Chapter 7
INHERITANCE
One reason to use inheritance is that it allows you to reuse code
from a previous project but gives you the flexibility to slightly
modify it if the old code doesn't do exactly what you need for the
new project. It doesn't make sense to start every new project from
scratch since some code will certainly be repeated in several
programs and you should strive to build on what you did previously.
Another reason for using inheritance is if the project requires the
use of several classes which are very similar but slightly
different.
In this chapter we will concentrate on the mechanism of inheritance
and how to build it into a program. A better illustration of why
you would use inheritance will be given in later chapters where we
will discuss some practical applications of object oriented
programming.
The principle of inheritance is available with several modern
programming languages and is handled slightly differently with
each. C++ allows you to inherit all or part of the methods of a
class, modify some, and add new ones not available in the parent
class. You have complete flexibility, and as usual, the method
used with C++ has been selected to result in the most efficient
code execution.
A SIMPLE CLASS TO START WITH
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Examine the file named VEHICLE.HPP for a simple ===============
class which we will use to begin our study of VEHICLE.HPP
inheritance in this chapter. There is nothing ===============
unusual about this class header, it has been
kept very simple. It consists of four simple
methods which can be used to manipulate data pertaining to our
vehicle. What each method does is not especially important at this
time. We will eventually refer to this as a superclass but for the
time being, we will simply use it like any other class to show that
it is indeed identical to the classes already studied.
Ignore lines 4, 5, and 17 until the end of this chapter where they
will be explained in detail. This file cannot be compiled or
executed because it is only a header file.
THE IMPLEMENTATION FOR VEHICLE
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Examine the file named VEHICLE.CPP and you will find that it is the
implementation of the vehicle class. The initialize() method
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assigns the values input as parameters to the ===============
wheels and weight variables. We have methods to VEHICLE.CPP
return the number of wheels and the weight, and ===============
finally, we have one that does a trivial
calculation to return the loading on each wheel.
We will have a few examples of methods that do some significant
processing later, but at this point, we are more interested in
learning how to set up the interface to the classes, so the
implementations will be kept trivial.
As stated above, this is a very simple class which will be used in
the next program. Later in this tutorial we will use it as a
superclass. You should compile this class at this time in
preparation for the next example program, but you cannot execute
it because there is no entry point.
USING THE VEHICLE CLASS
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The file named TRANSPRT.CPP uses the vehicle ================
class in exactly the same manner as we TRANSPRT.CPP
illustrated in the last chapter. This should be ================
an indication to you that this is truly nothing
more than a normal class as defined in C++. We
will make it a little special, however, by using it unmodified as
a superclass in the next few example files to illustrate
inheritance. Inheritance uses an existing class and adds
functionality to it to accomplish another, possibly more complex
job.
You should have no problem understanding the operation of this
program. It declares four objects of the vehicle class,
initializes them, and prints out a few of the data points to
illustrate that the vehicle class can be used as a simple class
because it is a simple class. We are referring to it as a simple
class as opposed to calling it a superclass or subclass as we will
do shortly.
If you thoroughly understand this program, you should compile and
execute it, remembering to link the vehicle object file with this
object file.
OUR FIRST SUBCLASS
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Examine the file named CAR.HPP for our first ===============
example of the use of a subclass. The vehicle CAR.HPP
class is inherited due to the ": public vehicle" ===============
added to line 4. This class named car is
composed of all of the information included in
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the superclass vehicle, and all of its own additional information.
Even though we did nothing to the class named vehicle, we made it
into a superclass because of the way we are using it here. To go
a step further, even though it will be used as a superclass in an
example program later in this chapter, there is no reason it cannot
continue to be used as a simple class in the previous example
program. In fact, it can be used as a single class and a
superclass in the same program. The question of whether it is a
simple class or a super class is answered by the way it is used.
A discussion of terminology is needed here. When discussing object
oriented programming in general, a class that inherits another is
called a subclass, but the proper term as defined for C++ is a
derived class. Since both terms are very descriptive, and most
writers tend to use the terms interchangeably, we will also use
both terms in this tutorial.
A superclass is a rather general class which can cover a wide range
of objects, whereas a subclass is somewhat more restricted but at
the same time more useful. For example if we had a superclass
named programming language and a subclass named C++, then we could
use the superclass to define Pascal, Ada, C++, or any other
programming language, but it would not tell us about the use of
classes in C++ because it can only give a general view of each
language. On the other hand, the subclass named C++ could define
the use of classes, but it could not be used to describe the other
languages because it is too narrow. A superclass tends to be more
general, and a subclass is more specific.
