TypeCode constants for the types defined in "myclass.idl".


ΓòÉΓòÉΓòÉ 8.4.4.4. Using the IDL basic type `any' ΓòÉΓòÉΓòÉ

Some Interface Repository methods and TypeCode functions return information
typed as the IDL basic type any. Usually this is done when a wide variety of
different types of data may need to be returned through a common interface. The
type any actually consists of a structure with two fields: a _type field and a
_value field. The _value field is a pointer to the actual datum that was
returned, while the _type field holds a TypeCode that describes the datum.

In many cases, the context in which an operation occurs makes the type of the
datum apparent. If so, there is no need to examine the TypeCode unless it is
simply as a consistency check. For example, when accessing the first parameter
of a tk_struct TypeCode, the type of the result will always be the name of the
structure (a string). Because this is known ahead of time, there is no need to
examine the returned TypeCode in the any_type field to verify that it is a
tk_string TypeCode. You can just rely on the fact that it is a string; or, you
can check the TypeCode in the _type field to verify it, if you so choose.

An IDL any type can be used in an interface as a way of bypassing the strong
type checking that occurs in languages like ANSI C and C++. Your compiler can
only check that the interface returns the any structure; it has no way of
knowing what type of data will be carried by the any during execution of the
program. Consequently, in order to write C or C++ code that accesses the
contents of the any correctly, you must always cast the _value field to reflect
the actual type of the datum at the time of the access.

Here is an example of a code fragment written in C that illustrates how the
casting must be done to extract various values from an any:

#include <som.h>    /* For "any" & "Environment" typedefs */
#include <somtc.h>  /* For TypeCode_kind prototype        */

any result;
Environment *ev;

printf ("result._value = ");
switch (TypeCode_kind (result._type, ev)) {

    case tk_string:
        printf ("%s\n", *((string *) result._value));
        break;

    case tk_long:
        printf ("%ld\n", *((long *) result._value));
        break;

    case tk_boolean:
        printf ("%d\n", *((boolean *) result._value));
        break;

    case tk_float:
        printf ("%f\n", *((float *) result._value));
        break;

    case tk_double:
        printf ("%f\n", *((double *) result._value));
        break;

    default:
        printf ("something else!\n");
}

Note: Of course, an any has no restriction, per se, on the type of datum that
it can carry. Frequently, however, methods that return an any or that accept an
any as an argument do place semantic restrictions on the actual type of data
they can accept or return. Always consult the reference page for a method that
uses an any to determine whether it limits the range of types that may be
acceptable.


ΓòÉΓòÉΓòÉ 9. The Metaclass Framework ΓòÉΓòÉΓòÉ

In SOM, classes are objects; metaclasses are classes and thus are objects, too.
Also depicted are the three primitive class objects of the SOM run time:
SOMClass, SOMobject, and SOMClassMgr.


Primitive objects of the SOM run time

The important point to be aware of here is that any class that is a subclass of
SOMClass is a metaclass This chapter describes metaclasses that are available
in SOMobjects Toolkit. There are two kinds of metaclasses:

Framework metaclasses-metaclasses for building new metaclasses and Utility
metaclasses - metaclasses to help you write applications

Briefly, the SOMobjects Toolkit provides the following metaclasses of each
category for use by programmers:

     Framework metaclasses:

    SOMMBeforeAfter Used to create a metaclass that has "before" and "after"
           methods for all methods (inherited or introduced) invoked on
           instances of its classes.

     Utility metaclasses:

    SOMMSingleInstance Used to create a class that may have at most one
           instance.

    SOMMTraced Provides tracing for every invocation of all methods on
           instances of its classes.

 The following sections describe each metaclass more fully.


  Class organization of the Metaclass Framework.


ΓòÉΓòÉΓòÉ 9.1. A note about metaclass programming ΓòÉΓòÉΓòÉ

SOM metaclasses are carefully constructed so that they compose (see Section 8.1
below). If you need to create a metaclass, you can introduce new class methods,
and new class variables, but you should not override any of the methods
introduced by SOMClass. If you need more than this, request access to the
experimental Cooperative metaclass used to implement the Metaclass Framework
metaclasses described in this chapter.


ΓòÉΓòÉΓòÉ 9.2. Framework Metaclasses for "Before/After" Behavior ΓòÉΓòÉΓòÉ

This section covers the following subjects:

     The 'SOMMBeforeAfter' metaclass
     Composition of before/after metaclasses
     Notes and advantages of 'before/after' usage


ΓòÉΓòÉΓòÉ 9.2.1. The 'SOMMBeforeAfter' metaclass ΓòÉΓòÉΓòÉ

SOMMBeforeAfter is a metaclass that allows the user to create a class for which
a particular method is invoked before each invocation of every method,and for
which a second method is invoked after each invocation. SOMMBeforeAfter defines
two methods: sommBeforeMethod and sommAfterMethod.  These two methods are
intended to be overridden in the child of SOMMBeforeAfter to define the
particular "before" and "after" methods needed for the client application.

For example, suppose a "Barking" metaclass (a subclass of SOMMBeforeAfter
overrides the sommBeforeMethod and sommAfterMethod with a method that emits one
bark when invoked. Thus, one can create the "BarkingDog" class, whose instances
(such as "Lassie") bark twice when "disturbed" by a method invocation.


A hierarchy of metaclasses

The SOMMBeforeAfter metaclass is designed to be subclassed; a subclass (or
child) of SOMMBeforeAfter is also a metaclass. The subclass overrides
sommBeforeMethod or sommAfterMethod or both. These (redefned) methods are
invoked before and after any method supported by instances of the subclass
(these methods are called primary methods). That is, they are invoked before
and after methods invoked on the ordinary objects that are instances of the
class objects that are instances of the subclass of SOMMBeforeAfter.

The sommBeforeMethod returns a boolean value. This allows the "before" method
to control whether the "after" method and the primary method get invoked. If
sommBeforeMethod returns TRUE, normal processing occurs. If FALSE is returned,
neither the primary method nor the corresponding sommAfterMethod is invoked. In
addition, no more deeply nested before/after methods are invoked (see
"Composition of before/after metaclasses" below). This facility can be used,
for example, to allow a before/after metaclass to provide secure access to an
object. The implication of this convention is that, if sommBeforeMethod is
going to return FALSE, it must do any post-processing that might otherwise be
done in the "after" method.

Caution: somInit and somFree are among the methods that get before/after
behavior. This implies the following two obligations are imposed on the
programmer of a SOMMBeforeAfter class. First, the implementation must guard
against sommBeforeMethod being called before somInit has executed, and the
object is not yet fully initialized. Second, the implementation must guard
against sommAfterMethod being called after somFree, at which time the object no
longer exists (see the example "C implementation for 'Barking' metaclass"
below).

The following example shows the IDL needed to create a Barking metaclass. Just
run the appropriate emitter to get an implementation binding, and then provide
the appropriate "before" behavior and "after" behavior.

SOM IDL for 'Barking' metaclass

#ifndef Barking_idl
#define Barking_idl

#include <sombacls.idl>
interface  Barking : SOMMBeforeAfter
{
#ifdef __SOMIDL__
implementation
{
  //# Class Modifiers
  filestem = barking;
  callstyle = idl;

  //# Method Modifiers
  sommBeforeMethod : override;
  sommAfterMethod : override;
};
#endif /* __SOMIDL__ */
};
#endif  /* Barking_idl */

The next example shows an implementation of the Barking metaclass in which no
barking occurs when somFree is invoked.

C implementation for 'Barking' metaclass

#define Barking_Class_Source
#include <barking.ih>

static char *somMN_somFree = "somFree";
static somId somId_somFree = &somMN_somFree;

SOM_Scope boolean  SOMLINK sommBeforeMethod(Barking somSelf,
                                            Environment *ev,
                                            SOMObject object,
                                            somId methodId,
                                            va_list ap)
{
    if ( !somCompareIds( methodId, somId_somFree )
       printf( "WOOF" );
}

SOM_Scope void  SOMLINK sommAfterMethod(Barking somSelf,
                                        Environment *ev,
                                        SOMObject object,
                                        somId methodId,
                                        somId descriptor,
                                        somToken returnedvalue,
                                        va_list ap)
{
    if ( !somCompareIds( methodId, somId_somFree )
       printf( "WOOF" );
}


ΓòÉΓòÉΓòÉ 9.2.2. Composition of before/after metaclasses ΓòÉΓòÉΓòÉ

When there are two before/after metaclasses -"Barking" (as before) and
"Fierce", which has a sommBeforeMethod and sommAfterMethod that both growl
(that is, both methods make a "grrrr" sound when executed). The preceding
discussion demonstrated how to create a "FierceDog " or a "BarkingDog ", but
has not yet addressed the question of how to compose these properties of fierce
and barking. Composability means having the ability to easily create either a
"FierceBarkingDog" that goes "grrr woof woof grrr " when it responds to a
method call or a "BarkingFierceDog " that goes "woof grrr grrr woof" when it
responds to a method call.


Example for composition of before or after metaclasses.

There are several ways to express such compositions. Figure 1 depicts SOM IDL
fragments for three techniques in which composition can be indicated by a
programmer. These are denoted as Technique 1, Technique 2, and Technique 3,
each of which creates a "FierceBarkingDog" class, named "FB-1", "FB-2", and
"FB-3", respectively, as follows:

     In Technique 1, a new metaclass ("FierceBarking") is created with both
      the "Fierce" and "Barking" metaclasses as parents. An instance of this
      new metaclass (that is, "FB-1") is a "FierceBarkingDog" (assuming "Dog "
      is a parent).

     In Technique 2, a new class is created which has parents that are
      instances of "Fierce" and "Barking" respectively. That is, "FB-2" is a
      "FierceBarkingDog" also (assuming "FierceDog" and "BarkingDog" do not
      further specialize "Dog").

     In Technique 3, "FB-3", which also is a "FierceBarkingDog", is created by
      declaring that its parent is a "BarkingDog" and that its explicit
      (syntactically declared) metaclass is "Fierce".

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 Figure 1.Three techniques for composing before/after metaclasses.

 Note that the explicit metaclass in the SOM IDL of "FB-1" is its derived
 class, "FierceBarking". The derived metaclass of "FB-2" is also
 "FierceBarking". Lastly, the derived metaclass of "FB-3" is  not  the
 metaclass explicitly specified in the SOM IDL; rather, it  too is
 "FierceBarking."


ΓòÉΓòÉΓòÉ 9.2.3. Notes and advantages of 'before/after' usage ΓòÉΓòÉΓòÉ

Notes on the dispatching of before/after methods:

     A before (after) method is invoked just once per primary method
      invocation.

     The dispatching of before/after methods is thread-safe.

     The dispatching of before/after methods is fast. The time overhead for
      dispatching a primary method is on the order of N times the time to
      invoke a before/after method as a procedure, where N is the total number
      of before/after methods to be applied.

 In conclusion, consider an example that clearly demonstrates the power of the
 composition of before/after metaclasses. Suppose you are creating a class
 library that will have n classes. Further suppose there are p properties that
 must be included in all combinations for all classes. Potentially, the library
 must have n2p classes. Let us hypothesize that (fortunately) all these
 properties can be captured by before/after metaclasses. In this case, the size
 of the library is n+p.

 The user of such a library need only produce those combinations necessary for
 a given application. In addition, note that there is none of the usual
 programming. Given the IDL for a combination of before/after metaclasses, the
 SOM compiler generates the implementation of the combination (in either C or
 C++) with no further manual intervention.


ΓòÉΓòÉΓòÉ 9.3. The 'SOMMSingleInstance' Metaclass ΓòÉΓòÉΓòÉ

Sometimes it is necessary to define a class for which only one instance can be
created. This is easily accomplished with the SOMMSingleInstance metaclass.
Suppose the class "Collie" is an instance of SOMMSingleInstance. The first call
to CollieNew creates the one possible instance of "Collie"; hence, subsequent
calls to CollieNew return the first (and only) instance.

Any class whose metaclass is SOMMSingleInstance gets this requisite behavior;
nothing further needs to be done. The first instance created is always returned
by the <className>New macro.

Alternatively, the method sommGetSingleInstance does the same thing as the
<className>New macro. This method invoked on a class object (for example,
"Collie") is useful because the call site explicitly shows that something
special is occurring and that a new object is not necessarily being created.
For this reason, one might prefer the second form of creating a single-instance
object to the first.

Instances of SOMMSingleInstance keep a count of the number of times somNew and
sommGetSingleInstance are invoked. Each invocation of somFree decrements this
count. An invocation of somFree does not actually free the single instance
until the count reaches zero.

SOMMSingleInstance overrides somRenew, somRenewNoInit, somRenewNoInitNoZero,
and somRenewNoZero so that a proxy is created in the space indicated in the
somRenew* call. This proxy redispatches all methods to the single instance,
which is always allocated in heap storage. Note that all of these methods
(somRenew*) increment the reference count; therefore, somFree should be called
on these objects, too. In this case, somFree decrements the reference and frees
the single instance (and, of course, takes no action with respect to the
storage indicated in the original somRenew* call).

If a class is an instance of SOMMSingleInstance, all of its subclasse are also
instances of SOMMSingleInstance. Be aware that this also means that each
subclass is allowed to have only a single instance. (This may seem obvious.
However, it is a common mistake to create a framework class that must have a
single instance, while at the same time expecting users of the framework to
subclass the single instance class. The result is that two single-instance
objects are created: one for the framework class and one for the subclass. One
technique that can mitigate this scenario is based on the use of
somSubstituteClass. In this case, the creator of the subclass must substitute
the subclass for the framework class - before the instance of the framework
class is created.)


ΓòÉΓòÉΓòÉ 9.4. The 'SOMMTraced' Metaclass ΓòÉΓòÉΓòÉ

SOMMTraced is a metaclass that facilitates tracing of method invocations. If
class "Collie" is an instance of SOMMTraced (if SOMMTraced is the metaclass of
"Collie"), any method invoked on an instance of "Collie" is traced. That is,
before the method begins execution, a message prints (to standard output)
giving the actual parameters. Then, after the method completes execution, a
second message prints giving the returned value. This behavior is attained
merely by being an instance of the SOMMTraced metaclass.

