Power-Programmierung/CD2.mdf/c/tutorial/cpptutor/text/chap05.txt

Text File | 1994-05-15 | 45.3 KB | 928 lines

                                                        Chapter 5
                                                    ENCAPSULATION

As mentioned in Chapter 1, object oriented programming will seem
very unnatural to a programmer with a lot of procedural
programming experience.  This chapter is the beginning of the
definition of object oriented programming, and we will study the
topic of encapsulation which is a "divide and conquer" technique.
As we stated earlier, there are a lot of new terms used with
object oriented programming.  Don't be intimidated by the new
terminology, we will study the terms one at a time in a
meaningful order.

Encapsulation is the process of forming objects which we will
discuss throughout this chapter.  An encapsulated object is often
called an abstract data type and it is what object oriented
programming is all about.  Without encapsulation, which involves
the use of one or more classes, there is no object oriented
programming.  Of course there are other topics concerning object
oriented programming, but this is the cornerstone.


WHY BOTHER WITH ENCAPSULATION?
-----------------------------------------------------------------
We need encapsulation because we are human, and humans make
errors.  When we properly encapsulate some code, we actually
build an impenetrable wall to protect the contained code from
accidental corruption due to the silly little errors that we are
all prone to make.  We also tend to isolate errors to small
sections of code to make them easier to find and fix.  We will
have a lot more to say about the benefits of encapsulation as we
progress through the tutorial.


NO INFORMATION HIDING
-----------------------------------------------------------------
The program named OPEN.CPP is a really stupid    ================
program because it does next to nothing, but         OPEN.CPP
it will be the beginning point for our           ================
discussion of encapsulation, otherwise known
as information hiding.  Information hiding is an important part
of object oriented programming and you should have a good grasp
of what it is by the time we finish this chapter.

A very simple structure is defined in lines 4 through 6 which
contains a single int type variable within the structure.  This
is sort of a silly thing to do, but it will illustrate the
problem we wish to overcome in this chapter.  Three variables are
declared in line 10, each of which contains a single int type
variable and each of the three variables are available for use
anywhere within the main function.  Each variable can be
assigned, incremented, read, modified, or have any number of

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                                        Chapter 5 - Encapsulation

operations performed on it.  A few of the operations are
illustrated in lines 13 through 21 and should be self explanatory
to anyone with a little experience with the C programming
language.

An isolated local variable named piggy is declared and used in
the same section of code to illustrate that there is nothing
magic about this code.  Figure 5-1 is a graphical representation
of the data space after execution of line 16.  Study this simple
program carefully because it is the basis for beginning our study
of encapsulation.  Be sure to compile and execute this program,
then we will go on to the next example program where we will see
our first example of real information hiding.


INFORMATION HIDING
-----------------------------------------------------------------
Examine the program named CLAS.CPP for our     ==================
first example of a program with a little            CLAS.CPP
information hiding contained in it.  This      ==================
program is identical to the last one except
for the way it does a few of its operations.  We will take the
differences one at a time and explain what is happening.  Keep in
mind that this is a trivial program and the safeguards built into
it are not needed for such a simple program but are used here to
illustrate how to use these techniques in a larger much more
complicated program.

The first difference is that we have a class instead of a
structure beginning in line 4 of this program.  The only
difference between a class and a structure is that a class begins
with a private section whereas a structure begins with a private
section.  The keyword class is used to declare a class as
illustrated here.

The class named one_datum is composed of the single variable
named data_store and two functions, one named set() and the other
named get_value().  A more complete definition of a class is a
group of variables, and one or more functions that can operate on
that data.  Stay with us, we will tie this all together in a
meaningful and useful way very soon.


WHAT IS A PRIVATE SECTION?
-----------------------------------------------------------------
All data at the beginning of a class defaults to private.
Therefore, the data at the beginning of the class cannot be
accessed outside of the class, it is hidden from any outside
access.  Therefore, the variable named data_store which is a part
of the object (an object will be defined completely later) named
dog1 declared in line 23, is not available for use anywhere in
the main program.  It is as if we have built a "brick wall"
around the variables to protect them from accidental corruption

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                                        Chapter 5 - Encapsulation

by outside programming influences.  It seems a little dumb to
declare a variable in the main program that we cannot use, but
that is exactly what we did.

