Power-Programmierung/CD2.mdf/c/tutorial/cpptutor/text/chap07.txt

Text File | 1994-05-15 | 22.0 KB | 464 lines

                                                        Chapter 7
                                                      INHERITANCE

One reason to use inheritance is that it permits 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.  Moreover, it is easy to make an error
if you try to modify the original class.  You are less likely to
make an error if you leave the original alone and only add to it.
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 members and 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
-----------------------------------------------------------------
Examine the file named VEHICLE.H for a simple   =================
class which we will use to begin our study of       VEHICLE.H
inheritance.  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 base class or parent class, 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.  Note that we
will explain the added keyword protected shortly.  Figure 7-1 is
a graphical representation of the vehicle class.

Ignore lines 4, 5, and 18 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.






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                                          Chapter 7 - Inheritance

THE IMPLEMENTATION FOR VEHICLE
-----------------------------------------------------------------
Examine the file named VEHICLE.CPP and you    ===================
will find that it is the implementation of        VEHICLE.CPP
the vehicle class.  The initialize() method   ===================
assigns the values input as parameters to
the wheels and weight variables.  We have methods to 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
base class.  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
-----------------------------------------------------------------
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 the vehicle
class 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 base class 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 values 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 base class or derived class 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 DERIVED CLASS
-----------------------------------------------------------------
Examine the file named CAR.H for our first      =================
example of the use of a derived class or              CAR.H
child class.  The vehicle class is inherited    =================
due to the ": public vehicle" added to line

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                                          Chapter 7 - Inheritance

7.  This derived class named car is composed of all of the
information included in the base class vehicle, and all of its
own additional information.  Even though we did nothing to the
class named vehicle, we made it into a base class because of the
way we are using it here.  To go a step further, even though it
will be used as a base class 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 base class in the same program.  The
question of whether it is a simple class or a base 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 often called a derived class or a child class, but the
most proper term as defined for C++, is a derived class.  Since
these terms are very descriptive, and most writers tend to use
the terms interchangeably, we will also use these terms in this
tutorial.  Likewise the proper C++ terminology for the inherited
class is to call it a base class, but parent class and super
class are sometimes used also.

A base class is a rather general class which can cover a wide
range of objects, whereas a derived class is somewhat more
restricted but at the same time more useful.  For example if we
had a base class named programming language and a derived class
named C++, then we could use the base class 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 derived
class 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 base class tends to be more general, and a derived class is
more specific.

In this case, the vehicle base class 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 derived
class using car as the base class, 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 derived class and a base class at
the same time, so it should be clear that these names refer to
how a class is used.



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                                          Chapter 7 - Inheritance

HOW DO WE DECLARE A DERIVED CLASS?
-----------------------------------------------------------------
Enough generalities about classes, let's get down to the
specifics.  A derived class is defined by including the header
file for the base class as is done in line 5, then the name of
the base class is given following the name of the derived class
separated by a colon as is illustrated in line 7.  Ignore the
keyword public immediately following the colon in this line.  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.  Figure 7-2 is a graphical representation of an
object of this class.

Note once again that the implementation for the base class 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 base class
must be available as a text file since the class definitions are
required in order to use the class.


THE CAR CLASS IMPLEMENTATION
-----------------------------------------------------------------
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 derived class 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 derived class or not, it is written in exactly the
same way as any other class implementation file.

The implementations for the two new methods are written in
exactly the same way as methods are written for any other class.
If you think you understand this file, you should compile it for
later use.

ANOTHER DERIVED CLASS
-----------------------------------------------------------------
Examine the file named TRUCK.H for an example   =================
of another class that uses the vehicle class         TRUCK.H
and adds to it.  Of course, it adds different   =================

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                                          Chapter 7 - Inheritance

things to it, because it will specialize in
those things that pertain to trucks.  In fact, it adds two more
variables and three more methods.  Once again, ignore the keyword
public following the colon in line 7 for a few minutes and we
will cover it in detail in the next chapter of this tutorial.
See figure 7-3 for a graphical representation of the truck class.

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 derived classes of the same base
class 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 base class, 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
child classes.


THE TRUCK IMPLEMENTATION
-----------------------------------------------------------------
Examine the file named TRUCK.CPP for the      ===================
implementation of the truck class.  It has         TRUCK.CPP
nothing unusual included in it.               ===================

You should have no problem understanding this implementation.
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.  Figure 7-4 is the class diagram for these three
classes illustrating the inheritance.  This form of class design
notation is generally accepted in the software development
community, although some authors use "clouds" rather than boxes
to indicate a class.  The double line with an arrow pointing from
the derived to the base class is in general use.  Many designers
draw the diagram with the base class at the top and the derived
classes below it, but that is really a matter of personal taste.


USING ALL THREE CLASSES
-----------------------------------------------------------------
Examine the program named ALLVEHIC.CPP for an  ==================
example that uses all three of the classes we     ALLVEHIC.CPP
have been discussing in this chapter.  It      ==================
uses the parent class vehicle to declare
objects and also uses the two child classes to 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

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                                          Chapter 7 - Inheritance

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.  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.  Finally, you can execute the complete
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.  If you are using an implementation of C++ that
has a project capability, it will make this a snap.


WHY THE #ifndef VEHICLE_H ?
-----------------------------------------------------------------
We promised to return to the strange looking preprocessor
directive in lines 4, 5 and 18 in the VEHICLE.H file, and this is
the time for it.  When we define the derived class car, we are
required to supply it with the full definition of the interface
to the vehicle class since car is a derived class 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,
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 VEHICLE_H.  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

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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 VEHICLE_H was chosen as the word because it is the name
of the file, with the period replaced by the underline.  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 provided you keep all files in the
same directory.  It would be good for you to get into the
practice of building the optional skip around all of your class
headers.  All class definition files in the remainder of this
tutorial will include this skip around to prevent multiple
inclusions and to be an example for you.  You should get into
the practice of adding the skip around to all of your class
headers no matter how trivial they may seem to be.


OUR FIRST PRACTICAL INHERITANCE
-----------------------------------------------------------------
Continuing where we started in chapter 5, we    =================
will inherit the date class into the file           NEWDATE.H
named NEWDATE.H and add a member variable and   =================
a new method to the class.  Actually, this is
not a good way to add the day_of_year to the date class since it
is available in the structure returned from the system call in
the date class.  However, we are more interested in illustrating
inheritance in a practical example than we are in developing a
perfect class, so we will live with this inefficiency.  You will
note that we add one variable and one method to create our new
class.

The program named NEWDATE.CPP contains the    ===================
implementation for the added method and           NEWDATE.CPP
should be simple for the student to           ===================
understand.  This class implementation uses
the array days[] from the date class implementation since it was
defined as a global variable there.  The method named
get_time_of_day() involves very simple logic but still adjusts
for leap years.


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                                          Chapter 7 - Inheritance

Finally, the example program named             ==================
TRYNDATE.CPP will use the new class in a          TRYNDATE.CPP
very simple way to illustrate that the         ==================
derived class is as easy to use as the
base class and in fact the main program has no way of knowing
that it is using a derived class.

You should compile and link this program to gain the experience
of doing so.  Remember that it will be necessary to link in the
object code for the original date class as well as the object
code from the newdate class and the main program.


PROGRAMMING EXERCISES
-----------------------------------------------------------------
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 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.H, 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.H (TRUCK.H), 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.

3.  Add a variable named sex of type char to the name class you
    developed in chapter 5 as well as methods to set and retrieve
    the value of this variable.  The only legal inputs are 'M' or
    'F'.  These additions should be done by inheriting the name
    class into the new class.


















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