Vendor1::v()

Vendor::f() calling B(p1p) Paste1::v() Vendor1::v() Vendor::f() delete mp ~Paste1() ~Vendor1() ~Vendor() ~MyBase()

The original library functions A( ) and B( ) still work the same (assuming the new v( ) calls its base-class version). The destructor is now virtual and exhibits the correct behavior.

Although this is a messy example, it does occur in practice and it’s a good demonstration of where multiple inheritance is clearly necessary: You must be able to upcast to both base classes.

Summary

The reason MI exists in C++ and not in other OOP languages is that C++ is a hybrid language and couldn’t enforce a single monolithic class hierarchy the way Smalltalk does. Instead, C++ allows many inheritance trees to be formed, so sometimes you may need to combine the interfaces from two or more trees into a new class.

If no “diamonds” appear in your class hierarchy, MI is fairly simple (although identical function signatures in base classes must be resolved). If a diamond appears, then you must deal with the problems of duplicate subobjects by introducing virtual base classes. This not only adds confusion, but the underlying representation becomes more complex and less efficient.

Multiple inheritance has been called the “goto of the 90’s”.22 This seems appropriate because, like a goto, MI is best avoided in normal programming, but can occasionally be very useful. It’s a “minor” but more advanced feature of C++, designed to solve problems that arise in special situations. If you find yourself using it often, you may want to take a look at your reasoning. A good Occam’s Razor is to ask, “Must I upcast to all of the base classes?” If not, your life will be easier if you embed instances of all the classes you don’t need to upcast to.

22 A phrase coined by Zack Urlocker.

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Exercises

1.These exercises will take you step-by-step through the traps of MI. Create a base class X with a single constructor that takes an int argument and a member function f( ), that takes no arguments and returns void. Now inherit X into Y and Z, creating constructors for each of them that takes a single int argument. Now multiply inherit Y and Z into A. Create an object of class A, and call f( ) for that object. Fix the problem with explicit disambiguation.

2.Starting with the results of exercise 1, create a pointer to an X called px, and assign to it the address of the object of type A you created before. Fix the problem using a virtual base class. Now fix X so you no longer have to call the constructor for X inside A.

3.Starting with the results of exercise 2, remove the explicit disambiguation for f( ), and see if you can call f( ) through px. Trace it to see which function gets called. Fix the problem so the correct function will be called in a class hierarchy.

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