If the class being traced is contained in the Interface Repository, actual
parameters are printed as part of the trace. If the class is not contained in
the Interface Repository, an ellipsis is printed.

To be more concrete, suppose that the class "Collie" is a child of "Dog" and is
an instance of SOMMTraced. Because SOMMTraced is the metaclass of "Collie," any
method invoked on "Lassie" (an instance of "Collie") is traced.

It is easy to use SOMMTraced: Just make a class an instance of SOMMTraced in
order to get tracing.

There is one more step for using SOMMTraced: Nothing prints unless the
environment variable SOMM_TRACED is set. If it is set to the empty string, all
traced classes print. If SOMM_TRACED is not the empty string, it should be set
to the list of names of classes that should be traced. For example, the
following command turns on printing of the trace for "Collie", but not for any
other traced class:

SET     SOMM_TRACED=Collie

The example below shows the IDL needed to create a traced dog class: Just run
the appropriate emitter to get an implementation binding.

SOM IDL for 'TracedDog' class

#include "dog.idl"
#include <somtrcls.idl>
interface TracedDog : Dog
{
#ifdef __SOMIDL__
implementation
{
  //# Class Modifiers
  filestem = trdog;
  metaclass = SOMMTraced;
};
#endif /* __SOMIDL__ */
};


ΓòÉΓòÉΓòÉ 9.5. Error Codes ΓòÉΓòÉΓòÉ

It is possible to receive the following messages from the Metaclass Framework
while an application is running.

 60001     An attempt was made to construct a class with SOMMSingleInstance as
           a metaclass constraint. (This may occur indirectly because of the
           construction of a derived metaclass). The initialization of the
           class failed because somInitMIClass defined by SOMMSingleInstance is
           in conflict with another metaclass that has overridden somNew. That
           is, some other metaclass has already claimed the right to return the
           value for somNew.

 60002     An attempt was made to construct a class with SOMMSingleInstance as
           a metaclass constraint. (This may occur indirectly because of the
           construction of a derived metaclass). The initialization of the
           class failed because somInitMIClass defined by SOMMSingleInstance is
           in conflict with another metaclass that has overridden somFree. That
           is, some other metaclass has already claimed this right to override
           somFree.

 60004     An invocation of somrRepInit was made with a logging type other than
           'o' or 'v'.

 60005     The sommBeforeMethod or the sommAfterMethod was invoke on a
           SOMRReplicbleObject whose logging type is other than 'o' or 'v'.
           This error cannot occur normally. The likely cause is that some
           method invoked on another object has overwritten this object's
           memory.

 60006     A Before/After Metaclass must override both sommBeforeMethod an
           sommAfterMethod. This message indicates an attempt to create a
           Before/After Metaclass where only one of the above methods is
           overridden.


ΓòÉΓòÉΓòÉ 10. The Event Management Framework ΓòÉΓòÉΓòÉ

The Event Management Framework is a central facility for registering all events
of an application. Such a registration facilitates grouping of various
application events and waiting on multiple events in a single event-processing
loop. This facility is used by the Replication Framework and by DSOM to wait on
their respective events of interest. The Event Management Framework must also
be used by any interactive application that contains DSOM or replicated
objects.


ΓòÉΓòÉΓòÉ 10.1. Event Management Basics ΓòÉΓòÉΓòÉ

The Event Management Framework consists of an Event Manager (EMan) class, a
Registration Data class and several Event classes. It provides a way to
organize various application events into groups and to process all events in a
single event-processing loop. The need for this kind of facility is seen very
clearly in interactive applications that also need to process some background
events (say, messages arriving from a remote process). Such applications must
maintain contact with the user while responding to events coming from other
sources.

One solution in a multi-threaded environment is to have a different thread
service each different source of events. For a single-threaded environment it
should be possible to recognize and process all events of interest in a single
main loop. EMan offers precisely this capability. EMan can be useful even when
multiple threads are available, because of its simple programming model. It
avoids contention for common data objects between EMan event processing and
other main-loop processing activity.


ΓòÉΓòÉΓòÉ 10.1.1. Model of EMan usage ΓòÉΓòÉΓòÉ

The programming model of EMan is similar to that of many GUI toolkits. The main
program initializes EMan and then registers interest in various types of
events. The main program ends by calling a non-returning function of EMan that
waits for events and dispatches them as and when they occur. In short, the
model includes steps that:

   1. Initialize the Event Manager,
   2. Register with EMan for all events of interest, and
   3. Hand over control to EMan to loop forever and to dispatch events.

 The Event Manager is a SOM object and is an instance of the SOMEEMan class.
 Since any application requires only one instance of this object, the SOMEEMan
 class is an instance of the SOMMSingleInstance class. Creation and
 initialization of the Event Manager is accomplished by a function call to
 SOMEEmanNew.

 Currently, EMan supports the four kinds of events described in the following
 topic. An application can register or unregister for events in a callback
 routine (explained below) even after control has been turned over to EMan.


ΓòÉΓòÉΓòÉ 10.1.2. Event types ΓòÉΓòÉΓòÉ

Event types are categorized as follows:

     Timer events

      These can be either one-time timers or interval timers.

     Sink events (sockets, file descriptors, and message queues)

      Sink events are not supported in Windows NT and Windows 95.

     Client events (any event that the application wants to queue with EMan)

      These events are defined, created, processed, and destroyed by the
      application. EMan simply acts as a place to queue these events for
      processing. EMan dispatches these client events whenever it sees them.
      Typically, this happens immediately after the event is queued.

     Work procedure events (procedures that can be called when there is no
      other event)

      These are typically background procedures that the application intends to
      execute when there are spare processor cycles. When there are no other
      events to process, EMan calls all registered work procedures. A work
      procedure event is called only after all other higher priority events
      have been called. A work procedure event is not called if there are no
      other events to be processed.

 The Event Management Framework is extendible (that is, other event types can
 be added to it) through subclassing. The event types currently supported by
 EMan are at a sufficiently low level so as to enable building other higher
 level application events on top of them. For example, you can build an X-event
 handler by simply registering the file descriptor for the X connection with
 EMan and getting notified when any X-event occurs.


ΓòÉΓòÉΓòÉ 10.1.3. Registration ΓòÉΓòÉΓòÉ

This topic illustrates how to register for an event type.


ΓòÉΓòÉΓòÉ 10.1.3.1. Callbacks ΓòÉΓòÉΓòÉ

The programmer decides what processing needs to be done when an event occurs
and then places the appropriate code either in a procedure or in a method of an
object. This procedure or method is called a callback. (The callback is
provided to EMan at the time of registration and is called by EMan when a
registered event occurs.) The signature of a callback is fixed by the framework
and must have one of the following three signatures:

void SOMLINK EMRegProc(SOMEEvent, void *);
void SOMLINK EMMethodProc(SOMObject, SOMEEvent, void *);
void SOMLINK EMMethodProcEv(SOMObject, Environment *Ev,
                            SOMEEvent, void *);
/* On OS/2, they all use "system" linkage  */
/* On Windows NT/95, they all use "__stdcall" linkage. */

The three specified prototypes correspond to a simple callback procedure, a
callback method using OIDL call style, and a callback method using IDL call
style. The parameter type SOMEEvent refers to an event object passed by EMan to
the callback.  Event objects are described below.

NOTE: When the callbacks are methods, EMan calls these methods using
Name-lookup Resolution (see Method Resolution in Chapter 5). One of the
implications is that at the time of registration EMan queries the target
object's class object to provide a method pointer for the method name supplied
to it. EMan uses this pointer for making event callbacks.


ΓòÉΓòÉΓòÉ 10.1.3.2. Event classes ΓòÉΓòÉΓòÉ

All event objects are instances of either the SOMEEvent class or a subclass of
it. The hierarchy of event classes is as follows:


SOMObject_________SOMEEvent_________ё__________SOMETimerEvent

                  ΓòÆ__________SOMEClientEvent

                  ΓòÆ__________SOMESinkEvent

                  ΓòÆ__________SOMEWorkProcEvent

When called by EMan, a callback expects the appropriate event instance as a
parameter. For example, a callback registered for a timer event expects a
SOMETimerEvent instance from EMan.


ΓòÉΓòÉΓòÉ 10.1.3.3. EMan parameters ΓòÉΓòÉΓòÉ

Several method calls in the Event Management Framework make use of bit masks
and constants as parameters (for example, EMSinkEvent or EMInputReadMask).
These methods are defined in the include file "eventmsk.h". When a user plans
to extend the Event Management Framework, care must be taken to avoid name and
value collisions with the definitions in "eventmsk.h". For convenience, the
contents of the "eventmsk.h" file are shown below.

#ifndef H_EVENTMASKDEF
#define H_EVENTMASKDEF

/* Event Types */
#define EMTimerEvent                    54
#define EMSignalEvent                   55
#define EMSinkEvent                     56

#define EMWorkProcEvent                 57

#define EMClientEvent                   58


/* Sink input/output condition mask */

#define EMInputReadMask                 (1L<0)
#define EMInputWriteMask                (1L<1)
#define EMInputExceptMask               (1L<2)

/* Process Event mask */

#define EMProcessTimerEvent             (1L<0)
#define EMProcessSinkEvent              (1L<1)
#define EMProcessWorkProcEvent          (1L<2)
#define EMProcessClientEvent            (1L<3)
#define EMProcessAllEvents              (1L<6)

#endif   /* H_EVENTMASKDEF */


ΓòÉΓòÉΓòÉ 10.1.3.4. Registering for events ΓòÉΓòÉΓòÉ

In addition to the event classes, the Event Management Framework uses a
registration data class (SOMEEMRegisterData) to capture all event-related
registration information. The procedure for registering interest in an event is
as follows:

   1. Create an instance of the SOMEEMRegisterData class (this will be referred
      to as a "RegData" object).

   2. Set the event type of "RegData."

   3. Set the various fields of "RegData" to supply information about the
      particular event for which an interest is being registered.

   4. Call the registration method of EMan, using "RegData" and the callback
      method information as parameters. The callback information varies,
      depending upon whether it is a simple procedure, a method called using
      OIDL call style, or a method called using IDL call style.

 The following code segment illustrates how to register input interest in a
 socket "sock" and provide a callback procedure "ReadMsg".

  data = SOMEEMRegisterDataNew( );       /* create a RegData object */
  _someClearRegData(data, Ev);
  _someSetRegDataEventMask(data,Ev,EMSinkEvent,NULL); /* Event type */
  _someSetRegDataSink(data, Ev, sock);     /* provide the socket id */
  _someSetRegDataSinkMask(data,Ev, EMInputReadMask );
                                                   /*input interest */
  regId = _someRegisterProc(some_gEMan,Ev,data,ReadMsg,"UserData" );
  /* some_gEMan points to EMan. The last parameter "userData" is any
     data the user wants to be passed to the callback procedure as a
     second parameter */


ΓòÉΓòÉΓòÉ 10.1.4. Unregistering for events ΓòÉΓòÉΓòÉ

One can unregister interest in a given event type at any time. To unregister,
you must provide the registration id returned by EMan at the time of
registration. Unregistering a non-existent event (such as, an invalid
registration id) is a no-op. The following example unregisters the socket
registered above:

_someUnRegister(some_gEMan, Ev, regId);


ΓòÉΓòÉΓòÉ 10.1.5. An example callback procedure ΓòÉΓòÉΓòÉ

The following code segment illustrates how to write a callback procedure:

void SOMLINK ReadMsg( SOMEEvent  event, void  *targetData )
{
int sock;
    printf( "Data = %s\n", targetData );
    switch( _somevGetEventType( event )) {
    case  EMSinkEvent:
        printf("callback: Perceived Sink Event\n");
        sock = _somevGetEventSink(event);
        /* code to read the message off the socket */
        break;
    default: printf("Unknown Event type in socket callback\n");
    }
}
/* On OS/2, "system" linkage is also required.  */


ΓòÉΓòÉΓòÉ 10.1.6. Generating client events ΓòÉΓòÉΓòÉ

While the other events are caused by the operating system (for example, Timer),
by I/O devices, or by external processes, client events are caused by the
application itself. The application creates these events and enqueues them with
EMan. When client events are dispatched, they are processed in a callback
routine just like any other event. The following code segment illustrates how
to create and enqueue client events.

clientEvent1 = SOMEClientEventNew();  /* create a client event */
_somevSetEventClientType( clientEvent1, Ev, "MyClientType" );
_somevSetEventClientData( clientEvent1, Ev,
                                   "I can give any data here");
/* assuming that "MyClientType" is already registered with EMan */
/* enqueue the above event with EMan */
_someQueueEvent(some_gEMan, Ev, clientEvent1);


ΓòÉΓòÉΓòÉ 10.1.7. Examples of using other events ΓòÉΓòÉΓòÉ

The sample program shipped with the Event Management Framework illustrates the
tasks listed below. (Due to its large size, the source code is not included
here.)

     Registering and unregistering for Timer events.

     Registering and unregistering for Workproc events.

   

     Registering and unregistering Client events.

   


ΓòÉΓòÉΓòÉ 10.1.8. Processing events ΓòÉΓòÉΓòÉ

After all registrations are finished, an application typically turns over
control to EMan and is completely event driven thereafter. Typically, an
application main program ends with the following call to EMan:

_someProcessEvents(some_gEMan, Ev);

An equivalent way to process events is to write a main loop and call
someProcessEvent from inside the main loop, as indicated:

while (1) { /* do forever */
      _someProcessEvent( some_gEMan, Ev, EMProcessTimerEvent  Γöé
                                         EMProcessSinkEvent   Γöé
                                         EMProcessClientEvent Γöé
                                         EMProcessWorkProcEvent );
      /***  Do other main loop work, as needed. ***/
}

The second way allows more precise control over what type of events to process
in each call. The example above enables all four types to be processed. The
required subset is formed by logically OR'ing the appropriate bit constants
(these are defined in "eventmsk.h)".  Another difference is that the second way
is a non-blocking call to EMan. That is, if there are no events to process,
control returns to the main loop immediately, whereas someProcessEvents is a
non-returning blocking call. For most applications, the first way of calling
EMan is better, since it does not waste processor cycles when there are no
events to process.


