chap05.txt

含有文章和源码

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




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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.

Figure 5-2 is a graphical illustration of the two objects available
for use within the calling program.  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 others data.

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.




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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.


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

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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 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.  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 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

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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        BOXES1.CPP
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 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,

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                                        Chapter 5 - Encapsulation

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