Figure 5-2 is a graphical representation of the class with its
"brick wall" built around the data to protect it.  You will
notice the small peep holes we have opened up to allow the user
to gain access to the functions.  The peep holes were opened by
declaring the functions in the public section of the class.


WHAT IS A PUBLIC SECTION?
-----------------------------------------------------------------
A new keyword, public, is introduced in line 6 which states that
anything following this keyword can be accessed from outside of
this class.  Because the two functions are defined following the
keyword public, they are both public and available for use by
any  calling program that is within the scope of this object.
This essentially opens two small peepholes in the solid wall of
protection that we built around the class.  You should keep in
mind that the private variable is not available to the calling
program.  Thus, we can only use the variable by calling one of
the two functions defined within the public part of the class.
These are called member functions because they are members of the
class.

Since we have declared two functions, we need to define them by
saying what each function will actually do.  This is done in
lines 11 through 19 where they are each defined in the normal
way, except that the class name is prepended onto the function
name and separated from it by a double colon.  These two function
definitions are called the implementation of the functions.  The
class name is required because we can use the same function name
in other classes and the compiler must know with which class to
associate each function implementation.

One of the key points to be made here is that the private data
contained within the class is available within the implementation
of the member functions of the class for modification or reading
in the normal manner.  You can do anything with the private data
within the function implementations which are a part of that
class, but the private data of other classes is hidden and not
available within the member functions of this class.  This is the
reason we must prepend the class name to the function names of
this class when defining them.  Figure 5-3 depicts the data space
following execution of line 29.

It would be well to mention at this point that it is legal to
include variables and functions in the private part, and
additional variables and functions in the public part also.  In
most practical situations, variables are included in only the
private part and functions are included in only the public part
of a class definition.  Occasionally, variables or functions are

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                                        Chapter 5 - Encapsulation

used in the other part.  This sometimes leads to a very practical
solution to a particular problem, but in general, the entities
are used only in the places mentioned.

In C++ we have three scopes of variables, local, file and class.
Local variables are localized to a single function.  File
variables, those that are defined outside of any function, are
available anywhere in a file following their definition.  A
variable with class scope is available anywhere within the scope
of a class and nowhere else.  The variable named data_store has a
class scope.

You must be very confused by this point since we have given a lot
of rules but few reasons for doing all of this.  Stay with us and
you will soon see that there are very practical reasons for doing
all of this.


MORE NEW TERMINOLOGY
-----------------------------------------------------------------
As with most new technologies, developers seem to delight in
making up new names for all aspects of their new pet.  Object
oriented programming is no different, so we must learn new names
for some of our old familiar friends if we are going to learn how
to effectively use it.  To help you learn this new programming
terminology, we will list a few of them here and begin using them
in the text to get you used to seeing and using them.  You will
not understand them all yet, but we need to introduce them early.

    A class is a grouping of data and methods (functions).  A
    class is very much like a structure type as used in ANSI-C,
    it is only a pattern to be used to create a variable which
    can be manipulated in a program.

    An object is an instance of a class, which is similar to a
    variable defined as an instance of a type.  An object is
    what you actually use in a program since it has values and
    can be changed.

    A method is a function contained within the class.  You
    will find the functions used within a class often referred
    to as methods in programming literature.

    A message is the same thing as a function call.  In object
    oriented programming, we send messages instead of calling
    functions.  For the time being, you can think of them as
    identical.  Later in this tutorial we will see that they
    are in fact slightly different.

With all the new terminology, we will continue our study of the
program named CLAS.CPP and show you how to use the class.  We can
now say that we have a class composed of one variable and two
methods.  The methods operate on the variable contained in the

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                                        Chapter 5 - Encapsulation

class when they receive messages to do so.  In this tutorial we
will use the terms object and variable interchangeably because
both names are very descriptive of what the object really is.

