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. However, 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
Example program ------> VEHICLE.H
Examine the file named VEHICLE.H for a simple class which we will use to begin our study of 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 19 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
Example program ------> VEHICLE.CPP
Examine the file named VEHICLE.CPP and you will find that it is the implementation of 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
Example program ------> TRANSPRT.CPP
The file named TRANSPRT.CPP uses the vehicle class in exactly the same manner as we 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
Example program ------> CAR.H
Examine the file named CAR.H for our first example of the use of a derived class or child class. The vehicle class is inherited due to the ": public vehicle" added to line 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 simple 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.
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. This defines public inheritance 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. 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
Example program ------> CAR.CPP
Examine the file named CAR.CPP which is the implementation file for the car class. The 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
Example program ------> TRUCK.H
Examine the file named TRUCK.H for an example of another class that uses the vehicle class and 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 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
Example program ------> TRUCK.CPP
Examine the file named TRUCK.CPP for the implementation of the truck class. It has 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.
USING ALL THREE CLASSES
Example program ------> ALLVEHIC.CPP
Examine the program named ALLVEHIC.CPP for an example that uses all three of the classes we 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 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 19 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 ALLVEHIC.CPP 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 redeclare the class vehicle. Of course it is the same declaration, but the system doesn't care, it simply doesn't allow redeclaration 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 declaration is skipped, but if the word is not defined, the declaration is included and VEHICLE.H 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 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
Example program ------> NEWDATE.H
Continuing where we started in chapter 5, we will inherit the date class into the file 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.
Example program ------> NEWDATE.CPP
The program named NEWDATE.CPP contains the implementation for the added method and should be easy 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. It doesn't even adjust for leap years. Once again, we are not really interested in writing a good date class, but in learning the mechanics of inheritance.
Example program ------> TRYNDATE.CPP
Finally, the example program named TRYNDATE.CPP will use the new class in a 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.
Advance to Chapter 8
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