ΓòÉΓòÉΓòÉ 10.1.9. Interactive applications ΓòÉΓòÉΓòÉ

Interactive applications need special attention when coupled with EMan. Once
control is turned over to EMan by calling someProcessEvents, a single-threaded
application (for example, on AIX) has no way of responding to keyboard input.
The user must register interest in "stdin" with EMan and provide a callback
function that handles keyboard input. In a multi-threaded environment (for
example, Windows NT), this problem can be solved by spawning a thread to
execute someProcessEvents and another to handle keyboard input. (These two
options are illustrated in the sample program shipped with the Event Management
Framework.)


ΓòÉΓòÉΓòÉ 10.2. Event Manager Advanced Topics ΓòÉΓòÉΓòÉ

This section contains the following subjects:

     Extending EMan
     Using EMan from C++
     Using EMan from other languages
     Tips on using EMan


ΓòÉΓòÉΓòÉ 10.2.1. Extending EMan ΓòÉΓòÉΓòÉ

The current event manager can be extended without having access to the source
code. Several other extensions are possible. For example, new event types can
be defined by subclassing either directly from SOMEEvent class or from any of
its subclasses in the framework. There are three main problems to solve in
adding a new event type:

     How to register a new event type with EMan?

     How to make EMan recognize the occurrence of the new event?

     How to make EMan create and send the new event object (a subclass of
      SOMEEvent) to the callback when the event is dispatched?

 Because the registration information is supplied with appropriate "set"
 methods of a RegData object, the RegData object should be extended to include
 additional methods.  This can be achieved by subclassing from
 SOMEEMRegisterData and building a new registration data class that has methods
 to "set" and "get" additional fields of information that are needed to
 describe the new event types fully. To handle registrations with instances of
 new registration data subclass, we must also subclass from SOMEEMan and
 override the someRegister and the someUnRegister methods. These methods should
 handle the information in the new fields introduced by the new registration
 data class and call parent methods to handle the rest.

 Making EMan recognize the occurrence of the new event is primarily limited by
 the primitive events EMan can wait on. Thus the new event would have to be
 wrapped in a primitive event that EMan can recognize. For example, to wait on
 a message queue on Windows NT /95 concurrently with other EMan events, a
 separate thread can be made to wait on the message queue and to enqueue a
 client event with EMan when a message arrives on this message queue. We can
 thus bring multiple event sources into the single EMan main loop.

 The third problem of creating new event objects unknown to EMan can be easily
 done by applying the previous technique of wrapping the new event in terms of
 a known event. In a callback routine of the known event, we can create and
 dispatch the new event unknown to EMan. Of course, this does introduce an
 intermediate callback routine which would not be needed if EMan directly
 understood the new event type.

 A general way of extending EMan is to look for newly defined event types by
 overriding someProcessEvent and someProcessEvents in a subclass of EMan.


ΓòÉΓòÉΓòÉ 10.2.2. Using EMan from C++ ΓòÉΓòÉΓòÉ

The Event Management framework can be used from C++ just like any other
framework in the SOMobjects Toolkit. You must ensure that the C++ usage
bindings (that is, the .xh files) are available for the Event Management
Framework classes. These .xh files are generated by the SOM Compiler in the
SOMobjects Toolkit when the -s option includes an xh emitter.


ΓòÉΓòÉΓòÉ 10.2.3. Using EMan from other languages ΓòÉΓòÉΓòÉ

The event manager and the other classes can be used from other languages,
provided usage bindings are available for them. These usage bindings are
produced from .idl files of the framework classes by the appropriate language
emitter.


ΓòÉΓòÉΓòÉ 10.2.4. Tips on using EMan ΓòÉΓòÉΓòÉ

The following are some do's and don'ts for EMan:

     EMan callback procedures or methods must return quickly. You cannot wait
      for long periods of time to return from the callbacks. If such long
      delays occur, then the application may not notice some subsequent events
      in time to process them meaningfully (for example, a timer event may not
      be noticed until long after it occurred).

     It follows from the previous tip that you should not do independent
      "select" system calls on file descriptors while inside a callback.  (This
      applies to sockets and message queues, as well.) In general, a callback
      should not do any blocking of system calls. If an application must do
      this, then it must be done with a small timeout value.

     Since EMan callbacks must return quickly, no callback should wait on a
      semaphore indefinitely. If a callback has to obtain some semaphores
      during its processing, then the callback should try to acquire all of
      them at the very beginning, and should be prepared to abort and return to
      EMan if it fails to acquire the necessary semaphores.

     EMan callback methods are called using name-lookup resolution. Therefore,
      the parameters to an EMan registration call must be such that the class
      object of the object parameter must be able to provide a pointer to the
      method indicated by the method parameter. Although this requirement is
      satisfied in a majority of cases, there are exceptions.  For example, if
      the object is a proxy (in the DSOM sense) to a remote object, then the
      "real" class object cannot provide a meaningful method pointer. Also note
      that, when somDispatch is overridden, the effect of such an override will
      not  apply  to the callback from EMan.  Do not use a method callback in
      these situations; instead, use a procedure callback.


ΓòÉΓòÉΓòÉ 10.3. Limitations ΓòÉΓòÉΓòÉ

The present implementation of the Event Management framework has the
limitations described below. For a more up-to-date list of limitations, refer
to the README file on EMan in the SOMobjects Developer Toolkit. Sink Events are
not supported on Windows NT or on Windows 95.


ΓòÉΓòÉΓòÉ 10.3.1. Use of EMan DLL ΓòÉΓòÉΓòÉ

The Windows NT/95 Event Manager Framework does not use a Sockets class, so
SOMSockets cannot be set in the environment. Note that the EMan will look at
its thread message queue so that messages will continue to be processed. It is
best, however, to always run the EMan in its own thread.


ΓòÉΓòÉΓòÉ 11. SOMobjects Error Codes ΓòÉΓòÉΓòÉ

This section covers the following subjects:

     SOM Kernel Error Codes
     DSOM Error Codes
     Metaclass Framework Error Codes


ΓòÉΓòÉΓòÉ 11.1. SOM Kernel Error Codes ΓòÉΓòÉΓòÉ

Following are error codes with messages/explanations for the SOM kernel and the
various frameworks of the SOMobjects Developer Toolkit.

 Value     Symbolic Name and Description

 20011     SOMERROR_CCNullClass
           The somDescendedFrom method was passed a null class argument.

 20029     SOMERROR_SompntOverflow
           The internal buffer used in somPrintf overflowed.

 20039     SOMERROR_MethodNotFound
           somFindMethodOk failed to find the indicated method.

 20049     SOMERROR_StaticMethodTableOverflow
           A Method-table overflow occurred in somAddStaticMethod.

 20059     SOMERROR_DefaultMethod
           The somDefaultMethod was called; a defined method probably was not
           added before it was invoked.

 20069     SOMERROR_MissingMethod
           The specified method was not defined on the target object.

 20079     SOMERROR_BadVersion
           An attempt to load, create, or use a version of a class-object
           implementation is incompatible with the using program.

 20089     SOMERROR_NullId
           The SOM_CheckId was given a null ID to check.

 20099     SOMERROR_OutOfMemory
           Memory is exhausted.

 20109     SOMERROR_TestObjectFailure
           The somObjectTest found problems with the object it was testing.

 20119     SOMERROR_FailedTest
           The somTest detected a failure; generated only by test code.

 20121     SOMERROR_ClassNotFound
           The somFindClass could not find the requested class.

 20131     SOMERROR_OldMethod
           An old-style method name was used; change to an appropriate name.

 20149     SOMERROR_CouldNotStartup
           The somEnvironmentNew failed to complete.

 20159     SOMERROR_NotRegistered
           The somUnloadClassFile argument was not a registered class.

 20169     SOMERROR_BadOverride
           The somOverrideSMethod was invoked for a method that was not defined
           in a parent class.

 20179     SOMERROR_NotImplementedYet
           The method raising the error message is not implemented yet.

 20189     SOMERROR_MustOverride
           The method raising the error message should have been overridden.

 20199     SOMERROR_BadArgument
           An argument to a core SOM method failed a validity test.

 20219     SOMERROR_NoParentClass
           During the creation of a class object, the parent class could not be
           found.

 20229     SOMERROR_NoMetaClass
           During the creation of a class object, the metaclass object could
           not be found.


ΓòÉΓòÉΓòÉ 11.2. DSOM Error Codes ΓòÉΓòÉΓòÉ

The  following table lists the error codes that may be encountered when using
DSOM.

 Value     Description

 30001     SOMDERROR_NoMemory
           Memory is exhausted.

 30002     SOMDERROR_NotImplemented
           Function or method has a null implementation.

 30003     SOMDERROR_UnexpectedNULL
           Internal error:  a pointer variable was found to be NULL,
           unexpectedly.

 30004     SOMDERROR_IO
           I/O error while accessing a file located in SOMDDIR.

 30005     SOMDERROR_BadVersion
           Internal error:  incorrect version of an object reference data
           table.

 30006     SOMDERROR_ParmSize
           Internal error:  a parameter of incorrect size was detected.

 30007     SOMDERROR_HostName
           Communications error:  unable to retrieve local host name.

 30008     SOMDERROR_HostAddress
           Communications error:  unable to retrieve local host address.

 30009     SOMDERROR_SocketCreate
           Communications error:  unable to create socket.

 30010     SOMDERROR_SocketBind
           Communications error:  unable to bind address to socket.

 30011     SOMDERROR_SocketName
           Communications error:  unable to query socket information.

 30012     SOMDERROR_SocketReceive
           Communications error:  unable to receive message from socket.

 30013     SOMDERROR_SocketSend
           Communications error: indicates socket error.

 30014     SOMDERROR_SocketIoctl
           Communications error:  unable to set socket blocking state.

 30015     SOMDERROR_SocketSelect
           Communications error:  unable to select on socket.

 30016     SOMDERROR_PacketSequence
           Communications error:  unexpected message packet received.

 30017     SOMDERROR_PacketTooBig
           Communications error:  packet too big for allocated message space.

 30018     SOMDERROR_AddressNotFound
           Uninitialized DSOM communications object.

 30019     SOMDERROR_NoMessages
           No messages available (and caller specified "no wait").

 30020     SOMDERROR_UnknownAddress
           Invalid client or server address.

 30021     SOMDERROR_RecvError
           Communications error during receive.

 30022     SOMDERROR_SendError
           Communications error in sending the request.

 30023     SOMDERROR_CommTimeOut
           Communications timeout.

 30024     SOMDERROR_CannotConnect
           Unable to initialize connection information.

 30025     SOMDERROR_BadConnection
           Invalid connection information detected.

 30026     SOMDERROR_NoHostName
           Unable to get host name.

 30027     SOMDERROR_BadBinding
           Invalid server location information in proxy object.

 30028     SOMDERROR_BadMethodName
           Invalid method name in request message.

 30029     SOMDERROR_BadEnvironment
           Invalid Environment value in request message.

 30030     SOMDERROR_BadContext
           Invalid Context object in request message.

 30031     SOMDERROR_BadNVList
           Invalid Named Value List (NVList).

 30032     SOMDERROR_BadFlag
           Bad flag in NVList item.

 30033     SOMDERROR_BadLength
           Bad length in NVList item.

 30034     SOMDERROR_BadObjref
           Invalid object reference.

 30035     SOMDERROR_NullField
           Unexpected null field in request message.

 30036     SOMDERROR_UnknownReposId
           Attempt to use Invalid Interface Repository ID.

 30037     SOMDERROR_NVListAccess
           Invalid NVList object in request message.

 30038     SOMDERROR_NVIndexError
           Attempt to use an out-of-range NVList index.

 30039     SOMDERROR_SysTime
           Error retrieving system time.

 30040     SOMDERROR_SystemCallFailed
           System call failed.

 30041     SOMDERROR_CouldNotStartProcess
           Unable to start a new process.

 30042     SOMDERROR_NoServerClass
           No SOMDServer (sub)class specified for server implementation.

 30043     SOMDERROR_NoSOMDInit
           Missing SOMD_Init call in program.

 30044     SOMDERROR_SOMDDIRNotSet
           SOMDDIR environment variable not set.

 30045     SOMDERROR_NoImplDatabase
           Could not open Implementation Repository database.

 30046     SOMDERROR_ImplNotFound
           Implementation not found in implementation repository.

 30047     SOMDERROR_ClassNotFound
           Class not found in implementation repository.

 30048     SOMDERROR_ServerNotFound
           Server not found in somdd's active server table.

 30049     SOMDERROR_ServerAlreadyExists
           Server already exists in somdd's active server table.

 30050     SOMDERROR_ServerNotActive
           Server is not active.

 30051     SOMDERROR_CouldNotStartSOM
           SOM initialization error.

 30052     SOMDERROR_ObjectNotFound
           Could not find desired object.

 30053     SOMDERROR_NoParentClass
           Unable to find / load parent class during proxy class creation.

 30054     SOMDERROR_DispatchError
           Unable to dispatch method.

 30055     SOMDERROR_BadTypeCode
           Invalid type code.

 30056     SOMDERROR_BadDescriptor
           Invalid method descriptor.

 30057     SOMDERROR_BadResultType
           Invalid method result type.

 30058     SOMDERROR_KeyInUse
           Internal object key is in use.

 30059     SOMDERROR_KeyNotFound
           Internal object key not found.

 30060     SOMDERROR_CtxInvalidPropName
           Illegal context property name.

 30061     SOMDERROR_CtxNoPropFound
           Could not find property name in context.

 30062     SOMDERROR_CtxStartScopeNotFound
           Could not find specified context start scope.

 30063     SOMDERROR_CtxAccess
           Error accessing context object.

 30064     SOMDERROR_CouldNotStartThread
           System error:  Could not start thread.

 30065     SOMDERROR_AccessDenied
           System error:  Access to a system resource (file, queue, shared
           memory, etc.) denied.