This is a small point but it could be easily overlooked.  Lines 7
and 8 of this program are actually the prototypes for the two
methods, and is our first example of the use of a prototype
within a class.  This is the reason we spent so much time
studying prototypes in the last chapter.  You will notice line 7
which says that the method named set requires one parameter of
type int and returns nothing, hence the return type is void.  The
method named get_value() however, according to line 8, has no
input parameters but returns an int type value to the caller.


SENDING A MESSAGE
-----------------------------------------------------------------
Following all of the definitions in lines 1 through 19, we
finally come to the program where we actually use the class.  In
line 23 we declare three objects of the class one_datum and name
the objects dog1, dog2, and dog3.  Each object contains a single
data point which we can set through use of one method or read its
value through use of the other method, but we cannot directly set
or read the value of the data point because it is hidden within
the "block wall" around the class.  In line 26, we send a message
to the object named dog1 instructing it to set its internal value
to 12, and even though this looks like a function call, it is
properly called sending a message to a method.  Remember that the
object named dog1 has a method associated with it called set()
that sets its internal value to the actual parameter included
within the message.  You will notice that the form is very much
like the means of accessing the elements of a structure.  You
mention the name of the object with a dot connecting it to the
name of the method.  In a similar manner, we send a message to
each of the other two objects dog2 and dog3 to set their values
to those indicated.

Lines 31 and 32 have been commented out because the operations
are illegal. The variable named data_store is private and
therefore not available to the code outside of the object
itself.  It should be obvious, but it will be pointed out that
the data contained within the object named dog1 is not available
within the methods of dog2 or dog3 because they are different
objects.  These rules are all devised to help you develop better
code more quickly and you will soon see how they help.

The other method defined for each object is used in lines 34
through 36 to illustrate how it can be used.  In each case,
another message is sent to each object and the returned result is
output to the monitor via the stream library.




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                                        Chapter 5 - Encapsulation

USING A NORMAL VARIABLE
-----------------------------------------------------------------
There is another variable named piggy declared and used
throughout this example program that illustrates that a normal
variable can be intermixed with the objects and used in the
normal manner.  The use of this variable should pose no problem
to you, so after you understand the program, be sure to compile
and execute it.  It would be a good exercise for you to remove
the comments from lines 31 and 32 to see what kind of error
message your compiler issues.

This program illustrates information hiding but it will not be
clear to you that it really does anything worthwhile until we
study the next two programs.  Be sure to compile and execute this
program, then remove the comments from lines 31 and 32 as
suggested to see the error messages issued.


A PROGRAM WITH PROBLEMS
-----------------------------------------------------------------
Examine the program named OPENPOLE.CPP for an  ==================
example of a program with a few serious           OPENPOLE.CPP
problems that will be overcome in the next     ==================
example program by using the principles of
encapsulation.

We have two structures declared, one being a rectangle and the
other a pole.  The data fields should be self explanatory with
the exception of the depth of the flagpole which is the depth it
is buried in the ground, the overall length of the pole is
therefore the sum of the length and the depth.

Figure 5-4 depicts the data space for this program after
execution of line 32.  Based on your experience with ANSI-C, you
should have no problem understanding exactly what this program
is doing, but you may be a bit confused at the meaning of the
result found in line 38 where we multiply the height of the
square with the width of the box.  This is perfectly legal to do
in ANSI-C or C++, but the result has no earthly meaning because
the data are for two different entities.  Likewise, the result
calculated in line 40 is even sillier because the product of the
height of the square and the depth of the flagpole has absolutely
no meaning in any physical system we can think up.

Wouldn't it be neat if we had a way to prevent such stupid things
from happening in a large production program.  If we had a good
program that defined all of the things we can do with a square
and another program that defined everything we could do with a
pole, and if the data could be kept mutually exclusive, we could
prevent these silly things from happening.

