VIRTUAL FUNCTIONS
Once again we are into a completely new topic with terminology which will be new to you. If you are new to object oriented programming, you should follow along in this chapter very carefully because every attempt has been made to define every detail of this new and somewhat intimidating topic. However, if you are well versed in object oriented programming, and simply learning C++ as a new language, you may wish to skip the first four programs in this chapter and go directly to the example program named VIRTUAL5.CPP and continue from there to the end of the chapter.
One term which must be defined is polymorphism, a rather large word that simply means similar when used in the context of object oriented programming. Objects are polymorphic if they have some similarities but are still somewhat different. We will see how it is used in the context of object oriented programming as we proceed through this chapter.
We have already studied operator overloading and function overloading in this tutorial, and they are a subtle form of polymorphism since in both cases, a single entity is used to refer to two or more things. The use of virtual functions can be a great aid in programming some kinds of projects as you will see in these two chapters.
A SIMPLE PROGRAM WITH INHERITANCE
Example program ------> VIRTUAL1.CPP
Examine the example program named VIRTUAL1.CPP for the basic program outline we will use for all discussion in this chapter. Since this program has nothing to do with virtual functions, the name may be somewhat misleading. It is named VIRTUAL1.CPP because it is part of a series of programs intended to illustrate the use of virtual functions. The last program in this chapter will illustrate the proper use of virtual functions.
The first program is very simple and you will recognize it as being similar to the programs studied earlier in this tutorial except that this program is greatly simplified in order to effectively instruct you in the use of a virtual function. You will notice that many of the methods from the last chapter have been completely dropped from this example for simplicity, and a new method has been added to the parent class, the method named message() in line 9. Throughout this chapter we will be studying the operation of the method named message() in the base class and the derived classes. For that reason, there is a method named message() in the car class as well as in the new class named boat in lines 30 through 36.
You will also notice that there is a lack of a method named message() in the truck class. This has been done on purpose to illustrate the use of the virtual method, or if you prefer, you can refer to it as a virtual function. You will recall that the method named message() from the base class is available in the truck class because the method from the base class is inherited with the keyword public included in line 21.
The method named message() in the base class and in the derived classes has been kept very simple on purpose. Once again, we are interested in the technique of the virtual method rather than a long complicated example.
The main program is as simple as the classes, one object of each of the classes is defined in lines 41 through 44 and the method named message() is called once for each object. The result of executing the program indicates that the method for each is called except for the object named semi, which has no method named message(). As discussed in the last chapter, the method named message() from the parent class is called and the data output to the monitor indicates that this did happen since it displays "Vehicle message" for the object named semi.
The data for the objects is of no concern in this chapter so all data is allowed to default to private type and none is inherited into the derived classes. Some of the data is left in the example program simply to make the classes look like classes. Based on your experience with C++ by now, you realize that the data could be removed since it is not used.
After you understand this program, compile and execute it to see if your compiler gives the same result of execution.
ADDING THE KEYWORD VIRTUAL
Example program ------> VIRTUAL2.CPP
As you examine the next example program named VIRTUAL2.CPP, you will notice that there is one small change in line 9. The keyword virtual has been added to the declaration of the method named message() in the parent class.
It may be a bit of a disappointment to you to learn that this program operates no differently than the last example program. This is because we are using objects directly and virtual methods have nothing to do with objects, only with pointers to objects as we will see soon. There is an additional comment in line 50 illustrating that since all four objects are of different classes, it is impossible to assign any object to any other object in this program. We will soon see that some pointer assignments are permitted between objects of dissimilar classes.
After you are sure that the fact that they are virtual functions, or methods, has nothing to do with the objects as they are instantiated, compile and execute this example program to see if your compiler results in the same output as that listed.
USING OBJECT POINTERS
Example program ------> VIRTUAL3.CPP
Examine the example program named VIRTUAL3.CPP and you will find a repeat of the first program but with a different main program.
In this program the keyword virtual has been removed from the method declaration in the parent class in line 9, and the main program defines pointers to the objects rather than defining the objects themselves in lines 41 through 44. Since we only defined pointers to the objects, we find it necessary to allocate the objects before using them by using the new operator in lines 46 through 53. Upon running the program, we find that even though we are using pointers to the objects we have done nothing different than what we did in the first program. Upon execution, we find that the program operates in exactly the same manner as the first example program in this chapter. This should not be surprising because a pointer to a method can be used to operate on an object in the same manner as an object can be manipulated.
Be sure to compile and execute this program before continuing on to the next example program. The observant student will notice that we failed to check the allocation to see that it did allocate the objects properly, and we also failed to deallocate the objects prior to terminating the program. As always, in such a simple program, it doesn't matter because the heap will be cleaned up automatically when we return to the operating system.
A POINTER AND A VIRTUAL FUNCTION
Example program ------> VIRTUAL4.CPP
The example program named VIRTUAL4.CPP is identical to the last program except for the addition of the keyword virtual to line 9 once again.
I hope you are not terribly disappointed to find that this program, including the keyword virtual, is still identical to the last program. Once again we are simply using pointers to each of the objects, and in every case the pointer is of the same type as the object to which it points. You will begin to see some changes in the next example program, so be patient, we are almost there.