 30066     SOMDERROR_BadParm
           System error:  invalid parameter supplied to a operating system
           call.

 30067     SOMDERROR_Interrupt
           System error:  Interrupted system call.

 30068     SOMDERROR_Locked
           System error:  Drive locked by another process.

 30069     SOMDERROR_Pointer
           System error:  Invalid physical address.

 30070     SOMDERROR_Boundary
           OS/2 system error:  ERROR_CROSSES_OBJECT_BOUNDARY.

 30071     SOMDERROR_UnknownError
           System error:  Unknown error on operating system call.

 30072     SOMDERROR_NoSpace
           System error:  No space left on device.

 30073     SOMDERROR_DuplicateQueue
           System error:  Duplicate queue name.

 30074     SOMDERROR_BadQueueName
           System error:  Invalid queue name.

 30075     SOMDERROR_DuplicateSem
           System error:  Duplicate semaphore name used.

 30076     SOMDERROR_BadSemName
           System error:  Invalid semaphore name.

 30077     SOMDERROR_TooManyHandles
           System error:  Too many files open (no file handles left).

 30078     SOMDERROR_BadAddrFamily
           System error:  Invalid address family.

 30079     SOMDERROR_BadFormat
           System error:  Invalid format.

 30080     SOMDERROR_BadDrive
           System error:  Invalid drive.

 30081     SOMDERROR_SharingViolation
           System error:  Sharing violation.

 30082     SOMDERROR_BadExeSignature
           System error:  Program file contains a DOS mode program or invalid
           program.

 30083     SOMDERROR_BadExe
           Executable file is invalid (linker errors occurred when program file
           was created).

 30084     SOMDERROR_Busy
           System error:  Segment is busy.

 30085     SOMDERROR_BadThread
           System error:  Invalid thread id.

 30086     SOMDERROR_SOMDPORTNotDefined
           SOMDPORT not defined.

 30087     SOMDERROR_ResourceExists
           System resource (file, queue, shared memory segment,. etc.) already
           exists.

 30088     SOMDERROR_UserName
           USER environment variable is not set.

 30089     SOMDERROR_WrongRefType
           Operation attempted on an object reference is incompatible with the
           reference type.

 30090     SOMDERROR_MustOverride
           This method has no default implementation and must be overridden.

 30091     SOMDERROR_NoSocketsClass
           Could not find/load Sockets class.

 30092     SOMDERROR_EManRegData
           Unable to register DSOM events with the Event Manager.

 30093     SOMDERROR_NoRemoteComm
           Remote communications is disabled (for Workstation DSOM).

 30096     SOMDERROR_GlobalAtomError
           On Windows only, an error occurred while adding a segment name to
           the Windows atom table.

 30097     SOMDERROR_NamedMemoryTableError
           On Windows only, an error occurred while creating or deleting a
           (named) shared memory segment.

 30098     SOMDERROR_WMQUIT
           On Windows only, indicates DSOM received a Windows WM_QUIT message.
           The developer of a server application should check for
           SOMDERROR_WMQUIT returned from method execute_request_loop and
           handle the error by cleaning up and exiting.

 30105     SOMDERROR_DuplicateImplEntry
           Implementation repository identifier already exists.  Add wait time
           between `regimpl' calls.

 30106     SOMDERROR_InvalidSOMSOCKETS
           SOMSOCKETS environment variable set incorrectly.

 30107     SOMDERR0R_IRNotFound
           Interface Repository not found.

 30108     SOMDERR0R_ClassNotInIR
           Attempt to create an object whose Class is not in the Interface
           Repository.

 30110     SOMDERROR_SocketError
           A communications socket error has occurred.  Make sure the DSOM
           daemon is running.

 30111     SOMDERROR_PacketError
           A communications packet error has occurred.

 30112     SOMDERROR_Marshal

 30113     SOMDERROR_NotProcessOwner
           This error code is returned when the somdd daemon is trying to kill
           the server process and if the owner of the server process is
           different from that of the somdd daemon.

 30114     SOMDERROR_ServerInactive
           The specified server is in the process of being activated.

 30115     SOMDERROR_ServerDisabled
           The server has been disabled by the program servmgr.

 30117     SOMDERROR_SOMDAlreadyRunning
           The DSOM daemon has been started when another daemon is already
           running.

 30118     SOMDERROR_ServerToBeDeleted
           An attempt has been made to start or connect to a server marked for
           deletion.

 30119     SOMDERROR_NoObjRefTable
           The Object Reference Table cannot be found.

 30120     SOMDERROR_UpdateImplDef
           The Implementation Repository cannot be updated.

 30138     SOMDERROR_NoImplDirectory
           The directory signified by SOMDDIR does not exist.

 30169     SOMDERROR_ServerNotStoppable
           An attempt has been made to stop a server (using the dsom stop
           command or a SOMDServerMgr object) registered as nonstoppable.

 XXXXX     SOMDERROR_OperatingSystem
           On AIX, this is the value of the C error variable "errno" defined in
           errno.h; on OS/2 and Windows, it is the DOS API return code.


ΓòÉΓòÉΓòÉ 11.3. Metaclass Framework Error Codes ΓòÉΓòÉΓòÉ

It is possible to receive the following messages from the Metaclass Framework
while an application is running.

 60001     An attempt was made to construct a class with SOMMSingleInstance as
           a metaclass constraint. (This may occur indirectly because of the
           construction of a derived metaclass). The initialization of the
           class failed because somInitMIClass defined by SOMMSingleInstance is
           in conflict with another metaclass that has overridden somNew. That
           is, some other metaclass has already claimed the right to return the
           value for somNew.

 60002     An attempt was made to construct a class with SOMMSingleInstance as
           a metaclass constraint. (This may occur indirectly because of the
           construction of a derived metaclass). The initialization of the
           class failed because somInitMIClass defined by SOMMSingleInstance is
           in conflict with another metaclass that has overridden somFree. That
           is, some other metaclass has already claimed this right to override
           somFree.

 60004     An invocation of somrRepInit was made with a logging type other than
           `o' or `v'.

 60005     The sommBeforeMethod or the sommAfterMethod was invoked on a
           SOMRReplicableObject whose logging type is other than `o' or `v'.
           This error cannot occur normally. The likely cause is that some
           method invoked on another object has overwritten this object's
           memory.

 60006     A Before/After Metaclass must override both sommBeforeMethod and
           sommAfterMethod.  This message indicates an attempt to create a
           Before/After Metaclass where only one of the above methods is
           overridden.


ΓòÉΓòÉΓòÉ 12. Implementing Sockets Subclasses ΓòÉΓòÉΓòÉ

Distributed SOM (DSOM) and the Replication Framework require basic message
services for inter-process communications. The Event Management Framework must
be integrated with the same communication services in order to handle
communications events.

To maximize their portability to a wide variety of local area network transport
protocols, the DSOM, Replication, and Event Management Frameworks have been
written to use a common communications interface, which is implemented by one
or more SOM class libraries using available local protocols.

The common communications interface is based on the "sockets" interface used
with TCP/IP, since its interface and semantics are fairly widespread and well
understood. The IDL interface is named Sockets. There is no implementation
associated with the Sockets interface by default; specific protocol
implementations are supplied by subclass implementations.

Note:  The Sockets classes supplied with the SOMobjects Developer Toolkit and
       run-time packages are only intended to support the DSOM, Replication,
       and Event Management Frameworks. These class implementations are not
       intended for general application usage.

 Available Sockets subclasses by SOMobjects product are as follows:

     For AIX:
           TCPIPSockets class for TCP/IP,
           IPXSockets class for Netware IPX/SPX, and
           NBSockets class for NetBIOS.
     For OS/2 and Windows:
           TCPIPSockets class (a) for TCP/IP for Windows or (b) for TCP/IP
           1.2.1 on OS/2,
           TCPIPSockets32 class for TCP/IP 2.0 on OS/2 only (see Note below),
           IPXSockets class for NetWare IPX/SPX, and
           NBSockets class for NetBIOS.

 Note:  The TCPIPSocket32 class gives greater performance over the TCPIPSockets
        class on OS/2, but requires the 32-bit version of TCPP/IP (version 2.0)
        rather than the 16-bit version of TCP/IP (version 1.2.1).

 Application developers may need to develop their own Sockets  subclass if the
 desired transport protocol or product version is not one of those supported by
 the SOMobjects run-time packages. This appendix explains how to approach the
 implementation of a Sockets subclass, if necessary.

 Warning: this may be a non-trivial exercise!


ΓòÉΓòÉΓòÉ 12.1. Sockets IDL interface ΓòÉΓòÉΓòÉ

The base Sockets interface is expressed in IDL in the file somssock.idl, listed
below. There is a one-to-one mapping between TCP/IP socket APIs and the methods
defined in the Sockets interface.

Please note the following:

     The semantics of each of the Sockets methods must be that of the
      corresponding TCP/IP call. Currently, only Internet address family
      (AF_INET) addresses are used by the frameworks.

      (The TCP/IP sockets API is not documented as part of the SOMobjects
      Developer Toolkit. The implementor is referred to the programming
      references for IBM TCP/IP for AIX or OS/2, or to similar references that
      describe the sockets interface for TCP/IP.)

     Data types, constants, and macros which are part of the Sockets interface
      are defined in a C include file, soms.h. This file is supplied with the
      SOMobjects Toolkit, and is not shown in this manual.

     The Sockets interface is expressed in terms of a 32-bit implementation.

     Some of the method parameters and return values are expressed using
      pointer types, for example:

            hostent *somsGethostent ();

      This has been done to map TCP/IP socket interfaces as directly as
      possible to their IDL equivalent. (Use of strict CORBA IDL was not a
      primary goal for the Sockets interface, since it is only used internally
      by the frameworks.)

     The Sockets class and its subclasses are  single instance classes.

 Following is a listing of the file somssock.idl. Each socket call is briefly
 described with a comment.

  // 96F8647, 96F8648 (C) Copyright IBM Corp. 1992, 1993
  // All Rights Reserved
  // Licensed Materials - Property of IBM

  #ifndef somssock_idl
  #define somssock_idl

  #include <somobj.idl>
  #include <snglicls.idl>

  interface Sockets : SOMObject
  {
      //# The following typedefs are fully defined in <soms.h>.
      typedef SOMFOREIGN sockaddr;
      #pragma modifier sockaddr : impctx="C", struct;
      typedef SOMFOREIGN iovec;
      #pragma modifier iovec : impctx="C", struct;
      typedef SOMFOREIGN msghdr;
      #pragma modifier msghdr : impctx="C", struct;
      typedef SOMFOREIGN fd_set;
      #pragma modifier fd_set : impctx="C", struct;
      typedef SOMFOREIGN timeval;
      #pragma modifier timeval : impctx="C", struct;
      typedef SOMFOREIGN hostent;
      #pragma modifier hostent : impctx="C", struct;
      typedef SOMFOREIGN servent;
      #pragma modifier servent : impctx="C", struct;
      typedef SOMFOREIGN in_addr;
      #pragma modifier in_addr : impctx="C", struct;

      long somsAccept (in long s, out sockaddr name, out long namelen);
      // Accept a connection request from a client.

      long somsBind (in long s, inout sockaddr name, in long namelen);
      // Binds a unique local name to the socket with descriptor s.

      long somsConnect (in long s, inout sockaddr name,
                        in long namelen);
      // For streams sockets, attempts to establish a connection
      // between two sockets.  For datagram sockets, specifies the
      // socket's peer.

      hostent *somsGethostbyaddr (in char *addr, in long addrlen,
                                  in long domain);
      // Returns a hostent structure for the host address specified on
      // the call.

      hostent *somsGethostbyname (in string name);
      // Returns a hostent structure for the host name specified on
      // the call.

      hostent *somsGethostent ();
      // Returns a pointer to the next entry in the hosts file.

      unsigned long somsGethostid ();
      // Returns the unique identifier for the current host.

      long somsGethostname (in string name, in long namelength);
      // Retrieves the standard host name of the local host.

      long somsGetpeername (in long s, out sockaddr name,
                           out long namelen);
      // Gets the name of the peer connected to socket s.

      servent *somsGetservbyname (in string name, in string protocol);
      // Retrieves an entry from the /etc/services file using the
      // service name as a search key.

      long somsGetsockname (in long s, out sockaddr name,
                            out long namelen);
      // Stores the current name for the socket specified by the s
      // parameter into the structure pointed to by the name
      // parameter.

      long somsGetsockopt (in long s, in long level, in long optname,
                          in char *optval, out long option);
      // Returns the values of socket options at various protocol
      // levels.

      unsigned long somsHtonl (in unsigned long a);
      // Translates an unsigned long integer from host-byte order to
      // network-byte order.

      unsigned short somsHtons (in unsigned short a);
      // Translates an unsigned short integer from host-byte order to
      // network-byte order.

      long somsIoctl (in long s, in long cmd, in char *data,
                      in long length);
      // Controls the operating characteristics of sockets.

      unsigned long somsInet_addr (in string cp);
      // Interprets character strings representing numbers expressed
      // in standard '.' notation and returns numbers suitable for use
      // as internet addresses.

      unsigned long somsInet_lnaof (in in_addr addr);
      // Breaks apart the internet address and returns the local
      // network address portion.

      in_addr somsInet_makeaddr (in unsigned long net,
                                 in unsigned long lna);
      // Takes a network number and a local network address and
      // constructs an internet address.

      unsigned long somsInet_netof (in in_addr addr);
      // Returns the network number portion of the given internet
      // address.

      unsigned long somsInet_network (in string cp);
      // Interprets character strings representing numbers expressed
      // in standard '.' notation and returns numbers suitable for use
      // as network numbers.

      string somsInet_ntoa (in in_addr addr);
      // Returns a pointer to a string expressed in the dotted-decimal
      // notation.

      long somsListen (in long s, in long backlog);
      // Creates a connection request queue of length backlog to queue
      // incoming connection requests, and then waits for incoming
      // connection requests.

      unsigned long somsNtohl (in unsigned long a);
      // Translates an unsigned long integer from network-byte order
      // to host-byte order.

      unsigned short somsNtohs (in unsigned short a);
      // Translates an unsigned short integer from network-byte order
      // to host-byte order.

      long somsReadv (in long s, inout iovec iov, in long iovcnt);
      // Reads data on socket s and stores it in a set of buffers
      // described by iov.

      long somsRecv (in long s, in char *buf, in long len,
                     in long flags);
      // Receives data on streams socket s and stores it in buf.

      long somsRecvfrom (in long s, in char *buf, in long len,
          in long flags, out sockaddr name, out long namelen);
      // Receives data on datagram socket s and stores it in buf.

      long somsRecvmsg (in long s, inout msghdr msg, in long flags);
      // Receives messages on a socket with descriptor s and stores
      // them in an array of message headers.

      long somsSelect (in long nfds, inout fd_set readfds,
                       inout fd_set writefds, inout fd_set exceptfds,
                       inout timeval timeout);
      // Monitors activity on a set of different sockets until a
      // timeout expires, to see if any sockets are ready for reading
      // or writing, or if an exceptional condition is pending.

      long somsSend (in long s, in char *msg, in long len,
                     in long flags);
      // Sends msg on streams socket s.

      long somsSendmsg (in long s, inout msghdr msg, in long flags);
      // Sends messages passed in an array of message headers on a
      // socket with descriptor s.

      long somsSendto (in long s, inout char msg, in long len,
                       in long flags, inout sockaddr to, in long tolen);
      // Sends msg on datagram socket s.

      long somsSetsockopt (in long s, in long level, in long optname,
                           in char *optval, in long optlen);
      // Sets options associated with a socket.

      long somsShutdown (in long s, in long how);
      // Shuts down all or part of a full-duplex connection.

      long somsSocket (in long domain, in long type,
                       in long protocol);
      // Creates an endpoint for communication and returns a socket
      // descriptor representing the endpoint.

      long somsSoclose (in long s);
      // Shuts down socket s and frees resources allocated to the
      // socket.

      long somsWritev (in long s, inout iovec iov, in long iovcnt);
      // Writes data on socket s.  The data is gathered from the
      // buffers described by iov.

      attribute long serrno;
      // Used to pass error numbers.