It should come as no real surprise to you that the next program
will do just those things for us and do it in a very elegant way.

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                                        Chapter 5 - Encapsulation

Before proceeding on to the next example program, you should
compile and execute this one even though it displays some silly
results.


OBJECTS PROTECT DATA
-----------------------------------------------------------------
Examine the program named CLASPOLE.CPP as an   ==================
example of data protection in a very simple       CLASPOLE.CPP
program.  In this program the rectangle is     ==================
changed to a class with the same two
variables which are now private, and two methods to handle the
private data.  One method is used to initialize the values of the
objects created and the other method returns the area of the
object.  The two methods are defined in lines 12 through 21 in
the manner described earlier in this chapter.  The pole is left
as a structure to illustrate that the two can be used together
and that C++ is truly an extension of ANSI-C.

In line 33 we declare two objects, once again named box and
square, but this time we cannot assign values directly to their
individual components because they are private elements of the
class. Figure 5-5 is a graphical illustration of the two objects
available for use within the calling program.  Lines 36 through
38 are commented out for that reason and the messages are sent to
the objects in lines 40 and 41 to tell them to initialize
themselves to the values input as parameters.  The flag_pole is
initialized in the same manner as in the previous program.  Using
the class in this way prevents us from making the silly
calculations we did in the last program, because we can only
calculate the area by using the data stored in one object.  The
compiler is now being used to prevent the erroneous calculations.
The end result is that the stupid calculations we did in the last
program are not possible in this program, so lines 50 through 53
have been commented out.  Once again, it is difficult to see the
utility of this in such a simple program.  In a large program,
using the compiler to enforce the rules can pay off in a big way.

Even though the square and the box are both objects of class
rectangle, their private data is hidden from each other such that
neither can purposefully or accidentally change the other's data.

This is the abstract data type mentioned earlier in this chapter,
a model with a set of private variables for data storage and a
set of operations that can be performed on that stored data.  The
only operations that can be performed on the data are those
defined by the methods, which prevents many kinds of erroneous or
silly operations.  Encapsulation and data hiding bind the data
and procedures, or methods, tightly together and limit the scope
and visibility of each.  Once again, we have the divide and
conquer technique in which an object is separated from the rest
of the code and carefully developed in complete isolation from


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it.  Only then is it integrated into the rest of the code with a
few very simple interfaces.


HAVE YOU EVER USED THIS TECHNIQUE BEFORE?
-----------------------------------------------------------------
A good example of the use of this technique is in the file
commands you have been using with ANSI-C.  The data in the file
is only available through the predefined functions provided by
your compiler writer.  You have no direct access to the actual
data because it is impossible for you to address the actual data
stored on the disk.  The data is therefore private data, as far
as you are concerned, but the available functions are very much
like methods in C++.  There are two aspects of this technique
that really count when you are developing software.  First, you
can get all of the data you really need from the file system
because the interface is complete, but secondly, you cannot get
any data that you do not need.  You are prevented from getting
into the file handling system and accidentally corrupting some
data stored within it.  You are also prevented from using the
wrong data because the functions available demand a serial access
to the data.

Another example is in the monitor and keyboard handling routines.
You are prevented from getting into the workings of them and
corrupting them accidentally, or on purpose if you have such a
bent, but once again, you are provided with all of the data
interfaces that you really need.

Suppose you are developing a program to analyze some
characteristics of flagpoles.  You would not wish to
accidentally use some data referring to where the flagpole
program was stored on your hard disk as the height of the
flagpole, nor would you wish to use the cursor position as the
flagpole thickness or color.  All code for the flagpole is
developed alone, and only when it is finished, is it available
for external use.  When using it, you have a very limited number
of operations which you can do with the class.  The fact that
the data is hidden from you protects you from accidentally doing
such a thing when you are working at midnight to try to meet a
schedule.  Once again, this is referred to as information hiding
and is one of the primary advantages of object oriented
programming over procedural techniques.

