Magnum One/Magnum One (Mid-American Digital) (Disc Manufacturing).iso/d12/cptutor1.arc/CHAP05.TXT

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                                                        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.


NO INFORMATION HIDING
_________________________________________________________________

The program named OPEN.CPP is a really stupid      ==============
program because it does next to nothing but it        OPEN.CPP
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 and what it will do for you by the time we finish this
chapter.

A very simple structure is defined in lines 4 through 6 which
contains only 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 anywhere
within the main function.  Each variable can be assigned,
incremented, read, modified, or have any number of 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.

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.

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                                        Chapter 5 - Encapsulation

INFORMATION HIDING
_________________________________________________________________

Examine the program named CLAS.CPP for our first   ==============
example of a program with a little information        CLAS.CPP
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 here.  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 has no private section automatically
defined.  Everything in a structure is defaulted to public by
definition of the C++ language, but you can declare a private
section in a structure if you so desire.  The keyword class is used
to declare a class as illustrated here.


WHAT IS A PRIVATE SECTION?
_________________________________________________________________

A private section of a class is a section of data which cannot be
accessed outside of the class, it is hidden from any outside
access.  Thus, 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 seems a little dumb to declare a variable in the main
program that we cannot use, but that is exactly what we did.

The class 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 PUBLIC SECTION?
_________________________________________________________________

A new keyword, public, is introduced in line 6 which states that
anything following this keyword can be accessed from the main
program or any function that is within the scope of an object of
the given class.  Because the two functions are defined following
the keyword public, they are both public and available for use in
the calling function or any other function that is within the scope
of the calling function.  You should keep in mind that the private
variable is not available to the calling program.  Thus, we can

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                                        Chapter 5 - Encapsulation

only use the variable by calling one of the two functions defined
as a 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.


In C++ we have three scopes of variables, local, file and class.
Local variables are localized to a single function and file
variables 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.

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.  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 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.


MORE NEW TERMINOLOGY
_________________________________________________________________

If we are going to learn how to effectively use object oriented
programming, we must learn the new terminology of the technique,
so we will list a few of them here and begin using them in the text
to get you used to seeing and using them.

      A class is a grouping of data and methods.  A class is
      very much like a 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.

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                                        Chapter 5 - Encapsulation


      An object is an instance of a class.  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 referred to
      as a method.

      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
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 on 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.  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.

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                                        Chapter 5 - Encapsulation

Lines 31 and 32 have been commented out because the operations are
illegal since the variable named data_store is private and 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.



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
before continuing on to the next example program.



A PROGRAM WITH PROBLEMS
_________________________________________________________________

Examine the program named OPENPOLE.CPP for an    ================
example of a program with a few serious problems   OPENPOLE.CPP
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 being 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.

Based on your experience with ANSI-C, you should have no problem
at all understanding exactly what the 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

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                                        Chapter 5 - Encapsulation

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
real world 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.
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
to return 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.  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.  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.



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                                        Chapter 5 - Encapsulation

This is the abstract data type mentioned earlier in this chapter,
a model with an allowable set of variables for data storage and a
set of allowable 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 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 either
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.  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 other methods.  Based on the discussion
given above you can see that object oriented programming is not
really new, since it has been used in some degree for as long as
computers have been popular.  The newest development, however, is

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                                        Chapter 5 - Encapsulation

in allowing the programmer to partition his programs in such a way
that he too can practice information hiding and reduce the
debugging time.



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 a silly example of protection, 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 program is identical to the last program
except that a constructor has been added as well as a destructor.
The constructor is declared in line 8 and defined in lines 14
through 18.  The constructor is called automatically by the C++
system when the object is declared and can therefore be of great
help in preventing the use of an uninitialized variable.  When the
object named box is declared in line 46, the constructor is called
automatically by the system which sets the values of its height and
width each to 6.  This is printed out for reference in lines 49 and

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                                        Chapter 5 - Encapsulation

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 since it is not
permitted to have a user defined return type.  It actually has a
predefined return type, a 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
since 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, otherwise known
as globally, 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 universal use.  This packaging is actually
fine for a very small program but is meant to illustrate to you

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                                        Chapter 5 - Encapsulation

how to split your program up into smaller more    ================
manageable files when you are developing a large     BOXES1.CPP
program or when you are part of a team            ================
developing a large system.  The last 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 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 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.HPP carefully, you will see    ===============
that it is only the class definition.  No             BOX.HPP
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.


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                                        Chapter 5 - Encapsulation

THE CLASS IMPLEMENTATION FILE
_________________________________________________________________

Examine the file named BOX.CPP for the            ===============
implementation of the methods declared in the         BOX.CPP
class header file.  Notice that the class header  ===============
file is included into this file in line 2 which
contains all of the prototypes for its methods.
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++
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 a object file began long before the method 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 program 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 object we defined previously is       BOXES2.CPP
used within this file.  In fact, these last      ================
three programs taken together are identical to
the program named BOXES1.CPP studied earlier.

The BOX.HPP file is included here, in line 3, since the definition
of the box is needed to declare three objects and use their
methods.  You should have no trouble seeing that this is a repeat

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                                        Chapter 5 - Encapsulation

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.

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.

If you are using TURBO C++, this is your first opportunity to use
a project file.  If you are using Zortech C++, you can use the make
facility included with your compiler.  Whichever one 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

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                                        Chapter 5 - Encapsulation

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 development
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 object 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.

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 some
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 no interaction with the outside world,
it is completely decoupled from its surroundings.

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.




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                                        Chapter 5 - Encapsulation

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 function can 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 all methods and data are automatically defaulted to be public
in a structure.  Of course you can make any of the data or methods
private but only if you are using a C++ compiler that supports C++
version 2.0 or higher.  Earlier versions of C++ did not permit a
private section in a structure.  The structure should be used only
for constructs that are truly structures.  If you are building even
the simplest objects, use classes to define them.


INITIALIZING THE class AND struct
_________________________________________________________________

The struct can be initialized with an aggregate in exactly the same
manner that a struct can be initialized in ANSI-C, but this is not
permitted for a class.  The class is allowed to contain private
members, so it cannot be initialized without access to the private
members.  It could have been possible to allow aggregate
initialization of classes without private members, but this would
have greatly complicated the compiler while adding little to the
capability since a class without a private member is contrary to
the entire thrust of object oriented programming.  The designers
of C++ chose therefore to prohibit aggregate initialization of
class variables in order to simplify the job of the compiler
writer.  This probably adds to the efficiency of the resulting
applications.  This really is no limitation because both struct and
class types can be initialized with constructors or initialization
methods.

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


                                                        Page 5-14

                                        Chapter 5 - Encapsulation


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.





































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