  #ifdef __SOMIDL__
      implementation
      {
      releaseorder:
          somsAccept, somsBind, somsConnect, somsGethostbyaddr,
          somsGethostbyname, somsGethostent, somsGethostid,
          somsGethostname, somsGetpeername, somsGetsockname,
          somsGetsockopt, somsHtonl, somsHtons, somsIoctl,
          somsInet_addr, somsInet_lnaof, somsInet_makeaddr,
          somsInet_netof, somsInet_network, somsInet_ntoa,
          somsListen, somsNtohl, somsNtohs, somsReadv,
          somsRecv, somsRecvfrom, somsRecvmsg, somsSelect,
          somsSend, somsSendmsg, somsSendto, somsSetsockopt,
          somsShutdown, somsSocket, somsSoclose, somsWritev,
          _set_serrno, _get_serrno, somsGetservbyname;

      //# Class modifiers
      callstyle=idl;
      metaclass = SOMMSingleInstance;
      majorversion=1; minorversion=1;
      dll="soms.dll";
      };
  #endif /* __SOMIDL__ */
  };
  #endif  /* somssock_idl */


ΓòÉΓòÉΓòÉ 12.2. IDL for a Sockets subclass ΓòÉΓòÉΓòÉ

Sockets subclasses inherit their entire interface from Sockets. All methods are
overridden.

For example, here is a listing of the TCPIPSockets IDL description.

// 96F8647, 96F8648 (C) Copyright IBM Corp. 1992, 1993
// All Rights Reserved
// Licensed Materials - Property of IBM

#ifndef tcpsock_idl
#define tcpsock_idl

#include <somssock.idl>
#include <snglicls.idl>

interface TCPIPSockets : Sockets
{
#ifdef __SOMIDL__
    implementation
    {
    //# Class modifiers
    callstyle=idl;
    majorversion=1; minorversion=1;
    dllname="somst.dll";
    metaclass=SOMMSingleInstance;
    //# Method modifiers
    somsAccept: override;
    somsBind: override;
    somsConnect: override;
    somsGethostbyaddr: override;
    somsGethostbyname: override;
    somsGethostent: override;
    somsGethostid: override;
    somsGethostname: override;
    somsGetpeername: override;
    somsGetservbyname: override;
    somsGetsockname: override;
    somsGetsockopt: override;
    somsHtonl: override;
    somsHtons: override;
    somsIoctl: override;
    somsInet_addr: override;
    somsInet_lnaof: override;
    somsInet_makeaddr: override;
    somsInet_netof: override;
    somsInet_network: override;
    somsInet_ntoa: override;
    somsListen: override;
    somsNtohl: override;
    somsNtohs: override;
    somsReadv: override;
    somsRecv: override;
    somsRecvfrom: override;
    somsRecvmsg: override;
    somsSelect: override;
    somsSend: override;
    somsSendmsg: override;
    somsSendto: override;
    somsSetsockopt: override;
    somsShutdown: override;
    somsSocket: override;
    somsSoclose: override;
    somsWritev: override;
    _set_serrno: override;
   _get_serrno: override;
   };
#endif /* __SOMIDL__ */
 };
#endif  /* tcpsock_idl */


ΓòÉΓòÉΓòÉ 12.3. Implementation considerations ΓòÉΓòÉΓòÉ

     Only the AF_INET address family must be supported. That is, the DSOM,
      Replication, and Event Manager frameworks all use Internet addresses and
      port numbers to refer to specific sockets.

     On OS/2, the SOMobjects run-time libraries were built using the C Set/2
      32-bit compiler. If the underlying subclass implementation uses a 16-bit
      subroutine library, conversion of the method call arguments may be
      required. (This mapping of arguments is often referred to as "thunking.")

     Sockets subclasses to be used in multi-threaded environments should be
      made thread-safe. That is, it is possible that concurrent threads may
      make calls on the (single) Sockets object, so data structures must be
      protected within critical regions, as appropriate.

     Valid values for the serrno attribute are defined in the file soms.h. The
      subclass implementation should map local error numbers into the
      appropriate corresponding Sockets error numbers.


ΓòÉΓòÉΓòÉ 12.4. Example code ΓòÉΓòÉΓòÉ

The following code fragment shows an example of the implementation of the
somsBind method of the TCPIPSockets subclass, for both AIX and OS/2. The sample
illustrates that, for TCP/IP, the implementation is basically a one-to-one
mapping of Sockets methods onto TCP/IP calls. For other transport protocols,
the mapping from the socket abstraction to the protocol's API may be more
difficult.

For AIX, the mapping from Sockets method to TCP/IP call is trivial.

SOM_Scope long  SOMLINK somsBind(TCPIPSockets somSelf,
                                 Environment *ev,
                                 long s, Sockets_sockaddr* name,
                                 long namelen)
{
    long rc;

    TCPIPSocketsMethodDebug("TCPIPSockets","somsBind");

    rc = (long) bind((int)s, name, (int)namelen);

    if (rc == -1)
       __set_serrno(somSelf, ev, errno);

    return rc;
}

On OS/2, however, the TCP/IP Release 1.2.1 library is a 16-bit library.
Consequently, many of the method calls require conversion ("thunking") of
32-bit parameters into 16-bit parameters, before the actual TCP/IP calls can be
invoked. For example, the function prototype for the somsBind method is defined
as:

SOM_Scope long  SOMLINK somsBind(TCPIPSockets somSelf,
                                 Environment *ev,
                                 long s, Sockets_sockaddr* name,
                                 long namelen);

whereas the file socket.h on OS/2 declares the bind function with the following
prototype:

short _Far16 _Cdecl bind(short /*s*/, void * _Seg16 /*name*/,
                         short /*len*/);

In this case, the pointer to the "name" structure, passed as a 32-bit address,
cannot be used directly in the bind call: a 16-bit address must be passed
instead. This can be accomplished by dereferencing the 32-bit pointer provided
by the "name" parameter in the somsBind call, copying the caller's
Sockets_sockaddr structure into a local structure ("name16"), and then passing
the address of the local structure ("&name16") as a 16-bit address in the bind
call.

SOM_Scope long  SOMLINK somsBind(TCPIPSockets somSelf,
                                 Environment *ev,
                                 long s, Sockets_sockaddr* name,
                                 long namelen)
{
    long rc;
    Sockets_sockaddr name16;

    TCPIPSocketsMethodDebug("TCPIPSockets","somsBind");

    /* copy user's parameter into a local structure */
    memcpy ((char *)&name16, (char *)((sockaddr32 *)name), namelen);
    rc = (long) bind((short)s, (void *)&name16, (short)namelen);

    if (rc == -1)
       __set_serrno(somSelf, ev, tcperrno());

    return rc;
}


ΓòÉΓòÉΓòÉ 13. Glossary ΓòÉΓòÉΓòÉ

Note:  In the following definitions, words shown in italics are terms for which
separate glossary entries are also defined.


ΓòÉΓòÉΓòÉ 13.1. abstract class ΓòÉΓòÉΓòÉ

abstract class

A class that is not designed to be instantiated, but serves as a base class for
the definition of subclasses. Regardless of whether an abstract class inherits
instance data and methods from parent classes, it will always introduce methods
that must be overridden in a subclass, in order to produce a class whose
objects are semantically valid.


ΓòÉΓòÉΓòÉ 13.2. affinity group ΓòÉΓòÉΓòÉ

affinity group

An array of class objects that were all registered with the SOMClassMgr object
during the dynamic loading of a class. Any class is a member of at most one
affinity group.


ΓòÉΓòÉΓòÉ 13.3. ancestor class ΓòÉΓòÉΓòÉ

ancestor class

A class from which another class inherits instance methods, attributes, and
instance variables, either directly or indirectly. A direct descendant of an
ancestor class is called a child class, derived class, or  subclass. A direct
ancestor of a class is called a parent class, base class, or superclass.


ΓòÉΓòÉΓòÉ 13.4. aggregate type ΓòÉΓòÉΓòÉ

aggregate type

A user-defined data type that combines basic types (such as, char, short,
float, and so on) into a more complex type (such as structs, arrays, strings,
sequences, unions, or enums).


ΓòÉΓòÉΓòÉ 13.5. apply stub ΓòÉΓòÉΓòÉ

apply stub

A procedure corresponding to a particular method that accepts as arguments: the
object on which the method is to be invoked, a pointer to a location in memory
where the method's result should be stored, a pointer to the method's
procedure, and the method's arguments in the form of a va_list. The apply stub
extracts the arguments from the va_list, invokes the method with its arguments,
and stores its result in the specified location. Apply stubs are registered
with class objects when instance methods are defined, and are invoked using the
somApply function. Typically, implementations that override somDispatch call
somApply to invoke a method on a va_list of arguments.


ΓòÉΓòÉΓòÉ 13.6. attribute ΓòÉΓòÉΓòÉ

attribute

A specialized syntax for declaring "set" and "get" methods. Method names
corresponding to attributes always begin with "_set_" or "_get_". An attribute
name is declared in the body of the interface statement for a class. Method
procedures for get/set methods are automatically defined by the SOM Compiler
unless an attribute is declared as "noget/noset". Likewise, a corresponding
instance variable is automatically defined unless an attribute is declared as
"nodata". IDL also supports "readonly" attributes, which specify only a "get"
method. (Contrast an attribute with an instance variable.)


ΓòÉΓòÉΓòÉ 13.7. auxiliary class data structure: ΓòÉΓòÉΓòÉ

auxiliary class data structure:

A structure provided by the SOM API to support efficient static access to
class-specific information used in dealing with SOM objects. The structure's
name is <className>CClassData. Its first component (parentMtab) is a list of
parent-class method tables (used to support efficient parent method calls). Its
second component (instanceDataToken) is the instance token for the class
(generally used to locate the instance data introduced by method procedures
that implement methods defined by the class).


ΓòÉΓòÉΓòÉ 13.8. base class ΓòÉΓòÉΓòÉ

base class

See parent class.


ΓòÉΓòÉΓòÉ 13.9. behavior (of an object) ΓòÉΓòÉΓòÉ

behavior (of an object)

The methods that an object responds to. These methods are those either
introduced or inherited by the class of the object. See also state.


ΓòÉΓòÉΓòÉ 13.10. bindings: ΓòÉΓòÉΓòÉ

bindings:

Language-specific macros and procedures that make implementing and using SOM
classes more convenient. These bindings offer a convenient interface to SOM
that is tailored to a particular programming language. The SOM Compiler
generates binding files for C and C ++. These binding files include an
implementation template for the class and two header files, one to be included
in the class's implementation file and the other in client programs.


ΓòÉΓòÉΓòÉ 13.11. BOA (basic object adapter) class: ΓòÉΓòÉΓòÉ

BOA (basic object adapter) class:

A CORBA interface (represented as an abstract class in DSOM), which defines
generic object-adapter (OA) methods that a server can use to register itself
and its objects with an ORB (object request broker). See also SOMOA (SOM object
adapter) class.


ΓòÉΓòÉΓòÉ 13.12. callback ΓòÉΓòÉΓòÉ

callback

A user-provided procedure or method to the Event Management Framework that gets
invoked when a registered event occurs. (See also event ).


ΓòÉΓòÉΓòÉ 13.13. casted dispatching ΓòÉΓòÉΓòÉ

casted dispatching

A form of method dispatching that uses casted method resolution; that is, it
uses a designated ancestor class of the actual target object's class to
determine what procedure to call to execute a specified method.


ΓòÉΓòÉΓòÉ 13.14. casted method resolution ΓòÉΓòÉΓòÉ

casted method resolution

A method resolution technique that uses a method procedure from the method
table of an ancestor of the class of an object (rather than using a procedure
from the method table of the object's own class).


ΓòÉΓòÉΓòÉ 13.15. child class ΓòÉΓòÉΓòÉ

child class

A class that inherits instance methods, attributes, and instance variables
directly from another class, called the parent class, base class, or
superclass, or indirectly from an ancestor class. A child class may also be
called a derived class or subclass.


ΓòÉΓòÉΓòÉ 13.16. class ΓòÉΓòÉΓòÉ

class

A way of categorizing objects based on their behavior (the methods they
support) and shape (memory layout). A class is a definition of a generic
object. In SOM, a class is also a special kind of object that can manufacture
other objects that all have a common shape and exhibit similar behavior. The
specification of what comprises the shape and behavior of a set of objects is
referred to as the "definition" of a class. New classes are defined in terms of
existing classes through a technique known as inheritance. See also class
object.