Based on the discussion given above you can see that object
oriented programming is not really new, since it has been used in
a small measure for as long as computers have been popular.  The
newest development, however, is in allowing the programmer to
partition his programs in such a way that he too can practice
information hiding and reduce the debugging time.




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                                        Chapter 5 - Encapsulation

WHAT DOES THIS COST?
-----------------------------------------------------------------
It should be clear that this technique will cost you something in
efficiency because every access to the elements of the object
will require the time and inefficiency of a call to a function,
or perhaps I should be more proper and refer to it as a method.
The time saved in building a large program, however, could easily
be saved in debug time when it comes time to iron out the last
few bugs.  This is because a program made up of objects that
closely match the application are much easier to understand than
a program that does not.

This is obviously such a small program that it is silly to try to
see any gain with this technique.  In a real project however, it
could be a great savings if one person developed all of the
details of the rectangle, programmed it, and made it available to
you to simply use.  This is exactly what has been done for you if
you consider the video monitor an object.  There is a complete
set of preprogrammed and debugged routines you can use to make
the monitor do anything you wish it to do, all you have to do is
study the interface to the routines and use them, expecting them
to work.  As we mentioned earlier, it is impossible for you to
multiply the size of your monitor screen by the depth of the flag
pole because that information is not available to you to use in a
corruptible way.

After you understand some of the advantages of this style of
programming, be sure to compile and execute this program.


CONSTRUCTORS AND DESTRUCTORS
-----------------------------------------------------------------
The file named CONSPOLE.CPP introduces         ==================
constructors and destructors and should be        CONSPOLE.CPP
examined at this time.  This example program   ==================
is identical to the last example except that
a constructor has been added as well as a destructor.  The
constructor always has the same name as the class itself and is
declared in line 8, then defined in lines 14 through 18.  The
constructor is called automatically by the C++ system when the
object is declared and prevents the use of an uninitialized
variable.  When the object named box is declared in line 46, the
constructor is called automatically by the system.  The
constructor sets the values of height and width each to 6 in the
object named box.  This is printed out for reference in lines 49
and 50.  Likewise, when the square is declared in line 46, the
values of the height and the width of the square are each
initialized to 6 when the constructor is called automatically.

A constructor is defined as having the same name as the class
itself.  In this case both are named rectangle.  The constructor
cannot have a return type associated with it because of the
definition of C++.  It actually has a predefined return type, a

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                                        Chapter 5 - Encapsulation

pointer to the object itself, but we will not be concerned about
this until much later in this tutorial.  Even though both objects
are assigned values by the constructor, they are initialized in
lines 58 and 59 to new values and processing continues.  Since we
have a constructor that does the initialization, we should
probably rename the method named initialize() something else but
it illustrates the concept involved here.

The destructor is very similar to the constructor except that it
is called automatically when each of the objects goes out of
scope.  You will recall that automatic variables have a limited
lifetime because they cease to exist when the enclosing block in
which they were declared is exited.  When an object is about to
be automatically deallocated, its destructor, if one exists, is
called automatically.  A destructor is characterized as having
the same name as the class but with a tilde prepended to the
class name.  A destructor has no return type.

A destructor is declared in line 11 and defined in lines 31
through 35.  In this case the destructor only assigns zeros to
the variables prior to their being deallocated, so nothing is
really accomplished.  The destructor is only included for
illustration of how it is used.  If some blocks of memory were
dynamically allocated within an object, a destructor should be
used to deallocate them prior to losing the pointers to them.
This would return their memory to the free store for further use
later in the program.

It is interesting to note that if a constructor is used for an
object that is declared prior to the main program, a global
variable, the constructor will actually be executed prior to the
execution of the main program.  In like manner, if a destructor
is defined for such a variable, it will execute following the
completion of execution of the main program.  This will not
adversely affect your programs, but it is interesting to make
note of.


