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CHAP08.TXT
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1992-01-20
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Chapter 8
MORE INHERITANCE
In the last chapter we developed a model using modes of
transportation to illustrate the concept of inheritance. In this
chapter we will use that model to illustrate some of the finer
points of inheritance and what it can be used for. If it has been
a while since you read and studied chapter 7, it would be good for
you to return to that material and review it in preparation for a
more detailed study of the topic of inheritance.
REORGANIZED FILE STRUCTURE
_________________________________________________________________
A close examination of the file named ================
INHERIT1.CPP will reveal that it is identical to INHERIT1.CPP
the program developed in chapter 7 named ================
ALLVEHIC.CPP except that the program text is
rearranged. The biggest difference is that some
of the simpler methods in the classes have been changed to inline
code to shorten the file considerably. In a practical programming
situation, methods that are this short should be programmed inline
since the actual code to return a simple value is shorter than the
code required to send a message to a non-inline method.
The only other change is the reordering of the classes and
associated methods with the classes all defined first, followed by
the main program. This puts all class interface definitions on a
single page to make the code easier to study. The implementations
for the methods are deferred until the end of the file where they
are available for quick reference but are not cluttering up the
class definitions which we wish to study carefully in this chapter.
This should be an indication to you that there is considerable
flexibility in the way the classes and methods can be arranged in
C++. Of course you realize that this violates the spirit of C++
and its use of separate compilation, but is only done here for
convenience. The best way to package all of the example programs
in this chapter is like the packaging illustrated in chapter 7.
As mentioned before, the two derived classes, car and truck, each
have a variable named passenger_load which is perfectly legal, and
the car class has a method of the same name, initialize(), as one
defined in the super-class named vehicle. The rearrangement of the
files in no way voids this allowable repeating of names.
After you have convinced yourself that this program is truly
identical to the program named ALLVEHIC.CPP from chapter 7, compile
and execute it with your compiler to assure yourself that this
Page 8-1
Chapter 8 - More Inheritance
arrangement is legal. Due to this means of code packaging, you
will not need a "make" file or a "project" capability to compile
and execute this code. This is to make it easy to compile and
execute the example programs in this chapter.
THE SCOPE OPERATOR
_________________________________________________________________
Because the method initialize() is defined in the derived car
class, it hides the method of the same name which is part of the
base class, and there may be times you wish to send a message to
the method in the base class for use in the derived class object.
This can be done by using the scope operator in the following
manner in the main program;
sedan.vehicle::initialize(4, 3500.0);
As you might guess, the number and types of parameters must agree
with those of the method in the base class because it will respond
to the message.
HIDDEN METHODS
_________________________________________________________________
Examine the file named INHERIT2.CPP carefully ================
and you will notice that it is a repeat of the INHERIT2.CPP
last example program with a few minor changes. ================
You will notice that the derived classes named
car and truck do not have the keyword public prior to the name of
the base class in the first line of each. The keyword public, when
included prior to the base class name, makes all of the methods
defined in the base class available for use in the derived class
just as if they were defined as part of the derived class.
Therefore, in the previous program, we were permitted to call the
methods defined as part of the base class from the main program
even though we were working with an object of one of the derived
classes. One example of when we did this, was when we sent a
message to the sedan to get its weight in an output statement of
the main program.
In the present program, without the keyword public prior to the
base class name, the only methods available for objects of the car
class, are those that are defined as part of the class itself, and
therefore we only have the methods named initialize() and
passengers() available for use with objects of class car. In this
program, the only inheritance is that of variables since the two
variables are inherited into objects of class car.
When we declare an object of type car, according to the definition
of the C++ language, it contains three variables. It contains the
Page 8-2
Chapter 8 - More Inheritance
one defined as part of its class named passenger_load and the two
that are part of its parent class, wheels and weight. All are
available for direct use within its methods because of the use of
the keyword protected in the base class. The variables are a part
of an object of class car when it is declared and are stored as
part of the object. We will show you the details of access to the
parent class variables within derived classes shortly in this
chapter. For now, we will return to the use of the subclasses in
this example program.
The observant student will notice that several of the output
statements have been commented out of the main program since they
are no longer legal or meaningful operations. Lines 57 through 59
have been commented out because the methods named get_weight() and
wheel_loading() are not inherited into the car class without the
keyword public in the car class definition. You will notice that
initialize() is still available but this is the one in the car
class, not the method of the same name in the vehicle class.
Moving on to the use of the truck class in the main program, we
find that lines 63 and 65 are commented out for the same reason as
given above, but lines 66 and 67 are commented out for an entirely
different reason. Even though the method named efficiency() is
available and can be called as a part of the truck class, it cannot
be used because we have no way to initialize the wheels or weight
of the truck objects. We can get the weight of the truck objects,
as we have done in line 106, but since the weight has no way to be
initialized, the result is meaningless and lines 66 and 67 are
commented out.
As you have surely guessed by now, there is a way around all of
these problems and we will cover them shortly. In the meantime,
be sure to compile and execute this example program to see that
your compiler gives the same result. It would be a good exercise
for you to reintroduce some of the commented out lines to see what
sort of an error message your compiler issues for these errors.
INITIALIZING ALL DATA
_________________________________________________________________
If you will examine the example program named ================
INHERIT3.CPP, you will find that we have fixed INHERIT3.CPP
the initialization problem that we left dangling ================
in the last example program.
The method named init_truck() now contains all four of the
parameters as input data which get transferred to the four
variables. Following the initialization, it is permissible to call
the semi.efficiency() method in line 67 and 68 of the main program.
Be sure to compile and execute this pro