ΓòÉΓòÉΓòÉ 13.17. class variable ΓòÉΓòÉΓòÉ

class variable

Instance data of a class object. All instance data of an object is defined
(through either introduction or inheritance) by the object's class. Thus, class
variables are defined by metaclasses.


ΓòÉΓòÉΓòÉ 13.18. class data structure ΓòÉΓòÉΓòÉ

class data structure

A structure provided by the SOM API to support efficient static access to
class-specific information used in dealing with SOM objects. The structure's
name is <className>ClassData. Its first component (classObject) is a pointer to
the corresponding class object. The remaining components (named after the
instance methods and instance variables) are method tokens or data tokens, in
order as specified by the class's implementation. Data tokens are only used to
support data (public and private) introduced by classes declared using OIDL;
IDL attributes are supported with method tokens.


ΓòÉΓòÉΓòÉ 13.19. class manager ΓòÉΓòÉΓòÉ

class manager

An object that acts as a run-time registry for all SOM class objects that exist
within the current process and which assists in the dynamic loading and
unloading of class libraries. A class implementor can define a customized class
manager by subclassing SOMClassMgr class to replace the SOM-supplied
SOMClassMgrObject. This is done to augment the functionality of the default
class-management registry (for example, to coordinate the automatic quiescing
and unloading of classes).


ΓòÉΓòÉΓòÉ 13.20. class method ΓòÉΓòÉΓòÉ

class method

(Also known as factory method or constructor.) A class method is a method that
a class object responds to (as opposed to an instance method). A class method
that class <X> responds to is provided by the metaclass of class <X>. Class
methods are executed without requiring any instances of class <X> to exist, and
are frequently used to create instances of the class.


ΓòÉΓòÉΓòÉ 13.21. class object ΓòÉΓòÉΓòÉ

class object

The run-time object representing a SOM class. In SOM, a class object can
perform the same behavior common to all objects, inherited from SOMObject.


ΓòÉΓòÉΓòÉ 13.22. client code ΓòÉΓòÉΓòÉ

client code

(Or client program or client.) An application program, written in the
programmer's preferred language, which invokes methods on o  bjects that are
instances of SOM classes. In DSOM, this could be a program that invokes a
method on a remote object.


ΓòÉΓòÉΓòÉ 13.23. constructor ΓòÉΓòÉΓòÉ

constructor

See class method.


ΓòÉΓòÉΓòÉ 13.24. context expression ΓòÉΓòÉΓòÉ

context expression

An optional expression in a method's IDL declaration, specifying identifiers
whose value (if any) can be used during SOM's method resolution  process and/or
by the target object as it executes the method procedure. If a context
expression is specified, then a related Context parameter is required when the
method is invoked. (This Context parameter is an implicit parameter in the IDL
specification of the method, but it is an explicit parameter of the method's
procedure.) No SOM-supplied methods require context parameters.


ΓòÉΓòÉΓòÉ 13.25. CORBA ΓòÉΓòÉΓòÉ

CORBA

The Common Object Request Broker Architecture established by the Object
Management Group. IBM's Interface Definition Language used to describe the
interface for SOM classes is fully compliant with CORBA standards.


ΓòÉΓòÉΓòÉ 13.26. daemon ΓòÉΓòÉΓòÉ

daemon

See DSOM daemon.


ΓòÉΓòÉΓòÉ 13.27. data token ΓòÉΓòÉΓòÉ

data token

A value that identifies a specific instance variable within an object whose
class inherits the instance variable (as a result of being derived, directly or
indirectly, from the class that introduces the instance variable). An object
and a data token are passed to the SOM run-time procedure, somDataResolve,
which returns is a pointer to the specific instance variable corresponding to
the data token. (See also instance token.)


ΓòÉΓòÉΓòÉ 13.28. derived class ΓòÉΓòÉΓòÉ

derived class

See subclass and subclassing.


ΓòÉΓòÉΓòÉ 13.29. derived metaclass ΓòÉΓòÉΓòÉ

derived metaclass

(Or SOM-derived metaclass.) A metaclass  that SOM creates automatically (often
even when the class implementor specifies an explicit metaclass) as needed to
ensure that, for any code that executes without method-resolution error on an
instance of a given class, the code will similarly execute without
method-resolution error on instances of any subclass of the given class. SOM's
ability to derive such metaclasses is a fundamental necessity in order to
ensure binary compatibility for client programs despite any subsequent changes
in class implementations.


ΓòÉΓòÉΓòÉ 13.30. descriptor ΓòÉΓòÉΓòÉ

descriptor

(Or method descriptor.) An ID representing the identifier of a method
definition or an attribute definition in the Interface Repository. The IR
definition contains information about the method's return type and the type of
its arguments.


ΓòÉΓòÉΓòÉ 13.31. directive ΓòÉΓòÉΓòÉ

directive

A message (a pre-defined character constant) received by a replica from the
Replication Framework. Indicates a potential failure situation.


ΓòÉΓòÉΓòÉ 13.32. dirty object ΓòÉΓòÉΓòÉ

dirty object

A persistent object that has been modified since it was last written to
persistent storage.


ΓòÉΓòÉΓòÉ 13.33. dispatch-function resolution ΓòÉΓòÉΓòÉ

dispatch-function resolution

Dispatch-function resolution is the slowest, but most flexible, of the three
method-resolution techniques SOM offers. Dispatch functions permit method
resolution to be based on arbitrary rules associated with an object's class.
Thus, a class implementor has complete freedom in determining how methods
invoked on its instances are resolved. See also dispatch method and dynamic
dispatching.


ΓòÉΓòÉΓòÉ 13.34. dispatch method ΓòÉΓòÉΓòÉ

dispatch method

A method (such as somDispatch or somClassDispatch) that is invoked (and passed
an argument list and the ID of another method) in order to determine the
appropriate method procedure to execute. The use of dispatch methods
facilitates dispatch-function resolution in SOM applications and enables method
invocation on remote objects in DSOM applications. See also dynamic
dispatching.


ΓòÉΓòÉΓòÉ 13.35. DLL ΓòÉΓòÉΓòÉ

DLL

Abbreviation for dynamic link library.


ΓòÉΓòÉΓòÉ 13.36. DSOM daemon (somdd) ΓòÉΓòÉΓòÉ

DSOM daemon (somdd)

The DSOM process (somdd) that locates and activates servers. The daemon
(actually known as the location services daemon) is primarily meant to provide
a client with the necessary communications information to allow the client to
connect with an implementation server.


ΓòÉΓòÉΓòÉ 13.37. dynamic dispatching ΓòÉΓòÉΓòÉ

dynamic dispatching

Method dispatching using dispatch-function resolution; the use of dynamic
method resolution at run time. See also dispatch- function resolution and
dynamic method.


ΓòÉΓòÉΓòÉ 13.38. Dynamic Invocation Interface (DII) ΓòÉΓòÉΓòÉ

Dynamic Invocation Interface (DII)

The CORBA-specified interface, implemented in DSOM, that is used to dynamically
build requests on remote objects. Note that DSOM applications can also use the
somDispatch method for dynamic method calls when the object is remote. See also
dispatch method.


ΓòÉΓòÉΓòÉ 13.39. dynamic link library ΓòÉΓòÉΓòÉ

dynamic link library

A piece of code that can be loaded (activated) dynamically. This code is
physically separate from its callers. DLLs can be loaded at load time or at run
time. Widely used term on OS/2, Windows, and, to some extent, AIX.


ΓòÉΓòÉΓòÉ 13.40. dynamic method ΓòÉΓòÉΓòÉ

dynamic method

A method that is not declared in the IDL interface statement for a class of
objects, but is added to the interface at run time, after which instances of
the class (or of its subclasses) will respond to the registered dynamic method.
Because dynamic methods are not declared, usage bindings for SOM classes cannot
support their use; thus, offset method resolution is not available. Instead,
name-lookup or dispatch-function method resolution must be used to invoke
dynamic methods. (There are currently no known uses of dynamic methods by any
SOM applications.) See also method and static method.


ΓòÉΓòÉΓòÉ 13.41. encapsulation ΓòÉΓòÉΓòÉ

encapsulation

An object-oriented programming feature whereby the implementation details of a
class are hidden from client programs, which are only required to know the
interface of a class (the signatures of its methods and the names of its
attributes) in order to use the class's methods and attributes.


ΓòÉΓòÉΓòÉ 13.42. encoder/decoder ΓòÉΓòÉΓòÉ

encoder/decoder

In the Persistence Framework, a class that knows how to read/write the
persistent object format of a persistent object. Every persistent object is
associated with an Encoder/Decoder, and an encoder/decoder object is created
for each attribute and instance variable. An Encoder/Decoder is supplied by the
Persistence Framework by default, or an application can define its own.


ΓòÉΓòÉΓòÉ 13.43. entry class ΓòÉΓòÉΓòÉ

entry class

In the Emitter Framework, a class that represents some syntactic unit of an
interface definition in the IDL source file.


ΓòÉΓòÉΓòÉ 13.44. Environment parameter ΓòÉΓòÉΓòÉ

Environment parameter

A CORBA-required parameter in all method procedures, it represents a memory
location where exception information can be returned by the object of a method
invocation. [Certain methods are exempt (when the class contains a modifier of
callstyle=oidl), to maintain upward compatibility for client programs written
using an earlier release.]


ΓòÉΓòÉΓòÉ 13.45. emitter ΓòÉΓòÉΓòÉ

emitter

Generically, a program that takes the output from one system and converts the
information into a different form. Using the Emitter Framework, selected output
from the SOM Compiler (describing each syntactic unit in an IDL source file) is
transformed and formatted according to a user-defined template. Example emitter
output, besides the implementation template and language bindings, might
include reference documentation, class browser descriptions, or "pretty"
printouts.


ΓòÉΓòÉΓòÉ 13.46. event ΓòÉΓòÉΓòÉ

event

The occurrence of a condition, or the beginning or ending of an activity that
is of interest to an application. Examples are elapse of a time interval,
sending or receiving of a message, and opening or closing a file. (See also
event manager and callback.)


ΓòÉΓòÉΓòÉ 13.47. event manager (EMan) ΓòÉΓòÉΓòÉ

event manager (EMan)

The chief component of the Event Management Framework that registers interest
in various events from calling modules and informs them through callbacks when
those events occur.


ΓòÉΓòÉΓòÉ 13.48. factory method ΓòÉΓòÉΓòÉ

factory method

See class method.


ΓòÉΓòÉΓòÉ 13.49. ID: ΓòÉΓòÉΓòÉ

ID:

See somId.


ΓòÉΓòÉΓòÉ 13.50. IDL source file ΓòÉΓòÉΓòÉ

IDL source file

A user-written .idl file, expressed using the syntax of the Interface
Definition Language (IDL), which describes the interface for a particular class
(or classes, for a module). The IDL source file is processed by the SOM
Compiler to generate the binding files specific to the programming languages of
the class implementor and the client application. (This file may also be called
the "IDL file," the "source file," or the "interface definition file.")


ΓòÉΓòÉΓòÉ 13.51. implementation ΓòÉΓòÉΓòÉ

implementation

(Or object implementation.) The specification of what instance variables
implement an object's state and what procedures implement its methods (or
behaviors). In DSOM, a remote object's implementation is also characterized by
its server implementation (a program).


ΓòÉΓòÉΓòÉ 13.52. Implementation Repository ΓòÉΓòÉΓòÉ

Implementation Repository

A database used by DSOM to store the implementation definitions of DSOM
servers.


ΓòÉΓòÉΓòÉ 13.53. implementation statement ΓòÉΓòÉΓòÉ

implementation statement

An optional declaration within the body of the interface  definition of a class
in a SOM IDL source file, specifying information about how the class will be
implemented (such as, version numbers for the class, overriding of inherited
methods, or type of method resolution to be supported by particular methods).
This statement is a SOM-unique statement; thus, it must be preceded by the term
"#ifdef __SOMIDL__" and followed by "#endif". See also interface declaration.


ΓòÉΓòÉΓòÉ 13.54. implementation template ΓòÉΓòÉΓòÉ

implementation template

A template file containing stub procedures for  methods that a class introduces
or overrides. The implementation template is one of the binding files generated
by the SOM Compiler when it processes the IDL source file containing class
interface declarations. The class implementor then customizes the
implementation, by adding language-specific code to the method procedures.


ΓòÉΓòÉΓòÉ 13.55. implicit method parameter ΓòÉΓòÉΓòÉ

implicit method parameter

A method parameter that is not included in the IDL interface specification of a
method, but which is a parameter of the method's procedure and which is
required when the method is invoked from a client program. Implicit parameters
include the required Environment parameter indicating where exception
information can be returned, as well as a Context parameter, if needed.


ΓòÉΓòÉΓòÉ 13.56. incremental update ΓòÉΓòÉΓòÉ

incremental update

A revision to an implementation template file that results from reprocessing of
the IDL source file by the SOM Compiler. The updated implementation file will
contain new stub procedures, added comments, and revised method prototypes
reflecting changes made to the method definitions in the IDL specification.
Importantly, these updates do not disturb existing code that the class
implementor has defined for the prior method procedures.


ΓòÉΓòÉΓòÉ 13.57. inheritance ΓòÉΓòÉΓòÉ

inheritance

The technique of defining one class (called a subclass, derived class, or child
class) as incremental differences from another class (called the  parent class,
base class, superclass, or ancestor class). From its parents, the subclass
inherits variables and methods for its instances. The subclass can also provide
additional instance variables and methods. Furthermore, the subclass can
provide new procedures for implementing inherited methods. The subclass is then
said to override the parent class's methods. An overriding method procedure can
elect to call the parent class's method procedure. (Such a call is known as a
parent method call.)


ΓòÉΓòÉΓòÉ 13.58. inheritance hierarchy ΓòÉΓòÉΓòÉ

inheritance hierarchy

The sequential relationship from a root class to a subclass, through which the
subclass inherits instance methods, attributes, and  instance variables from
all of its ancestors, either directly or indirectly. The root class of all SOM
classes is SOMObject.


ΓòÉΓòÉΓòÉ 13.59. instance ΓòÉΓòÉΓòÉ

instance

(Or object instance or just object.) A specific object, as distinguished from a
class of objects. See also object.


