Power-Programmierung/CD2.mdf/c/tutorial/cpptutor/text/chap11.txt

Text File | 1994-05-15 | 19.6 KB | 406 lines

                                                       Chapter 11
                                           MORE VIRTUAL FUNCTIONS

This chapter will actually be a continuation of the topics
covered in the last chapter but this will be a fuller explanation
of what virtual functions are and how they can be used in a
program.  We will present a simple database program with a
virtual function to show how it can be used, then we will go on
to illustrate a more complex use of the virtual function in a
manner that finally illustrates its utility and reason for
existence.


HOW TO START AN OOP PROJECT
-----------------------------------------------------------------
The observant student will notice that we begin our use of object
oriented programming by identifying an object, or in this case, a
class of objects and even some subordinate objects, which we
completely define.  When we get to the main program we then have
a simple job with the remaining needs and they are completed
using standard procedural programming techniques which we are
familiar with.  This is the way to begin any object oriented
programming project, by first identifying a few objects that can
be separated conveniently from the rest of the code, programming
them, then writing the main program.  It should be added that,
for your first project using objects, do not try to make
everything an object.  Select a few objects and after gaining
experience with object oriented programming techniques, use more
objects on future projects.  Most programmers use too many
objects for their first project and write very obtuse,
unreadable code.


THE PERSON HEADER FILE
-----------------------------------------------------------------
Examine the file named PERSON.H for the        ==================
definition file for the person class.  This         PERSON.H
class definition should cause you no problem   ==================
to understand since there is nothing new
here.  The only thing that should be mentioned about this class
is that the protected mode is used for the variables so that they
are readily available in the derived classes which will inherit
this class.


THE PERSON IMPLEMENTATION
-----------------------------------------------------------------
The implementation for the person class is     ==================
given here and it is a little strange in the       PERSON.CPP
way it is written and used.  The intent of     ==================
this program is that the virtual method named
display() in this file will never be used, but it is required by

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the C++ compiler to be used for a default in case some of the
subclasses do not have this function available.  In the main
program we will be careful to never call this function due to the
nature of the program we are writing.  Keep in mind that C++
requires an implementation of all virtual functions even if they
are never used.  In this case the message is obviously intended
to be output as an error message.

Be sure to compile this program prior to going on to the next
class definitions.


THE SUPERVISOR HEADER
-----------------------------------------------------------------
The file named SUPERVSR.H contains the class   ==================
definitions for the three derived classes,         SUPERVSR.H
supervisor, programmer, and secretary.  These  ==================
were all placed in a single file for two
reasons.  The first reason is to simply illustrate to you that
this can be done, and secondly, to allow some of the files to be
combined on the disk and to require fewer compilations by you
prior to executing the resulting program.  This is actually a
good way to combine these files since they are all derived
classes of a common class.  It is a matter of style or personal
taste.

You will notice that all three of these classes contain a method
named display() and all have the same return value of void, and
all have the same number of parameters as the parent class's
method of the same name.  All of this equality is required
because they will all be called by the same call statement.  You
will also notice that the other method in each class has the same
name, but different numbers and types of formal parameters which
prevents this method from being used as a virtual method.

The remainder of this file is simple and you should be able to
read the code and understand it completely.  Once again, this
file cannot be compiled or executed.


THE SUPERVISOR IMPLEMENTATION
-----------------------------------------------------------------
The file named SUPERVSR.CPP contains the       ==================
implementation for the three classes.  If         SUPERVSR.CPP
you spend a little time studying the code,     ==================
you will find that each of the methods named
init_data() simply initializes all fields to those passed in as
the actual arguments in a very simple manner.  The method named
display(), however, outputs the stored data in different ways for
each class since the data is so different in each of the classes.
Even though the interface to these three methods is identical,
the actual code is significantly different.  There is no reason
code besides output could not have been used, but the output is

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so visible when the program is executed that it was chosen for
this illustration.

This file should be compiled at this time in preparation for the
next example program which will use all four classes as defined
in these four files.


THE FIRST CALLING PROGRAM
-----------------------------------------------------------------
The file named EMPLOYEE.CPP is the first       ==================
program that uses the classes developed in        EMPLOYEE.CPP
this chapter, and you will find that it is a   ==================
very simple program.

We begin with an array of ten pointers, each pointing to the base
class.  As you recall from the last chapter, this is very
important when using virtual functions, the pointer must point to
the base class.  The pointers that will be stored in this array
will all point to objects of the derived classes however.  When
we use the resulting pointers to refer to the methods, the system
will choose the method at run time, not at compile time as nearly
all of our other programs have been doing.

