Chapter 9
MULTIPLE INHERITANCE
AND FUTURE DIRECTIONS
C++ version 2.0 was released by AT&T during the summer of 1989, and
the major addition to the language is multiple inheritance, the
ability to inherit data and methods from more than one class into
a subclass. Multiple inheritance and a few of the other additions
to the language will be discussed in this chapter along with a few
of the expected future directions of the language.
Several companies have announced their intention of marketing C++
compilers early in 1990, but as of this writing, Borland
International and Zortech are the only two major companies to
actually deliver a product to the marketplace. The examples in
this chapter have all been compiled and executed using the Borland
C++ compiler version 1.00 but the Zortech compiler was on back
order at the time of this writing, so the example programs could
not be tested with it.
After completing this tutorial, you should have enough experience
with the language to study additional new constructs on your own
as they are implemented by the various compiler writers. We will
update the entire tutorial as soon as practical following
procurement of any new compiler, but hopefully the language will
not change rapidly enough now to warrant an update oftener than
twice a year. Please feel free to contact us for information on
updates to the Coronado Enterprises C++ tutorial.
MULTIPLE INHERITANCE
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The major addition to the C++ language with the release of version
2.0 is the ability to inherit methods and variables from two or
more parent classes when building a new class. This is called
multiple inheritance, and is purported by many people to be a major
requirement for an object oriented programming language. Some
writers have expressed doubts as to the utility of multiple
inheritance, and we are inclined to agree with them. To illustrate
the validity of this, it was not easy to think up a good example
of the use of multiple inheritance as an illustration for this
chapter. In fact, the resulting example is sort of a forced
example that really does nothing useful. It does however,
illustrate the mechanics of the use of multiple inheritance with
C++, and that is our primary concern at this time.
The biggest problem with multiple inheritance involves the
inheritance of variables or methods from two or more parent classes
with the same name. Which method should be chosen as the inherited
variable or method if two or more have the same name? This will
be illustrated in the next few example programs.
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SIMPLE MULTIPLE INHERITANCE
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An examination of the file named MULTINH1.CPP ================
will reveal the definition of two very simple MULTINH1.CPP
classes in lines 4 through 27 named moving_van ================
and driver.
In order to keep the program as simple as possible, all of the
member methods are defined as inline functions. This puts the code
for the methods where it is easy to find and study. You will also
notice that all variables in both classes are declared to be
protected so they will be readily available for use in any class
which inherits them. The code for each class is kept very simple
so that we can concentrate on studying the interface to the methods
rather than spending time trying to understand complex methods.
As mentioned previously, chapter 12 will illustrate the use of non-
trivial methods.
In line 30, we define another class named driven_truck which
inherits all of the data and all of the methods from both of the
previously defined classes. In the last two chapters, we studied
how to inherit a single class into another class, and to inherit
two or more classes, the same technique is used except that we use
a list of inherited classes separated by commas as illustrated in
line 30. The observant student will notice that we use the keyword
public prior to the name of each inherited class in order to be
able to freely use the methods within the subclass. In this case,
we didn't define any new variables, but we did introduce two new
methods into the subclass in lines 32 through 39.
We declared an object named chuck_ford which presumably refers to
someone named Chuck who is driving a Ford moving van. The object
named chuck_ford is composed of four variables, three from the
moving_van class, and one from the driver class. Any of these four
variables can be manipulated in any of the methods defined within
the driven_truck class in the same way as in a singly inherited
situation. A few examples are given in lines 47 through 56 of the
main program and the diligent student should be able to add
additional output messages to this program if he understands the
principles involved.
All of the rules for private or protected variables and public or
private method inheritance as used with single inheritance extends
to multiple inheritance.
DUPLICATED METHOD NAMES
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You will notice that both of the parent classes have a method named
initialize(), and both of these are inherited into the subclass
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Chapter 9 - Multiple Inheritance
with no difficulty. However, if we attempt to send a message to
one of these methods, we will have a problem, because the system
does not know which we are referring to. This problem will be
solved and illustrated in the next example program.
Before going on to the next example program, it should be noted
that we have not declared any objects of the two parent classes in
the main program. Since the two parent classes are simply normal
classes themselves, it should be apparent that there is nothing
magic about them and they can be used to define and manipulate
objects in the usual fashion. You may wish to do this to review
your knowledge of simple classes and objects of those classes.
Be sure to compile and execute this program after you understand
its operation completely.
MORE DUPLICATE METHOD NAMES
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The second example program in this chapter named ================
MULTINH2.CPP, illustrates the use of classes MULTINH2.CPP
with duplicate method names being inherited into ================
a subclass.