ΓòÉΓòÉΓòÉ 13.60. instance method ΓòÉΓòÉΓòÉ

instance method

A method valid for an object instance (as opposed to a class method, which is
valid for a class object). An instance method that an object responds to is
defined by its class or inherited from an ancestor class.


ΓòÉΓòÉΓòÉ 13.61. instance token ΓòÉΓòÉΓòÉ

instance token

A data token that identifies the first instance variable among those introduced
by a given class. The somGetInstanceToken method invoked on a class object
returns that class's instance token.


ΓòÉΓòÉΓòÉ 13.62. instance variables ΓòÉΓòÉΓòÉ

instance variables

(Or, instance data.) Variables declared for use within the method procedures of
a class. An instance variable is declared within the body of the implementation
statement in a SOM IDL source file. An instance variable is "private" to the
class and should not be accessed by a client program. (Contrast an instance
variable with an attribute.)


ΓòÉΓòÉΓòÉ 13.63. interface ΓòÉΓòÉΓòÉ

interface

The information that a client must know to use a class  namely, the names of
its attributes and the signatures of its methods . The interface is described
in a formal language (the Interface Definition Language, IDL) that is
independent of the programming language used to implement the class's methods.


ΓòÉΓòÉΓòÉ 13.64. interface declaration ΓòÉΓòÉΓòÉ

interface declaration

(Or interface statement.) The statement in the IDL source file that specifies
the name of a new class and the names of its parent class(es). The "body" of
the interface declaration defines the signature of each new method and any
attribute(s) associated with the class. In SOM IDL, the body may also include
an implementation statement (where instance variables are declared or a
modifier is specified, for example to overridea method).


ΓòÉΓòÉΓòÉ 13.65. Interface Definition Language (IDL) ΓòÉΓòÉΓòÉ

Interface Definition Language (IDL)

The formal language (independent of any programming language) by which the
interface for a class of objects is defined in a .idl file, which the SOM
Compiler then interprets to create an implementation template file and binding
files. SOM's Interface Definition Language is fully compliant with standards
established by the Object Management Group's Common Object Request Broker
Architecture (CORBA).


ΓòÉΓòÉΓòÉ 13.66. Interface Repository (IR) ΓòÉΓòÉΓòÉ

Interface Repository (IR)

The database that SOM optionally creates to provide persistent storage of
objects representing the major elements of interface definitions. Creation and
maintenance of the IR is based on information supplied in the IDL source file.
The SOM IR Framework supports all interfaces described in the CORBA standard.


ΓòÉΓòÉΓòÉ 13.67. Interface Repository Framework ΓòÉΓòÉΓòÉ

Interface Repository Framework

A set of classes that provide methods whereby executing programs can access the
persistent objects of the Interface Repository to discover everything known
about the programming interfaces of SOM classes.


ΓòÉΓòÉΓòÉ 13.68. IR ΓòÉΓòÉΓòÉ

IR

Abbreviation for Interface Repository.


ΓòÉΓòÉΓòÉ 13.69. location services daemon (somdd) ΓòÉΓòÉΓòÉ

location services daemon (somdd)

A process whose primary purpose is to give DSOM clients the communications
information that they need to connect with an implementation server.


ΓòÉΓòÉΓòÉ 13.70. macro ΓòÉΓòÉΓòÉ

macro

An alias for executing a sequence of hidden instructions; in SOM, typically the
means of executing a command known within a binding file created by the SOM
Compiler.


ΓòÉΓòÉΓòÉ 13.71. metaclass ΓòÉΓòÉΓòÉ

metaclass

A class whose instances are classes. In SOM, any class descended from SOMClass
is a metaclass. The methods a class inherits from its metaclass are sometimes
called class methods (in Smalltalk) or factory methods (in Objective-C) or
constructors. See also class method.


ΓòÉΓòÉΓòÉ 13.72. metaclass incompatibility ΓòÉΓòÉΓòÉ

metaclass incompatibility

A situation where a subclass does not include all of the class variables or
respond to all of the class methods of its ancestor classes. This situation can
easily arise in OOP systems that allow programmers to explicitly specify
metaclasses, but is not allowed to occur in SOM. Instead, SOM automatically
prevents this by creating and using derived metaclasses whenever necessary.


ΓòÉΓòÉΓòÉ 13.73. method ΓòÉΓòÉΓòÉ

method

A combination of a procedure and a name, such that many different procedures
can be associated with the same name. In object-oriented programming, invoking
a method on an object causes the object to execute a specific method procedure.
The process of determining which method procedure to execute when a method is
invoked on an object is called method resolution. (The CORBA standard uses the
term "operation" for method invocation). SOM supports two different kinds of
methods: static methods and dynamic methods. See also static method and dynamic
method.


ΓòÉΓòÉΓòÉ 13.74. method descriptor ΓòÉΓòÉΓòÉ

method descriptor

See descriptor.


ΓòÉΓòÉΓòÉ 13.75. method ID ΓòÉΓòÉΓòÉ

method ID

A number representing a zero-terminated string by which SOM uniquely represents
a method name. See also somId.


ΓòÉΓòÉΓòÉ 13.76. method pointer ΓòÉΓòÉΓòÉ

method pointer

A pointer type that identifies one method procedure on a single class. Method
pointers are not ensured to be persistent among multiple processes.


ΓòÉΓòÉΓòÉ 13.77. method procedure ΓòÉΓòÉΓòÉ

method procedure

A function or procedure, written in an arbitrary programming language, that
implements a method of a class. A method procedure is defined by the class
implementor within the implementation template file generated by the SOM
Compiler.


ΓòÉΓòÉΓòÉ 13.78. method prototype ΓòÉΓòÉΓòÉ

method prototype

A method declaration that includes the types of the arguments. Based on method
definitions in an IDL source file, the SOM Compiler generates method prototypes
in the implementation template. A class implementor uses the method prototype
as a basis for writing the corresponding method procedurecode. The method
prototype also shows all arguments and their types that are required to invoke
the method from a client program.


ΓòÉΓòÉΓòÉ 13.79. method resolution ΓòÉΓòÉΓòÉ

method resolution

The process of selecting a particular method procedure, given a method name and
an object instance. The process results in selecting the particular
function/procedure that implements the abstract method in a way appropriate for
the designated object. SOM supports a variety of method-resolution mechanisms,
including offset method resolution, name-lookup resolution, and
dispatch-function resolution.


ΓòÉΓòÉΓòÉ 13.80. method table ΓòÉΓòÉΓòÉ

method table

A table of pointers to the  method procedures that implement the methods that
an object supports. See also method token.


ΓòÉΓòÉΓòÉ 13.81. method token ΓòÉΓòÉΓòÉ

method token

A value that identifies a specific method introduced by a class. A method token
is used during method resolution to locate the method procedure that implements
the identified method. The two basic method-resolution procedures are
somResolve (which takes as arguments an object and a method token, and returns
a pointer to a procedure that implements the identified method on the given
object) and somClassResolve (which takes as arguments a class and a method
token, and returns a pointer to a procedure that implements the identified
method on an instance of the given class).


ΓòÉΓòÉΓòÉ 13.82. modifier ΓòÉΓòÉΓòÉ

modifier

Any of a set of statements that control how a class, an attribute, or a method
will be implemented. Modifiers can be defined in the implementation statement
of a SOM IDL source file. The implementation statement is a SOM-unique
extension of the CORBA specification. [User-defined modifiers can also be
specified for use by user-written emitters or to store information in the
Interface Repository, which can then be accessed via methods provided by the
Interface Repository Framework.]


ΓòÉΓòÉΓòÉ 13.83. module ΓòÉΓòÉΓòÉ

module

The organizational structure required within an IDL source file that contains
interface declarations for two (or more) classes that are not a class-metaclass
pair. Such interfaces must be grouped within a module declaration.


ΓòÉΓòÉΓòÉ 13.84. multiple inheritance ΓòÉΓòÉΓòÉ

multiple inheritance

The situation in which a class is derived from (and inherits interface and
implementation from) multiple parent classes.


ΓòÉΓòÉΓòÉ 13.85. name-lookup method resolution ΓòÉΓòÉΓòÉ

name-lookup method resolution

Similar to the method resolution techniques employed by Objective-C and
Smalltalk. It is significantly slower than offset resolution. Name-lookup
resolution, unlike offset resolution, can be used when the name of the method
to be invoked is not known until run time, or the method is added to the class
interface at run time, or the name of the class introducing the method is not
known until run time.


ΓòÉΓòÉΓòÉ 13.86. naming scope: ΓòÉΓòÉΓòÉ

naming scope:

See scope.


ΓòÉΓòÉΓòÉ 13.87. object ΓòÉΓòÉΓòÉ

object

(Or object instance  or just instance.) An entity that has state (its data
values) and behavior (its methods). An object is one of the elements of data
and function that programs create, manipulate, pass as arguments, and so forth.
An object is a way to encapsulate state and behavior. Encapsulation permits
many aspects of the implementation of an object to change without affecting
client programs that depend on the object's behavior. In SOM, objects are
created by other objects called classes.


ΓòÉΓòÉΓòÉ 13.88. object adapter (OA) ΓòÉΓòÉΓòÉ

object adapter (OA)

A CORBA term denoting the primary interface a server implementation uses to
access ORB functions; in particular, it defines the mechanisms that a server
uses to interact with DSOM, and vice versa. This includes server
activation/deactivation, dispatching of methods, and authentication of the
principal making a call. The basic object adapter described by CORBA is defined
by the BOA (basic object adapter) abstract class; DSOM's primary object adapter
implementation is provided by the SOMOA (SOM Object Adapter) class.


ΓòÉΓòÉΓòÉ 13.89. object definition ΓòÉΓòÉΓòÉ

object definition

See class.


ΓòÉΓòÉΓòÉ 13.90. object implementation ΓòÉΓòÉΓòÉ

object implementation

See implementation.


ΓòÉΓòÉΓòÉ 13.91. object instance ΓòÉΓòÉΓòÉ

object instance

See instance and object.


ΓòÉΓòÉΓòÉ 13.92. object reference ΓòÉΓòÉΓòÉ

object reference

A CORBA term denoting the information needed to reliably identify a particular
object. This concept is implemented in DSOM with a proxy object in a client
process, or a SOMDObject in a server process. See also proxy object and
SOMDObject.


ΓòÉΓòÉΓòÉ 13.93. object request broker (ORB) ΓòÉΓòÉΓòÉ

object request broker (ORB)

See ORB.


ΓòÉΓòÉΓòÉ 13.94. offset method resolution: ΓòÉΓòÉΓòÉ

offset method resolution:

The default mechanism for performing method resolution in SOM, because it is
the fastest (nearly as fast as an ordinary procedure call). It is roughly
equivalent to the C ++ "virtual function" concept.  Using offset method
resolution requires that the name of the method to be invoked must be known at
compile time, the name of the class that introduces the method must be known at
compile time (although not necessarily by the programmer), and the method to be
invoked must be a static method.


ΓòÉΓòÉΓòÉ 13.95. OIDL ΓòÉΓòÉΓòÉ

OIDL

The original language used for declaring SOM classes. The acronym stands for
Object Interface Definition Language. OIDL is still supported by SOM release 2,
but it does not include the ability to specify multiple inheritance classes.


ΓòÉΓòÉΓòÉ 13.96. one-copy serializable ΓòÉΓòÉΓòÉ

one-copy serializable

The consistency property of the Replication Framework which states that the
concurrent execution of methods on a replicated object is equivalent to the
serial execution of those same methods on a nonreplicated object.


ΓòÉΓòÉΓòÉ 13.97. OOP ΓòÉΓòÉΓòÉ

OOP

An acronym for "object-oriented programming."


ΓòÉΓòÉΓòÉ 13.98. operation ΓòÉΓòÉΓòÉ

operation

See method.


ΓòÉΓòÉΓòÉ 13.99. operation logging ΓòÉΓòÉΓòÉ

operation logging

In the Replication Framework, a technique for maintaining consistency among
replicas of a replicated object, whereby the execution of a method that updates
the object is repeated at the site of each replica.


ΓòÉΓòÉΓòÉ 13.100. ORB (object request broker) ΓòÉΓòÉΓòÉ

ORB (object request broker)

A CORBA term designating the means by which objects transparently make requests
(that is, invoke methods) and receive responses from objects, whether they are
local or remote. With SOMobjects Developer Toolkit and Runtimes, this
functionality is implemented in the DSOM Framework. Thus, the DSOM (Distributed
SOM) system is an ORB. See also BOA (basic object adapter) class and SOMOA (SOM
object adapter) class.


ΓòÉΓòÉΓòÉ 13.101. overridden method ΓòÉΓòÉΓòÉ

overridden method

A method defined by a parent class and reimplemented (redefined or overridden)
in the current class.


ΓòÉΓòÉΓòÉ 13.102. override ΓòÉΓòÉΓòÉ

override

(Or overriding method.) The technique by which a class replaces (redefines) the
implementation of a method that it inherits from one of its parent classes. An
overriding method can elect to call the parent class's method procedure as part
of its own implementation. (Such a call is known as a parent method call.)


ΓòÉΓòÉΓòÉ 13.103. parent class ΓòÉΓòÉΓòÉ

parent class

A class from which another class inherits instance methods, attributes, and
instance variables. A parent class is sometimes called a base class or
superclass.


ΓòÉΓòÉΓòÉ 13.104. parent method call ΓòÉΓòÉΓòÉ

parent method call

A technique where an overriding method calls the method procedure of its parent
class as part of its own implementation.


ΓòÉΓòÉΓòÉ 13.105. persistent object ΓòÉΓòÉΓòÉ

persistent object

An object whose state can be preserved beyond the termination of the process
that created it. Typically, such objects are stored in files.


ΓòÉΓòÉΓòÉ 13.106. polymorphism ΓòÉΓòÉΓòÉ

polymorphism

An object-oriented programming feature that may take on different meanings in
different systems. Under various definitions of polymorphism, (a) a method or
procedure call can be executed using arguments of a variety of types, or (b)
the same variable can assume values of different types at different times, or
(c) a method name can denote more than one method procedure.  The SOM system
reflects the third definition (for example, when a SOM class overrides a parent
class definition of a method to change its behavior). The term literally means
"having many forms."


