HPAVC/HPAVC CD-ROM.iso/CPTUTT22.ZIP/CHAP08.TXT

Text File | 1992-01-20 | 17KB | 413 lines

                                                        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

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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

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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 program following your detailed
study of it.


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WHAT IS PROTECTED DATA?
_________________________________________________________________

Examine the program named INHERIT4.CPP for an    ================
example we will use to define 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 base class were totally available in all
classes inheriting that base class, 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.  The keyword private can
be used as illustrated in lines 19 and 30 but adds nothing due to
the fact that class members default to private by definition.

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.  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 derived classes inheriting this class.

     protected - The variables and methods are not available to any
          outside calling routines, but they are directly available
          to any derived class inheriting this class.

     public - All variables and methods are freely available to all
          outside calling routines and to all derived classes.

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.

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WHAT IS PRIVATE DATA?
_________________________________________________________________

Examine the file named INHERIT5.CPP where the    ================
data is allowed to use the private default.  In    INHERIT5.CPP
this program, the data is not available for use  ================
in the derived classes, so the only way the data
in the base class can be used is through use of
messages to methods in the base class.

It seems a little silly to have to call methods in the base class
to get to the data which is actually a part of the derived class,
but that is the way C++ is defined to work.  This would indicate
to you that you should spend some time thinking about how any class
you define will be used.  If you think somebody may wish to inherit
your class into a new class and expand it, you should make the data
members protected so they can be easily used in the new class.  Be
sure to compile and execute this program.



INHERITING CONSTRUCTORS
_________________________________________________________________

Examine the example program named INHERIT6.CPP   ================
for yet another variation to our basic program,    INHERIT6.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.

We will say much more about constructors used with inheritance in
the next chapter of this tutorial.  Be sure to compile and execute
this example program.


POINTERS TO AN OBJECT AND AN ARRAY OF OBJECTS
_________________________________________________________________

Examine the example program named INHERIT7.CPP for examples of the
use of an array of objects and a pointer to an object.  In this


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program, the objects are instantiated from an    ================
inherited class and the intent of this program     INHERIT7.CPP
is to illustrate that there is nothing magic     ================
about a derived class.

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 52.  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 56 through 62.  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 65, that we do not declare an object of
type truck but a pointer to an object of type truck.  In order to
use the pointer, we must give it something to point at which we do
in line 67 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 68 through 72, and will be further illustrated in the
example program of chapter 12 in this tutorial.

Finally, we deallocate the object in line 73.  You should spend
enough time with this program to thoroughly understand the new
material presented here, then compile and execute it.


THE NEW TIME CLASS
_________________________________________________________________

We began a series of nontrivial classes in chapter 5 where we
developed a date class, then a time class, and finally a newdate
class in the last chapter.  Now it is your turn to add to this
series.  Your assignment is to develop the newtime class which
inherits the time class and adds a new member variable named
seconds_today and a method to calculate the value of seconds since
midnight to fill the variable.

A complete solution to this problem will be found in the ANSWERS
directory on the distribution disk.  The files named NEWTIME.H,
NEWTIME.CPP, and TRYNTIME.CPP are the solution files.  It would be
a good exercise for you to attempt to write this new class before
you look at the example solution.









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PROGRAMMING EXERCISES
_________________________________________________________________

1.   Remove the comment delimiters from lines 65 through 67 of
     INHERIT2.CPP to see what kind of results are returned.  Remove
     them from line 57 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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