If you study the code, you will find that a new method has been
added to all three of the classes named cost_per_full_day(). This
was done intentionally to illustrate how the same method name can
be used in all three classes. The class definitions are no problem
at all, the methods are simply named and defined as shown. The
problem comes when we wish to use one of the methods since they are
all the same name and they have the same numbers and types of
parameters and identical return types. This prevents some sort of
an overloading rule to disambiguate the message sent to one or more
of the methods.
The method used to disambiguate the method calls are illustrated
in lines 60, 64, and 68 of the main program. The solution is to
prepend the class name to the method name with the double colon as
used in the method implementation definition. This is referred to
as qualifying the method name. Actually, you could qualify all
method calls, but if the names are unique, the compiler can do it
for you and make your code easier to write and read.
Be sure to compile and execute this program and study the results.
The observant student will notice that there is a slight
discrepancy in the results given in lines 79 through 81, since the
first two values do not add up to the third value exactly. This
is due to the limited precision of the float variable but should
cause no real problem.
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DUPLICATED VARIABLE NAMES
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If you will examine the example program named ================
MULTINH3.CPP, you will notice that each subclass MULTINH3.CPP
has a variable with the same name. ================
According to the rules of inheritance, an object
of the driven_truck class will have two variables with the same
name, weight. This would be a problem if it weren't for the fact
that C++ has defined a method of accessing each one in a well
defined way. You have probably guessed that we will use
qualification to access each variable. Lines 38 and 45 illustrate
the use of the variables. It may be obvious, but it should be
explicitly stated, that there is no reason that the subclass itself
cannot have a variable of the same name as those inherited from the
parent classes. In order to access it, you simply use
qualification.
It should be apparent to you that once you understand single
inheritance, multiple inheritance is nothing more than an extension
of the same rules. Of course, if you inherit two methods or
variables of the same name, you must use qualification to allow the
compiler to select the correct one.
FUTURE DIRECTIONS OF C++
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An ANSI committee has been formed to write an ANSI standard for
C++. They first met in the Spring of 1990 and are expected to
complete the standard in about three years. Until the new standard
is released, the C++ language is expected to stay fairly stable.
However, due to the nature of compiler writers and their desire to
slightly improve their offerings over their competitors, you can
bet that the language will not remain static during this three year
period.
Many small changes have been added during the past year that barely
affect the casual programmer, or even the heavy user of the
language. You can be sure that the language will evolve slowly and
surely into a very usable and reliable language. There are two
areas that should be discussed in a little detail because they will
add so much to the language in future years, exception handling and
parameterized types.
FUTURE DIRECTIONS - EXCEPTION HANDLING
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A future version of C++ will have some form of exception handling
to allow the programmer to trap errors and prevent the system from
completely shutting down when a fatal error occurs. The Ada
language allows the programmer to trap any error that occurs, even
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system errors, execute some recovery code, and continue on with the
program execution in a very well defined way. Bjarne Stroustrup
has announced that some form of exception handling will be
implemented but he has not stated what form it would take as of
this writing.
FUTURE DIRECTIONS - PARAMETERIZED TYPES
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Many times, when developing a program, you wish to perform some
operation on more than one data type. For example you may wish to
sort a list of integers, another list of floating point numbers,
and a list of alphabetic strings. It seems silly to have to write
a separate sort function for each of the three types when all three
are sorted in the same logical way. With parameterized types, you
will be able to write a single sort routine that is capable of
sorting all three of the lists.
This is already available in the Ada language as the generic
package or procedure. Because it is available in Ada, there is a
software components industry that provides programmers with
prewritten and thoroughly debugged software routines that work with
many different types. When this is available, there will be a
components industry for C++ and precoded, debugged and efficient
source code will be available off the shelf to perform many of the
standard operations. These operations will include such things as
sorts, queues, stacks, lists, etc.
Bjarne Stroustrup has announced that parameterized types will be
available in a future version of C++ but he has not announced the
details of how they would be implemented. He has presented a paper
with details of one way to implement them, but this is only a
suggestion, not a specification.
WHAT SHOULD BE YOUR NEXT STEP?
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Once again, we have reached a major milestone in C++ programming.
With the ability to use inheritance, you have nearly all of the
tools you need to effectively use the object oriented programming
techniques of C++ and you would do well to stop studying again and
begin programming. The only topic left with C++ is virtual methods
which are used for dynamic binding or polymorphism. This will be
covered in the next two chapters. The vast majority of all
programming can be done without dynamic binding, and in attempting
to force it into every program, you could wind up with an
unreadable mess.
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