1. Multiple Inheritance in C++
How to do it
Why to do it
This could still be slicked up some
Copyright © Curt Hill

2. Multiple Inheritance Recall the grad class
It inherited behavior from both student and person
This is not multiple inheritance
Multiple inheritance is the inheriting of behavior from two different classes
These classes are not usually in the same inheritance tree
Copyright © Curt Hill

3. Syntax Just about what you might expect class C: public A, public B {
The C class inherits all the properties and methods of both A and B
Class C is interchangeable with both A and B classes
Copyright © Curt Hill

4. Characteristics of Multiple Inheritance
When it is good it is wonderful
A good example is iostreams
Inherit from a string formatting object (stream) and a file I/O object (io)
Thus have characteristics of both
When it is bad it is horrendous
C++ is the main language to retain multiple inheritance
Smalltalk had it then removed it
Java never had – uses Interface instead
Eiffel seems to have done the most with it
Copyright © Curt Hill

5. Problems with Multiple Inheritance
Main problem is name clashes and what to do with them
Suppose two classes have a method X
A new class is a derivation of both
How do we get one X over the other
When both ancestors are from same tree problems compound
Copyright © Curt Hill

6. Bad Example Consider: class grademployee: public grad, public employ{…
This looks reasonable, but now there are two names, two birth dates, two genders
How do we distinguish between the access to the grad set_name or the employ set_name?
We may use the scope resolution operator but it still gets ugly
Copyright © Curt Hill

7. Accessing Names
In the previous example we might want to override set_name
A normally stupid thing to do
Thus we get: void grademployee::set_name (char * n){ strcpy(grad::name, n); strcpy(employ::name, n);
What is the point of two names?
Copyright © Curt Hill

8. Why use? We often have a class that may serve in several roles
These roles are implemented by classes and we have functions that use these classes
Another indicator is to use multiple inheritance when it will reduce the number of ifs and switches in the code
It is often preferable to use abstract base classes instead
Not always possible
Copyright © Curt Hill

9. Example Every person functions in many roles Two of these include
As a member of a family
As an employee
The person then has as its properties: name etc.
It then does multiple inheritance on the Family and Employee class
It is now with an isa relationship to both of those
Example project
Copyright © Curt Hill

10. Demo Commentary A Dev multiple inheritance demo exists
Dev does have some problems with large numbers of files
Readding the file may be needed
The Person class is multiply derived from Family and Employment
Thus it may be treated as any one of these
We see this in the call to SummarizeMS
Copyright © Curt Hill

11. Commentary In the SummarizeMS call a person object is passed
What it actually expects is a Family object
This could also be done with composition
Then we would have to develop one or more methods to direct calls to the property
Copyright © Curt Hill

12. Conclusion Multiple inheritance gives us some more power
It is much less common to use multiple inheritance than single inheritance
Yet IOStreams show a good reason to use
We should now look at a demo with family and employee
Copyright © Curt Hill