In this case, the vehicle superclass can be used to declare objects
that represent trucks, cars, bicycles, or any number of other
vehicles you can think up. The class named car however can only
be used to declare an object that is of type car because we have
limited the kinds of data that can be intelligently used with it.
The car class is therefore more restrictive and specific than the
vehicle class. The vehicle class is more general than the car
class.
If we wished to get even more specific, we could define a subclass
of car and name it sports_car and include such information as
red_line_limit for the tachometer which would be silly for the
family station wagon. The car class would therefore be used as a
subclass and a superclass at the same time, so it should be clear
that these names refer to how a class is used.
HOW DO WE DECLARE A SUBCLASS?
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Enough generalities about classes, let's get down to the specifics.
A subclass is defined by including the header file for the
superclass as is done in line 2, then the name of the superclass
is given following the name of the subclass separated by a colon
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as is illustrated in line 4. Ignore the keyword public immediately
following the colon in this line. It is optional and we will study
it in detail in the next chapter. All objects declared as being
of class car therefore are composed of the two variables from the
class vehicle because they inherit those variables, and the single
variable declared in the class car named passenger_load.
An object of this class will have three of the four methods of
vehicle and the two new ones declared here. The method named
initialize() which is part of the vehicle class will not be
available here because it is hidden by the local version of
initialize() which is a part of the car class. The local method
will be used if the name is repeated allowing you to customize your
new class.
Note once again that the implementation for the superclass only
needs to be supplied in its compiled form. The source code for the
implementation can be hidden for economic reasons to aid software
developers. Hiding the source code also allows the practice of
information hiding. The header for the superclass must be
available as a text file since the class definitions are required
in order to use the class.
THE CAR CLASS IMPLEMENTATION
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Examine the file named CAR.CPP which is the ===============
implementation file for the car class. The CAR.CPP
first thing you should notice is that this file ===============
has no indication of the fact that it is a
subclass of any other file, that can only be
determined by inspecting the header file for the class. Since we
can't tell if it is a subclass or not, it is written in exactly the
same way as any other class implementation file.
Line 8 contains a new construct. In order to initialize the
variables which are a part of the superclass, we must send a
message to the superclass. This is because the variables are
private and directly available only within the class itself. We
will see another way to do this in the next chapter of this
tutorial.
If you think you understand this file, you should compile it for
later use. Keep in mind that you must have already compiled the
vehicle class prior to this time in order to get a good compilation
of this subclass.
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ANOTHER SUBCLASS
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Examine the file named TRUCK.HPP for an example ===============
of another class that uses the vehicle class and TRUCK.HPP
adds to it. Of course, it adds different things ===============
to it because it will specialize in those things
that pertain to trucks. In fact it adds two
more variables and three methods. Once again, ignore the keyword
public following the colon in line 4 for a few minutes and we will
cover it in detail in the next chapter of this tutorial.
A very important point that must be made is that the car class and
the truck class have absolutely nothing to do with each other, they
only happen to be subclasses of the same superclass or parent class
as it is sometimes called.
Note that both the car and the truck classes have methods named
passengers() but this causes no problems and is perfectly
acceptable. If classes are related in some way, and they certainly
are if they are both derived classes of a common superclass, you
would expect them to be doing somewhat similar things. In this
situation there is a good possibility that a method name would be
repeated in both subclasses.
THE TRUCK IMPLEMENTATION
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Examine the file named TRUCK.CPP for the ===============
implementation of the truck class. It also has TRUCK.CPP
a few unusual things included in it. In line 13 ===============
of the second method, it uses a call to its
superclass to get the weight of the truck. This
is because the weight is private and therefore not available to
the subclass but must be accessed via one of the superclass
methods. There is a way to make it available here, but we will
save that definition until the next chapter of this tutorial.
You should have no problem understanding the remainder of this
program. Your assignment at this point is to compile it in
preparation for our example program that uses all three of the
classes defined in this chapter.
USING ALL THREE CLASSES
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Examine the example program named ALLVEHIC.CPP ================
for an example that uses all three of the ALLVEHIC.CPP
classes we have been discussing in this chapter. ================
It uses the superclass vehicle to declare
objects and also uses the two subclasses to
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declare objects. This was done to illustrate that all three
classes can be used in a single program.
All three of the header files for the classes are included in lines
3 through 5 so the program can use the components of the classes.