OBJECT PACKAGING
-----------------------------------------------------------------
Examine the file named BOXES1.CPP for an       ==================
example of how not to package an object for        BOXES1.CPP
universal use.  This packaging is actually     ==================
fine for a very small program, but is meant
to illustrate to you how to split your program up into smaller,
more manageable files when you are developing a large program or
when you are part of a team developing a large system.  The next
three example programs in this chapter will illustrate the proper
method of packaging a class.

This program is very similar to the last one with the pole
structure dropped and the class named box.  The class is defined
in lines 4 through 12, the implementation of the class is given

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                                        Chapter 5 - Encapsulation

in lines 15 through 34, and the use of the class is given in
lines 37 through 50.  With the explanation we gave about the last
example program, the diligent student should have no problem
understanding this program in detail.


INLINE IMPLEMENTATION
-----------------------------------------------------------------
The method in line 10 contains the implementation for the method
as a part of the declaration because it is very simple, and
because it introduces another new topic which you will use often
in C++ programming.  When the implementation is included in the
declaration, it will be assembled inline wherever this function
is called leading to much faster code.  This is because there is
no overhead to accomplish the call to the method.  In some cases
this will lead to code that is both smaller and faster.  This is
yet another illustration of the efficiency built into the C++
programming language.

Compile and execute this program in preparation for our study of
the next three examples which are a repeat of this program in a
slightly different form.


THE CLASS HEADER FILE
-----------------------------------------------------------------
If you examine BOX.H carefully, you will see      ===============
that it is only the class definition.  No              BOX.H
details are given of how the various methods      ===============
are implemented except of course for the
inline method named get_area().  This gives the complete
definition of how to use the class with no implementation details.
You would be advised to keep a hardcopy of this file available as
we study the next two files.  You will notice that it contains
lines 4 through 12 of the previous example program named
BOXES1.CPP.  This is called the class header file and cannot be
compiled or executed.


THE CLASS IMPLEMENTATION FILE
-----------------------------------------------------------------
Examine the file named BOX.CPP for the          =================
implementation of the methods declared in            BOX.CPP
the class header file.  Notice that the class   =================
header file is included into this file in
line 2 which contains the prototypes for its methods and the
definitions of the variables to be manipulated.  The code from
lines 15 through 34 of BOXES1.CPP is contained in this file which
is the implementation of the methods declared in the class named
box.

This file can be compiled but it cannot be executed because there
is no main entry point which is required for all ANSI-C or C++

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programs.  When it is compiled, the object code will be stored in
the current directory and available for use by other programs.
It should be noted here that the result of compilation is usually
referred to as an object file because it contains object code.
This use of the word object has nothing to do with the word
object as used in object oriented programming.  It is simply a
matter of overloading the use of the word.  The practice of
referring to the compiled result as an object file began long
before the technique of object oriented programming was ever
considered.

The separation of the definition and the implementation is a
major step forward in software engineering.  The definition file
is all the user needs in order to use this class effectively in a
program.  He needs no knowledge of the actual implementation of
the methods.  If he had the implementation available, he may
study the code and find a trick he could use to make the overall
program slightly more efficient, but this would lead to
nonportable software and possible bugs later if the implementor
changed the implementation without changing the interface.  The
purpose of object oriented programming is to hide the
implementation in such a way that the implementation can not
affect anything outside of its own small and well defined
boundary or interface.

You should compile this implementation file now and we will use
the result with the next example program.


USING THE BOX OBJECT
-----------------------------------------------------------------
Examine the file named BOXES2.CPP and you      ==================
will find that the class we defined                BOXES2.CPP
previously is used within this file.  In       ==================
fact, these last three programs taken
together are identical to the program named BOXES1.CPP studied
earlier.  The BOX.H file is included here, in line 3, since the
definition of the box class is needed to declare three objects
and use their methods.  You should have no trouble seeing that
this is a repeat of the previous program and will execute in
exactly the same way.  There is a big difference in BOXES1.CPP
and BOXES2.CPP as we will see shortly.