We allocate six objects in lines 16 through 39, initialize them
to some values using the methods named init_data(), then assign
the pointers to the members of the array of pointers to person.
Finally, in lines 41 and 42, we call the methods named display()
to display the stored data on the monitor.  You will notice that
even though we only use one method call in line 42, we actually
send messages to each of the three methods named display() in the
subclasses.  This is true dynamic binding because if we were to
change the values of some of the pointers in the array, we would
then call different methods with the same pointers.

Be sure to compile and execute this program before continuing on
in this chapter.  You will recall that the linking step requires
you to combine several files in order to satisfy all system
calls.  After you have done that, we will use the same objects in
another way to show how they can be reused.


THE LINKED LIST CLASS
-----------------------------------------------------------------
Examination of the file named ELEMLIST.H will  ==================
reveal the definition of two more classes          ELEMLIST.H
which will be used to build a linked list of   ==================
employees to illustrate a more practical way
to use the dynamic binding we have been studying in this chapter.
The two classes were put in the same file because they work
together so closely and neither is of much value without the
other.  You will notice that the elements of the linked list do
not contain any data, only a pointer to the person class that we

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developed for the last program, so that the linked list will be
composed of elements of the person class without modifying the
class itself.

There are two interesting constructs used here that must be
pointed out before going on to the next program.  The first is
the partial declaration given in line 8 which allows us to refer
to the class named employee_list before we actually define it.
The complete declaration for the class is given in lines 22
through 29.  The second construct of interest is the friend class
listed in line 17 where we give the entire class named
employee_list free access to the variables which are a part of
the employee_element class.  This is necessary because the method
named add_person() must access the pointers contained in
employee_element.  We could have defined an additional method as
a part of employee_element and used this method to refer to the
pointers but it was felt that these two classes work so well
together that it is not a problem to open a window between the
classes.  We still have complete privacy from all other programs
and classes declared as parts of this program.

Note that the single method included in the employee_element
class is implemented in inline code.  Two of the methods of
employee_list are still open so we need an implementation for
this class.


THE LINKED LIST IMPLEMENTATION
-----------------------------------------------------------------
The file named ELEMLIST.CPP is the             ==================
implementation for the linked list classes        ELEMLIST.CPP
and should be self explanatory if you          ==================
understand how a singly linked list operates.
All new elements are added to the end of the current list.  This
was done to keep it simple but an alphabetic sorting mechanism
could be added to sort the employees by name if desired.

The method to display the list simply traverses the list and
calls the method named display() in line 30 once for each element
of the list.

It is important for you to take notice that in this entire class,
there is no mention made of the existence of the three derived
classes, only the base class named person is mentioned.  The
linked list therefore has no hint that the three subclasses even
exist, but in spite of that, we will see this class send messages
to the three subclasses as they are passed through this logic.
That is exactly what dynamic binding is, and we will have a
little more to say about it after we examine the calling program.





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USING THE LINKED LIST
-----------------------------------------------------------------
At this time you should examine the example    ==================
program named EMPLOYE2.CPP for our best           EMPLOYE2.CPP
example of dynamic binding in this tutorial,   ==================
yet the program is kept very simple.

This program is very similar to the example program named
EMPLOYEE.CPP with a few changes to better illustrate dynamic
binding.  In line 7 we declare an object of the class
employee_list to begin our linked list.  This is the only copy of
the list we will need for this program.  For each of the
elements, we allocate the data, fill it, and send it to the
linked list to be added to the list where we allocate another
linked list element to point to the new data, and add it to the
list.  The code is very similar to the last program down through
line 40.

In line 43 we send a message to the display_list() method which
outputs the entire list of personnel.  You will notice that the
linked list class defined in the files named ELEMLIST.H and
ELEMLIST.CPP are never informed in any way that the subclasses
even exist but they dutifully pass the pointers to these
subclasses to the correct methods and the program runs as
expected.

If you changed PERSON.H to use a pure virtual function, it will
still work with this program just as we discussed earlier.


WHAT GOOD IS ALL OF THIS?
-----------------------------------------------------------------
Now that we have the program completely debugged and working,
suppose that we wished to add another class to the program.  For
example, suppose we wished to add a class named consultant
because we wished to include some consultants in our business.
We would have to write the class of course and the methods within
the classes, but the linked list doesn't need to know that the
new class is added, so it doesn't require any changes in order to
update the program to handle consultant class objects.  In this
particular case, the linked list is very small and easy to
understand, but suppose the code was very long and complex as
with a large database.  It would be very difficult to update
every reference to the subclasses and add another subclass to
every list where they were referred to, and this operation would
be very error prone.  In the present example program, the linked
list would not even have to be recompiled in order to add the new
functionality.