Notice that the implementations of the three classes are not in
view here and do not need to be in view. This allows the code to
be used without access to the source code for the actual
implementation of the class. However, it should be clear that the
header file definition must be available.
In this example program, only one object of each class is declared
and used but as many as desired could be declared and used in order
to accomplish the programming task at hand. You will notice how
clean and uncluttered the source code is for this program since the
classes were developed, debugged, and stored away previously, and
the interfaces were kept very simple. There is nothing new here
so you should have no trouble understanding the operation of this
program.
Compiling and executing this program will take a bit of effort but
the process is not complicated. The three classes and the main
program can be compiled in any order desired. All four must be
compiled prior to linking the four resulting object (or binary)
files together prior to any attempt to execute the final program.
Be sure you do the required steps to compile and execute this
program because the effective use of C++ will require you to
compile many separate files and link them together. This is
because of the nature of the C++ language, but it should not be a
burden if a good "make" capability exists with your compiler.
WHY THE #ifndef VEHICLEHPP ?
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We promised to return to the strange looking preprocessor directive
in lines 4, 5 and 17 in the VEHICLE.HPP file, and this is the time
for it. When we define the subclass car, we are required to supply
it with the full definition of the interface to the vehicle class
since car is a subclass of vehicle and must know all about its
parent. We do that by including the vehicle class into the car
class, and the car class can be compiled. The vehicle class must
also be included in the header file of the truck class for the same
reason.
When we get to the main program, we must inform it of the details
of all three classes, so all three header files must be included
as is done in lines 3 through 5 of ALLVEHIC.CPP, but this leads to
a problem. When the preprocessor gets to the car class, it
includes the vehicle class because it is listed in the car class
header file, but since the vehicle class was already included in
line 3 of ALLVEHIC.CPP, it is included twice and we attempt to
redefine the class vehicle. Of course it is the same definition,
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but the system doesn't care, it simply doesn't allow redefinition
of a class. We allow the double inclusion of the file and at the
same time prevent the double inclusion of the class by building a
bridge around it using the word VEHICLEHPP. If the word is already
defined, the definition is skipped, but if the word is not defined,
the definition is included and the word is defined at that time.
The end result is the actual inclusion of the class only once even
though the file is included more than once. You should have no
trouble understanding the logic of the includes if you spend a
little time studying this program sequence.
Even though ANSI-C allows multiple definitions of entities,
provided the definitions are identical, C++ does not permit this.
The primary reason is because the compiler would have great
difficulty in knowing if it has already made a constructor call for
the redefined entity, if there is one. A multiple constructor call
for a single object could cause great havoc, so C++ was defined to
prevent any multiple constructor calls by making it illegal to
redefine any entity. This is not a problem in any practical
program.
The name VEHICLEHPP was chosen as the word because it is the name
of the file, with the period omitted. If the name of the file is
used systematically in all of your class definitions, you cannot
have a name clash because the filename of every class must be
unique. It would be good for you to get into the practice of
building the optional skip around all of your classes.
MULTIPLE INHERITANCE
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Early implementations of C++ do not allow multiple inheritance,
inheriting data and methods from more than one parent class. The
newest versions of C++ permit multiple inheritance since the latest
definition of the language (AT&T version 2.0 of C++), includes it
as a part of its syntax. One of the biggest problems with multiple
inheritance is the question of what should be done when both parent
classes have methods of the same name. Some means must be
available to decide which of the methods will actually be
inherited.
Chapter 9 of this tutorial will discuss multiple inheritance.
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PROGRAMMING EXERCISES
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1. Add another object of the vehicle class to ALLVEHIC.CPP named
bicycle, and do some of the same operations as were done to
the unicycle. You will only need to recompile the main
program and link all four files together to get an executable
file, the three classes will not require recompilation.
2. Add the optional skip around the header files of the classes
named car and truck. Then recompile all four files and relink
them to get an executable file.
3. Add a new method to the truck class to return the total weight
of the truck plus its payload and add code to ALLVEHIC.CPP to
read the value out and display it on the monitor. This will
require an addition to TRUCK.HPP, another addition to
TRUCK.CPP, and of course the changes to the main program named
ALLVEHIC.CPP. The answer is given as three files named
CH07_3A.HPP (TRUCK.HPP), CH07_3B.CPP (TRUCK.CPP) and the
changed main program is found in CH07_3C.CPP in the answer
directory on the distribution disk for this tutorial.
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