A very important distinction must be made at this point.  We are
not merely calling functions and changing the terminology a
little to say we are sending messages.  There is an inherent
difference in the two operations.  Since the data for each object
is tightly bound up in the object, there is no way to get to the
data except through the methods and we send a message to the
object telling it to perform some operation based on its
internally stored data.  However, whenever we call a function, we
take along the data for it to work with as parameters since it
doesn't contain its own data.

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                                        Chapter 5 - Encapsulation


Be sure to compile and execute this program, but when you come to
the link step, you will be required to link this program along
with the result of the compilation when you compiled the class
named box.  The file is probably named BOX.OBJ that must be
linked with this file.  You may need to consult the documentation
for your C++ compiler to learn how to do this.  Even if it seems
to be a lot of trouble to learn how to link several files
together, it will be worth your time to do so now because we will
be linking several more multifile C++ programs in the remainder
of this tutorial.

Depending on your compiler, this is your first opportunity to use
either a project file, or the "make" facility included with your
compiler.  Regardless of which C++ compiler you are using, it
would pay you to stop and learn how to use the multifile
technique provided with your compiler because you will need to
use it several times before the end of this tutorial.  The nature
of C++ tends to drive the programmer to use many files for a
given programming project and you should develop the habit early.


INFORMATION HIDING
-----------------------------------------------------------------
The last three example programs illustrate a method of
information hiding that can have a significant impact on the
quality of software developed for a large project.  Since the
only information the user of the class really needs is the class
header, that is all he needs to be given.  The details of
implementation can be kept hidden from him to prevent him from
studying the details and possibly using a quirk of programming to
write some rather obtuse code.  Since he doesn't know exactly
what the implementor did, he must follow only the definition
given in the header file.  This can have a significant impact on
a large project.  As mentioned earlier, accidental corruption of
data is prevented also.

Another reason for hiding the implementation is economic.  The
company that supplied you with your C++ compiler gave you many
library functions but did not supply the source code to the
library functions, only the interface to each function.  You know
how to use the file access functions but you do not have the
details of implementation, nor do you need them.  Likewise a
class library industry can develop which supplies users with
libraries of high quality, completely developed and tested
classes, for a licensing fee of course.  Since the user only
needs the interface defined, he can be supplied with the
interface and the object (compiled) code for the class and can
use it in any way he desires.  The suppliers source code is
protected from accidental or intentional compromise and he can
maintain complete control over it.



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                                        Chapter 5 - Encapsulation

It is very important that you understand the principles covered
in this chapter before proceeding on to the next chapter.  If you
feel you are a little weak in any of the areas covered here, you
should go over them again before proceeding on.  A point that
should be made here that may be obvious to you, is that it
requires a considerable amount of forethought to effectively use
classes.


ABSTRACT DATA TYPES
-----------------------------------------------------------------
We mentioned the abstract data type at the beginning of this
chapter and again briefly midway through, and it is time to
describe it a little more completely.  An abstract data type is a
group of data, each of which can store a range of values, and a
set of methods or functions that can operate on that data.  Since
the data are protected from any outside influence, it is
protected and said to be encapsulated.  Also, since the data is
somehow related, it is a very coherent group of data that may be
highly interactive with each other, but with little interaction
outside the scope of its class.

The methods, on the other hand, are coupled to the outside world
through the interface, but there are a limited number of contacts
with the outside world and therefore a weak coupling with the
outside.  The object is therefore said to be loosely coupled to
the outside world.  Because of the tight coherency and the loose
coupling, ease of maintenance of the software is greatly
enhanced.  The ease of maintenance may be the greatest benefit of
object oriented programming.

It may bother you that even though the programmer may not use the
private variables directly outside of the class, they are in
plain sight and he can see what they are and can probably make a
good guess at exactly how the class is implemented.  The
variables could have been hidden completely out of sight in
another file, but because the designers of C++ wished to make the
execution of the completed application as efficient as possible,
the variables were left in the class definition where they can be
seen but not used.