Once again, it would be best for you to compile and execute this program.
The four previous programs were meant to instruct you in what virtual functions do not do. The next two will show you what virtual functions do.
A SINGLE POINTER TO THE PARENT CLASS
Example program ------> VIRTUAL5.CPP
Examine the example program named VIRTUAL5.CPP where we almost use a virtual method. Be just a little patient because we are almost ready to use a virtual method.
You will notice that this is another copy of our program with the keyword virtual omitted from line 9 and with a totally different main program. In this program, we only define a single pointer to a class and the pointer is pointing to the base class of the class hierarchy. We will use the single pointer to refer to each of the four classes and observe what the output of the method named message() is.
A little digression is in order to understand how we can use a pointer which has been declared to point to one class, to actually refer to another class. If we referred to a vehicle (in the real world, not necessarily in this program), we could be referring to a car, a truck, a motorcycle, or any other kinds of transportation, because we are referring to a very general form of an object. If however, we were to refer to a car, we are excluding trucks, motorcycles, and all other kinds of transportation, because we are referring to a car specifically. The more general term of vehicle can therefore refer to many kinds of vehicles, but the more specific term of car can only refer to a single kind of vehicle, namely a car.
We can apply the same thought process in C++ and say that if we have a pointer to a vehicle, we can use that pointer to refer to any of the more specific objects, and that is indeed legal in C++ according to the definition of the language. In a like manner, if we have a pointer to a car, we cannot use that pointer to reference any of the other classes including the vehicle class because the pointer to the car class is too specific and restricted to be used on any of the other classes.
THE C++ POINTER RULE
The rule as given in C++ terms is as follows. A pointer declared as pointing to a base class can be used to point to an object of a derived class of that base class, but a pointer to a derived class cannot be used to point to an object of the base class or to any of the other derived classes of the base class. In our program therefore, we are allowed to declare a pointer to the vehicle class which is the base class, and use that pointer to refer to objects of the base class or any of the derived classes.
This is exactly what we do in the main program. We define a single pointer which points to the vehicle class and use it to point to objects of each of the classes in the same order as in the last four programs. In each case, we allocate the object, send a message to the method named message() and deallocate the object before going on to the next class. You will notice that when we send the four messages, we are sending the message to the same method, namely the method named message() which is a part of the vehicle base class. This is because the pointer has a class associated with it. Even though the pointer is actually pointing to four different classes in this program, the program acts as if the pointer is always pointing to an object of the base class because the pointer is of the type of the base class.
The next program will finally do something you have not seen in any C program or in any C++ program in this tutorial up to this point. After you compile and execute the current program, we will go on to study our first virtual function.
AN ACTUAL VIRTUAL FUNCTION
Example program ------> VIRTUAL6.CPP
We finally come to an example program with a virtual function that operates as a virtual function and exhibits dynamic binding or polymorphism as it is called. This is in the program named VIRTUAL6.CPP.
This program is identical to the last example program except that the keyword virtual is added to line 9 to make the method named message() a virtual function. You will notice that the keyword virtual only appears in the base class, all classes that derive this class will have the corresponding method automatically declared virtual by the system. In this program, we will once again use the single pointer to the base class and allocate, use, then delete an object of each of the four available classes using the identical code we used in the last program. However, because of the addition of the keyword virtual in line 9, this program acts entirely different from the last example program.
Since the method named message() is declared to be a virtual method in its declaration in the base class, anytime we refer to this method with a pointer to the base class, we actually execute the method associated with one of the derived classes. But this is true only if there is a method available in the derived class and if the pointer is actually pointing to that derived class. When the program is executed, the output reflects the same output we saw in the other cases when we were actually calling the methods in the derived classes, but now we are using a pointer of the base class type to make the calls.
You will notice that in lines 44, 48, 52, and 56, even though the code is identical in each line, the system is making the decision of which method to actually call based on the type of the pointer when each message is sent. The decision of which method to call is not made during the time when the code is compiled but when the code is executed. This is dynamic binding and can be very useful in some programming situations. In fact, there are only three different calls made because the class named truck does not have a method named message(), so the system simply uses the method from the base class to satisfy the message passed. For this reason, a virtual function must have an implementation available in the base class which will be used if one is not available in one or more of the derived classes. Note that the message is actually sent to a pointer to the object, but this is splitting hairs and should not be overly emphasized at this time.
It is probably not obvious, but the observant student will note that the structure of the virtual function in the base class and each of the derived classes is identical. The return type and the number and types of the parameters must be identical for all functions, since a single statement can be used to call any of them.
IS THIS REALLY SIGNIFICANT?
This program probably does not seem to do much when you first approach it, but the dynamic binding is a very useful construct and will be illustrated in the next chapter with a rather simple program that uses the technique of dynamic binding to implement a personnel list for a small company.
If the keyword virtual is used, the system will use late binding which is done at run time, but if the keyword is not included, early binding will be used. What these words actually mean is that with late binding, the compiler does not know which method will actually respond to the message because the type of the pointer is not known at compile time. With early binding, however, the compiler decides at compile time what method will respond to the message sent to the pointer.
Be sure to compile and execute this example program before continuing on to the next chapter where we will see a practical example of the use of this technique.
PROGRAMMING EXERCISES
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