ΓòÉΓòÉΓòÉ 13.107. principal ΓòÉΓòÉΓòÉ

principal

The user on whose behalf a particular (remote) method call is being performed.


ΓòÉΓòÉΓòÉ 13.108. procedure ΓòÉΓòÉΓòÉ

procedure

A small section of code that executes a limited, well-understood task when
called from another program. In SOM, a method procedure is often referred to as
a procedure. See also method procedure.


ΓòÉΓòÉΓòÉ 13.109. process ΓòÉΓòÉΓòÉ

process

A series of instructions (a program or part of a program) that a computer
executes in a multitasking environment.


ΓòÉΓòÉΓòÉ 13.110. proxy object ΓòÉΓòÉΓòÉ

proxy object

In DSOM, a SOM object in the client's address space that represents a remote
object. The proxy object has the same interface  as the remote object, but each
method invoked on the proxy is overridden by a dispatch method that forwards
the invocation request to the remote object. Under DSOM, a proxy object is
created dynamically and automatically in the client whenever a remote method
returns a pointer to an object that happens to be remote.


ΓòÉΓòÉΓòÉ 13.111. readers and writers ΓòÉΓòÉΓòÉ

readers and writers

In the Replication Framework, different processes can access the same
replicated object in different modes. A "reader" is a process that does not
intend to update the object, but wants to continually watch the object as other
processes update it. A "writer" is a process that wants to update the object,
as well as continually watch the updates performed by others.


ΓòÉΓòÉΓòÉ 13.112. receiver ΓòÉΓòÉΓòÉ

receiver

See target object.


ΓòÉΓòÉΓòÉ 13.113. redispatch stub ΓòÉΓòÉΓòÉ

redispatch stub

A procedure, corresponding to a particular method, which has the same signature
as the method's procedure but which invokes somDispatch to dispatch the method.
The somOverrideMtab method can be used to replace the procedure pointers in a
class's method table with the corresponding redispatch stubs. This is done when
overriding somDispatch to customize method resolution so that all static method
invocations will be routed through somDispatch for selection of an appropriate
method procedure. (Dynamic methods have no entries in the method table, so they
cannot be supported with redispatch functionality.)


ΓòÉΓòÉΓòÉ 13.114. reference data ΓòÉΓòÉΓòÉ

reference data

Application-specific data that a server uses to identify or describe an object
in DSOM. The data, represented by a sequence of up to 1024 bytes, is registered
with DSOM when a server creates an object reference. A server can later ask
DSOM to return the reference data associated with an object reference. See also
object reference.


ΓòÉΓòÉΓòÉ 13.115. replica ΓòÉΓòÉΓòÉ

replica

When an object is replicated among a set of processes (using the Replication
Framework), each process is said to have a replica of the object. From the view
point of any application model, the replicas together represent a single
object.


ΓòÉΓòÉΓòÉ 13.116. replicated object ΓòÉΓòÉΓòÉ

replicated object

An object for which replicas (copies) exist.  See replica.


ΓòÉΓòÉΓòÉ 13.117. run-time environment ΓòÉΓòÉΓòÉ

run-time environment

The data structures, objects, and global variables that are created,
maintained, and used by the functions, procedures, and methods in the SOM
run-time library.


ΓòÉΓòÉΓòÉ 13.118. scope ΓòÉΓòÉΓòÉ

scope

(Or naming scope.) That portion of a program within which an identifier name
has "visibility" and denotes a unique variable. In SOM, an IDL source file
forms a scope. An identifier can only be defined once within a scope;
identifiers can be redefined within a nested scope. In a .idl file, modules,
interface statements, structures, unions, methods, and exceptions form nested
scopes.


ΓòÉΓòÉΓòÉ 13.119. serializable ΓòÉΓòÉΓòÉ

serializable

See one-copy serializable.


ΓòÉΓòÉΓòÉ 13.120. server ΓòÉΓòÉΓòÉ

server

(Or server implementation.) In DSOM, a process, running in a distributed
environment, that executes the implementation of an  object. DSOM provides a
default server implementation that can dynamically load SOM class libraries,
create SOM objects, and make those objects accessible to clients. Developers
can also write application-specific servers for use with DSOM.


ΓòÉΓòÉΓòÉ 13.121. server object ΓòÉΓòÉΓòÉ

server object

In DSOM, every server has an object that defines methods for managing objects
in that server. These methods include object creation, object destruction, and
maintaining mappings between object references and the objects they reference.
A server object must be an instance of the class SOMDServer (or one of its
subclasses). See also object reference and SOMDObject.


ΓòÉΓòÉΓòÉ 13.122. shadowing ΓòÉΓòÉΓòÉ

shadowing

In the Emitter Framework, a technique that is required when any of the entry
classes are subclassed. Shadowing causes instances of the new subclass(es)
(rather than instances of the original entry classes) to be used as input for
building the object graph, without requiring a recompile of emitter framework
code. Shadowing is accomplished by using the macro SOM_SubstituteClass.


ΓòÉΓòÉΓòÉ 13.123. signature ΓòÉΓòÉΓòÉ

signature

The collection of types associated with a method (the type of its return value,
if any, as well as the number, order, and type of each of its arguments).


ΓòÉΓòÉΓòÉ 13.124. sister class object ΓòÉΓòÉΓòÉ

sister class object

A duplicate of a class object that is created in order to save a copy of the
class's original method table before replacing the method table to customize
method resolution. The sister class object is created so that some original
method procedures can be called by the replacement method procedures.


ΓòÉΓòÉΓòÉ 13.125. Sockets class ΓòÉΓòÉΓòÉ

Sockets class

A class that provides a common communications interface to Distributed SOM, the
Replication Framework, and the Event Management Framework. The Sockets class
provides the base interfaces (patterned after TCP/IP sockets); the subclasses
TCPIPSockets, NBSockets, and IPXSockets provide actual implementations for
TCP/IP, Netbios, and Netware IPX/SPX, respectively.


ΓòÉΓòÉΓòÉ 13.126. SOM Compiler ΓòÉΓòÉΓòÉ

SOM Compiler

A tool provided by the SOM Toolkit that takes as input the interface definition
file for a class (the .idl file) and produces a set of binding files that make
it more convenient to implement and use SOM classes.


ΓòÉΓòÉΓòÉ 13.127. SOMClass ΓòÉΓòÉΓòÉ

SOMClass

One of the three primitive class objects of the SOM run-time environment.
SOMClass is the root (meta)class from which all subsequent metaclasses are
derived. SOMClass defines the essential behavior common to all SOM class
objects.


ΓòÉΓòÉΓòÉ 13.128. SOMClassMgr ΓòÉΓòÉΓòÉ

SOMClassMgr

One of the three primitive class objects of the SOM run-time environment.
During SOM initialization, a single instance (object) of SOMClassMgr is
created, called SOMClassMgrObject. This object maintains a directory of all SOM
classes that exist within the current process, and it assists with dynamic
loading and unloading of class libraries.


ΓòÉΓòÉΓòÉ 13.129. SOM-derived metaclass: ΓòÉΓòÉΓòÉ

SOM-derived metaclass:

See derived metaclass.


ΓòÉΓòÉΓòÉ 13.130. SOMDObject ΓòÉΓòÉΓòÉ

SOMDObject

The class that implements the notion of a CORBA "object reference" in DSOM. An
instance of SOMDObject contains information about an object's server
implementation and interface, as well as a user-supplied identifier.


ΓòÉΓòÉΓòÉ 13.131. somId ΓòÉΓòÉΓòÉ

somId

A pointer to a number that uniquely represents a zero-terminated string. Such
pointers are declared as type somId. In SOM, somId's are used to represent
method names, class names, and so forth.


ΓòÉΓòÉΓòÉ 13.132. SOMObject ΓòÉΓòÉΓòÉ

SOMObject

One of the three primitive class objects of the SOM run-time environment.
SOMObject is the root class for all SOM (sub)classes. SOMObject defines the
essential behavior common to all SOM objects.


ΓòÉΓòÉΓòÉ 13.133. SOMOA (SOM object adapter) class ΓòÉΓòÉΓòÉ

SOMOA (SOM object adapter) class

In DSOM, a class that dispatches methods on a server's objects, using the SOM
Compiler and run-time support. The SOMOA class implements methods defined in
the abstract BOA class (its base class). See also BOA class.


ΓòÉΓòÉΓòÉ 13.134. somSelf ΓòÉΓòÉΓòÉ

somSelf

Within method procedures  in the implementation file for a class, a parameter
pointing to the target object that is an instance of the class being
implemented.  it is local to the method procedure.


ΓòÉΓòÉΓòÉ 13.135. somThis ΓòÉΓòÉΓòÉ

somThis

Within method procedures, a local variable that points to a data structure
containing the instance variables introduced by the class. If no instance
variables are specified in the SOM IDL source file, then the somThis assignment
statement is commented out by the SOM Compiler.


ΓòÉΓòÉΓòÉ 13.136. state (of an object) ΓòÉΓòÉΓòÉ

state (of an object)

The data (attributes, instance variables and their values) associated with an
object. See also behavior.


ΓòÉΓòÉΓòÉ 13.137. static method ΓòÉΓòÉΓòÉ

static method

Any method that can be accessed through offset method resolution.  Any method
declared in the IDL specification of a class is a static method. See also
method and dynamic method.


ΓòÉΓòÉΓòÉ 13.138. stub procedures ΓòÉΓòÉΓòÉ

stub procedures

Method procedures in the implementation template generated by the SOM Compiler.
They are procedures whose bodies are largely vacuous, to be filled in by the
implementor.


ΓòÉΓòÉΓòÉ 13.139. subclass ΓòÉΓòÉΓòÉ

subclass

A class that inherits instance methods, attributes, and instance variables
directly from another class, called the parent class, base class, superclass,
or indirectly from an ancestor class. A subclass may also be called a child
class or derived class.


ΓòÉΓòÉΓòÉ 13.140. subclassing ΓòÉΓòÉΓòÉ

subclassing

The process whereby a new class, as it is created (or derived), inherits
instance methods, attributes, and instance variables from one or more
previously defined ancestor classes. The immediate parent class(es) of a new
class must be specified in the class's interface declaration. see also
inheritance.


ΓòÉΓòÉΓòÉ 13.141. superclass ΓòÉΓòÉΓòÉ

superclass

See parent class.


ΓòÉΓòÉΓòÉ 13.142. symbol ΓòÉΓòÉΓòÉ

symbol

In the Emitter Framework, any of a (standard or user-defined) set of names
(such as, className) that are used as placeholders when building a text
template to pattern the desired emitter output. When a template is emitted, the
symbols are replaced with their corresponding values from the emitter's symbol
table. Other symbols (such as, classSN) have values that are used by
section-emitting methods to identify major sections of the template (which are
correspondingly labeled as "classS" or by a user-defined name).


ΓòÉΓòÉΓòÉ 13.143. target object ΓòÉΓòÉΓòÉ

target object

(Or receiver.) The object responding to a method call. The target object is
always the first formal parameter of a method procedure. For SOM's C-language
bindings, the target object is the first argument provided to the method
invocation macro, _methodName.


ΓòÉΓòÉΓòÉ 13.144. usage bindings ΓòÉΓòÉΓòÉ

usage bindings

The language-specific binding files for a class that are generated by the SOM
Compiler for inclusion in client programs using the class.


ΓòÉΓòÉΓòÉ 13.145. value logging ΓòÉΓòÉΓòÉ

value logging

In the Replication Framework, a technique for maintaining consistency among
replicas of a replicated object, whereby the new value of the object is
distributed after the execution of a method that updates the object.


ΓòÉΓòÉΓòÉ 13.146. view-data paradigm ΓòÉΓòÉΓòÉ

view-data paradigm

A Replication Framework construct similar to the Model-View-Controller paradigm
in SmallTalk. The "view" object contains only presentation-specific
information, while the "data" object contains the state of the application. The
"view" and "data" are connected by means of an "observation" protocol that lets
the "view" be notified whenever the "data" changes.


ΓòÉΓòÉΓòÉ 13.147. writers ΓòÉΓòÉΓòÉ

writers

See readers and writers.


ΓòÉΓòÉΓòÉ 14. Bibliography ΓòÉΓòÉΓòÉ

This bibliography lists the publications that make up the IBM VisualAge for C++
library and related publications. The list of related publications is not
exhaustive but should be adequate for most VisualAge for C++ users.


ΓòÉΓòÉΓòÉ 14.1. The IBM VisualAge for C++ Library ΓòÉΓòÉΓòÉ

The following books are part of the IBM VisualAge for C++ library.

     Installation Guide and Product Overview, S33H-5030

     User's Guide, S33H-5031

     Programming Guide, S33H-5032

     Visual Builder User's Guide, S33H-5034

     Visual Builder Parts Reference, S33H-5035

     Building VisualAge for C++ Parts for Fun and Profit, S33H-5036

     Open Class Library User's Guide, S33H-5033

     Open Class Library Reference, S33H-5039

     Language Reference, S33H-5037

     C Library Reference, S33H-5038

     SOM Programming Guide, S33H-5043

     SOM Programming Reference, S33H-5044


ΓòÉΓòÉΓòÉ 14.2. C and C++ Related Publications ΓòÉΓòÉΓòÉ

     Portability Guide for IBM C, SC09-1405

     American National Standard for Information Systems / International
      Standards Organization - Programming Language C (ANSI/ISO
      9899-1990[1992])


ΓòÉΓòÉΓòÉ 14.2.1. Non-IBM Publications ΓòÉΓòÉΓòÉ

Many books have been written about the C++ language and related programming
topics. The authors use varying approaches and emphasis. The following is a
sample of some non-IBM C++ publications that are generally available.  This
sample is not an exhaustive list.  IBM does not specifically recommend any of
these books, and other C++ books may be available in your locality.

     The Annotated C++ Reference Manual by Margaret A. Ellis and Bjarne
      Stroustrup, Addison-Wesley Publishing Company.

     C++ Primer by Stanley B. Lippman, Addison-Wesley Publishing Company.

     Object-Oriented Design with Applications by Grady Booch,
      Benjamin/Cummings.

     Object-Oriented Programming Using SOM and DSOM by Christina Lau, Van
      Nostrand Reinhold.