FRIEND FUNCTIONS
-----------------------------------------------------------------
A function outside of a class can be defined to be a friend
function by the class which gives the friend free access to the
private members of the class.  This in effect, opens a small hole
in the protective shield of the class, so it should be used very
carefully and sparingly.  There are cases where it helps to make
a program much more understandable and allows controlled access
to the data.  Friend functions will be illustrated in some of the
example programs later in this tutorial.  It is mentioned here
for completeness of this section.  A single isolated function can

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                                        Chapter 5 - Encapsulation

be declared as a friend, as well as members of other classes, and
even entire classes can be given friend status if needed in a
program.  Neither a constructor nor a destructor can be a friend
function.


THE struct IN C++
-----------------------------------------------------------------
The struct is still useable in C++ and operates just like it does
in ANSI-C with one addition.  You can include methods in a
structure that operate on data in the same manner as in a class,
but methods and data are automatically defaulted to be public at
the beginning of a structure.  Of course you can make any of the
data or methods private by defining a private section within the
structure.  The structure should be used only for constructs that
are truly structures.  If you are building even the simplest
objects, you are advised to use classes to define them.


A VERY PRACTICAL CLASS
-----------------------------------------------------------------
The examples of encapsulation used in this chapter have all been
extremely simple in order to illustrate the mechanics of
encapsulation.  Since it would be expedient to study a larger
example, the date class is given for your instruction.  The date
class is a complete nontrivial class which can be used in any
program to get the current date and print it as an ASCII string
in any of four predefined formats.  It can also be used to store
any desired date and format it for display.

Examine the file named DATE.H which is the       ================
header file for the date class.  This file is         DATE.H
so well commented that we don't have much        ================
else to say about it.  If you understand the
principles covered in this chapter you should have no problem
understanding this class.  One thing that is new to you is the
reserved word protected which is used in line 12.  We will define
this word in a couple of chapters.  Until then, pretend that it
means the same thing as private and you will be close enough for
this present example.  The code in lines 8 and 9 along with line
55 will be explained shortly.  For the present time, simply
pretend those lines of code are not there.  Also the keyword
static as used in lines 16 and 17 will be explained later.  These
new constructs are added because we plan to use this class later
when we study inheritance.

You should spend the time necessary to completely understand this
class header, with the exception of the new things added, before
going on to the implementation for this class.

The file named DATE.CPP is the implementation    ================
for the date class and once again, there is          DATE.CPP
nothing unusual or difficult about this code.    ================

                                                        Page 5-15

                                        Chapter 5 - Encapsulation

It uses very simple logic to store and format
the date in a usable manner.  You should study this code until
you understand it completely before going on to the next example
which will use the date class in a main program.

The very simple program named USEDATE.CPP is    =================
a main program that uses the date class to         USEDATE.CPP
list the current date and another date on the   =================
monitor.  Once again, you should have no
problem understanding this program so nothing more will be said
about it.

You should spend the time necessary to understand these three
files because they are the starting point for a practical track
in the next few chapters.  This class will be used in conjunction
with others to illustrate single and multiple inheritance.  Even
though you do not understand all of the details of these files,
spend enough time that you are comfortable with the structure and
the major points of them.

We will continue our discussion of encapsulation in the next chapter.


PROGRAMMING EXERCISES
-----------------------------------------------------------------
1.  Add a method to CLAS.CPP which will supply the square of the
    stored value.  Include some code in the main program to read
    and display the squared values.

2.  Continuing with CLAS.CPP, add a constructor to initialize the
    stored value to 10 and add a few lines of code to the main
    program to display the values immediately following the
    object definition.

3.  Add an output statement to the rectangle constructor of the
    program named CONSPOLE.CPP and another to the destructor to
    prove to yourself that they really are called by the system
    when we said they are.

4.  Write a more comprehensive program to use the date class
    presented at the end of this chapter.

5.  Write a name class which is somewhat similar to the date
    class which can store any name in three parts and return the
    full name in any of several different formats such as the
    following;

      John Paul Doe
      J. P. Doe
      Doe, John Paul
        and any other formats you desire.

    If this is carefully planned, it could be useful to you
    someday.
                                                        Page 5-16