It should be clear to you that it would be possible to actually
define new types, dynamically allocate them, and begin using them
even while the program was executing if we properly partitioned
the code into executable units operating in parallel.  This would

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not be easy, but it could be done for a large database that was
tracking the inventory for a large retail store, or even for an
airlines reservation system.  You probably have little difficulty
understanding the use of dynamically allocated memory for data,
but dynamically allocating classes or types is new and difficult
to grasp, but the possibility is there with dynamic binding.


AN APPLICATION FRAMEWORK
-----------------------------------------------------------------
The example program named APPFRAM1.CPP         ==================
illustrates the method used to write an           APPFRAM1.CPP
application framework.  If you do much         ==================
serious programming, you will encounter one
because they provide so many benefits for the MS Windows
programmer.  There are application frameworks available for DOS
too, so most programmers should be familiar with them.

The class named CForm is the base class for our trivial but
important example, and it consists of four methods but no data
members.  The diligent student will note that the method named
display_form calls the other three members to actually do the
work of displaying our little form on the monitor.  There is
nothing magic about this program except that it is the framework
for a very interesting concept used by all of the application
frameworks currently available.  Note that three of the methods
are declared as virtual in lines 8 through 10.

The interesting part occurs when we inherit the class into our
new class named CMyForm in line 22 and write new methods for two
of the base class methods.  We have inherited as much
functionality from the base class as we liked and wrote new
methods for those that didn't serve our purpose as originally
written.  When we finally execute an object of the new class in
line 34 of the main program, we do indeed use part of the base
class, and override those parts that we explicitly wrote methods
for.

This is not too appealing in such a simple example, but if we
consider how this is used in a real application framework, it is
a very useful construct.  The writer of an application framework
will write a complete program that does all of the necessary
housekeeping and windows maintenance chores and partition it in a
number of virtual methods much like we did here.  Of course, the
framework will be composed of a large body of code to do all of
those chores.  In much the same manner that we picked which parts
of this little form display program we wished to use and which to
override, we pick those parts of the framework that we wish to
keep as is and override those parts that we wish change.  A large
body of code is therefore already written for us and ready for
our use.



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The application framework can include many other preprogrammed
functions ready for our use, such as editor code for edit
windows, data verification code for dialogue windows, and ready
to use message window code.  You will find it to be a very
rewarding study to spend time examining the capabilities of one
or more of the commercially available application frameworks.


A PURE VIRTUAL FUNCTION
-----------------------------------------------------------------
The example program named APPFRAM2.CPP         ==================
illustrates a pure virtual function.  A pure      APPFRAM2.CPP
virtual function is declared by assigning the  ==================
value of zero to the function as illustrated
in line 9.  A class containing one or more pure virtual functions
cannot be used to define an object.  The class is therefore only
useful as a base class to be inherited into a useable derived
class.  It is sometimes called an abstract class.

Every derived class must include a function for each pure virtual
function that is inherited from the base class.  This assures
that there will be a function available for each call and none
will ever need to be answered by the base class, which it cannot
do since it does not have an implementation for a pure virtual
function.  You cannot create an object of any class which
contains one or more pure virtual functions, because there is
nothing to answer a message if one is sent to the pure virtual
method.  The compiler will enforce the two rules mentioned in
this paragraph.  If a class inherits an abstract class without
providing an override for the pure virtual function, then it too
becomes an abstract class and cannot be used to define an object.

You will notice that the present example uses an abstract base
class which makes it illegal to use an object of the base class
as we did in the last example program.  For that reason, some of
the main program is commented out.

You will find abstract classes used in many commercially
available libraries and in application frameworks.  Be sure to
compile and execute this program, then make some modifications to
see what the compiler indicates if you try to violate some of the
rules we have delineated here.

PROGRAMMING EXERCISE
-----------------------------------------------------------------
1.  Add a new class named consultant to the files named
    SUPERVSR.H and SUPERVSR.CPP, then add code to EMPLOYE2.CPP to
    exercise the new class.  Note that you do not need to
    recompile the linked list class in order to execute the new
    code and use the new class.  Even without recompiling the
    linked list class it is capable of storing and passing the
    new class of data provided of course that the new class is
    referred to using a pointer to the parent class.

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