Power-Programmierung/CD2.mdf/c/tutorial/cpptutor/text/chap10.txt

Text File | 1994-05-15 | 17.2 KB | 348 lines

                                                       Chapter 10
                                                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, simply
learning to use C++, 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
-----------------------------------------------------------------
Examine the example program named              ==================
VIRTUAL1.CPP for the basic program outline        VIRTUAL1.CPP
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 in the last
chapter 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 8.  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 27 through 32.



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                                   Chapter 10 - Virtual Functions

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 19.  You will also
notice that the use of the keyword public in lines 12 and 27
actually do nothing because the only method available in the base
class is also available in the derived classes.  There are no
methods actually inherited.  Leaving the keyword in the header
poses no problem however, so it will be left there for your
study.

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 declared in lines 37 through 40 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
-----------------------------------------------------------------
As you examine the next example program        ==================
named VIRTUAL2.CPP, you will notice that          VIRTUAL2.CPP
there is one small change in line 8.  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 46
illustrating that since all four objects are of different

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                                   Chapter 10 - Virtual Functions

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
-----------------------------------------------------------------
Examine the example program named              ==================
VIRTUAL3.CPP and you will find a repeat of        VIRTUAL3.CPP
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 8, and the main program declares pointers to the objects
rather than declaring the objects themselves in lines 37 through
40.  Since we only declared pointers to the objects we find it
necessary to allocate the objects before using them by using the
new operator in lines 42 through 49.  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 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
-----------------------------------------------------------------
The example program named VIRTUAL4.CPP is      ==================
identical to the last program except for the      VIRTUAL4.CPP
addition of the keyword virtual to line 8      ==================
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.

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                                   Chapter 10 - Virtual Functions

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
-----------------------------------------------------------------
Examine the example program named              ==================
VIRTUAL5.CPP where we almost use a virtual        VIRTUAL5.CPP
method.  Be just a little patient because we   ==================
are almost ready to actually use a virtual
method.

You will notice that this is another copy of our program with the
keyword virtual omitted from line 8 and with a totally different
main program.  In this program, we only declare 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 (remembering that a pointer is
actually a reference), 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

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                                   Chapter 10 - Virtual Functions

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 declare 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 parent class
because the pointer is of the type of the parent 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
-----------------------------------------------------------------
We finally come to an example program with a   ==================
virtual function that operates as a virtual       VIRTUAL6.CPP
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
8 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 8, 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 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.


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                                   Chapter 10 - Virtual Functions

You will notice that in lines 40, 44, 48, and 52, 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
-----------------------------------------------------------------
1.  Modify VIRTUAL3.CPP to deallocate the objects prior to
    terminating the program.

2.  Add a message() method to the truck class of VIRTUAL6.CPP to
    observe the use of the new method instead of defaulting to
    the parent class method.

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