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
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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 actually 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.
As mentioned before, the two subclasses, 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
arrangement is legal.
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THE SCOPE OPERATOR
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Because the method initialize() is defined in the car subclass, it
hides the method of the same name which is part of the parent
class, and there may be times you wish to send a message to the
method in the parent class for use in the subclass 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 parent class because it will
respond to the message.
HIDDEN METHODS
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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 parent class in the first line of each. The keyword public,
when included prior to the parent's name, makes all of the methods
defined in the parent class available for use in the derived class
just as if they were defined as part of the subclass. Therefore,
in the previous program, we were permitted to call the methods
defined as part of the parent 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
parent 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 but even they are
not directly available as will soon be seen.
When we declare an object of type car, according to the definition
of the C++ language, it contains three variables. It contains the
one defined as part of its class named passenger_load and the two
that are part of its parent class, wheels and weight. The only
variable that is available for direct use within its methods is the
one defined as part of its own class, the other two are effectively
hidden from its methods. You will note that there is no way in
this program that we can ever use the variables named wheels or
weight directly in either an external program or one of the methods
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of this class. The variables are a part of an object of class car
when each is declared and is stored as part of the object, but the
only way to use them is through use of the methods defined as part
of the parent class. They are initialized in line 86 to illustrate
the means used to access them.
We will show you a way to access the parent class variables
directly within local methods 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 56 through 58
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 62 and 64 are commented out for the same reason as
given above, but lines 65 and 66 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 104, by using the scope resolution
operator, but since the weight has no way to be initialized, the
result is meaningless and lines 65 and 66 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
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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 and it calls the method named initialize()
of class vehicle within its implementation. You will notice that
we must call the method using the scope resolution operator in line
97 since there is no object to call, only the class. Following the
initialization, it is permissible to call the semi.efficiency()
method in line 65 and 66 of the main program.
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Be sure to compile and execute this program following your detailed
study of it.
WHAT IS PROTECTED DATA?
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Examine the program named INHERIT4.CPP for our ================
first example of the use of protected data. INHERIT4.CPP
Just to make the program more versatile, we have ================
returned to the use of the keyword public prior
to the name of the parent classes in lines 18
and 29 of the class definitions.
If the data within a superclass were totally available in all
classes inheriting that superclass, it would be a simple matter for
a programmer to inherit the superclass into a derived class and
have free access to all data in the parent class. This would
completely override the protection afforded by the use of
information hiding. For this reason, the data in a class are not
automatically available to the methods of an inheriting class.
There are times when you may wish to automatically inherit all
variables directly into the subclasses and have them act just as
though they were defined as a part of those classes also. For this
reason, the designer of C++ has provided the keyword protected.
In the present example program, the keyword protected is given in
line 5 so that all of the data of the vehicle class can be directly
imported into any derived classes but are not available outside of
the class or derived classes. All data are automatically defaulted
to private type if no specifier is given, as in all earlier
programs in this chapter. The keyword private can be used as
illustrated in lines 19 and 30 but adds nothing due to the default.
You will notice that the variables named wheels and weight are
available to use in the method named initialize() in lines 85
through 91 just as if they were declared as a part of the car class
itself, since they are used directly. We can now state the rules
for the three means of defining variables and methods.
private - The variables and methods are not available to any
outside calling routines, and they are not available
to any subclasses inheriting this class.
protected - The variables and methods are not available to any
outside calling routines, but they are available to
any subclass inheriting this class.
public - All variables and methods are freely available to all
outside calling routines and to all subclasses.
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You will note that these three means of definition can also be used
in a struct type. The only difference with a struct is that
everything defaults to public until one of the other keywords is
used.
Be sure to compile and execute this program before continuing on
to the next example program.
INHERITING CONSTRUCTORS
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Examine the example program named INHERIT5.CPP ================
for yet another variation to our basic program, INHERIT5.CPP
this time adding constructors. ================
The vehicle class has a constructor to
initialize the number of wheels and the weight to the indicated
values and has no surprising constructs. The car and truck classes
each have a constructor also to initialize their unique variables
to some unique values. If you jump ahead to the main program, you
will find that the initializing statements are commented out for
each of the objects so we must depend on the constructors to
initialize the variables. The most important thing to glean from
this example program is the fact that when one of the constructors
is called for a derived class, the constructor is also called for
the parent class. In fact, the constructor for the parent class
will be called before the constructor for the derived class is
called. All of the data will be initialized, including the data
inherited from the parent class.
Be sure to compile and execute this example program.
POINTERS TO AN OBJECT AND AN ARRAY OF OBJECTS
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Examine the final example program in this ================
chapter named INHERIT6.CPP for examples of the INHERIT6.CPP
use of an array of objects and a pointer to an ================
object.
The program is identical to the first program in this chapter until
we get to the main program where we find an array of 3 objects of
class car declared in line 51. It should be obvious that any
operation that is legal for a simple object is legal for an object
that is part of an array, but we must be sure to tell the system
which object of the array we are interested in by adding the array
subscript as we do in lines 55 through 61. The operation of this
portion of the program should be very easy for you to follow, so
we will go on to the next construct of interest.
You will notice, in line 64, that we do not declare an object of
type truck but a pointer to an object of type truck. In order to
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use the pointer, we must give it something to point at which we do
in line 66 by dynamically allocating an object. Once the pointer
has an object to point to, we can use the object in the same way
we would use any object, but we must use the pointer notation to
access any of the methods of the object. This is illustrated for
you in lines 67 through 71, and will be further illustrated in the
example programs of chapters 12 and 13 of this tutorial.
Finally, we deallocate the object in line 72. You should spend
enough time with this program to thoroughly understand the new
material presented here, then compile and execute it.
PROGRAMMING EXERCISES
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1. Remove the comment delimiters from lines 65 and 66 of
INHERIT2.CPP to see what kind of results are returned. Remove
them from line 56 to see what kind of an error is reported by
the compiler for this error.
2. Add cout statements to each of the constructors of
INHERIT5.CPP to output messages to the monitor so you can see
the order of sending messages to the constructors.
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