[CPP] Multiple Inheritance

Multiple Inheritance

  • In Java you can implement multiple interfaces but only extend one class
  • In C++ you can inherit from(extend) multiple classes
  • You will inherit all of the behaviour(i.e. function implementations), not just the interface.
  • But be careful of multiple inheritance, because
    • there are dangers, and confusing elements
    • there may be easier ways(e.g. composition)

There are multiple ways to support reuse of classes by new classes

Composition/Aggregation
  • Models the “has a” or “is a part of” relationship
  • Composition is a stronger form
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    • You need to expose the methods manually because they are on component objects
Inheritance
  • “is a” or “is a type of”
  • ImplementationL Make the “type of” a sub-class
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Don’t use C-style casting to keep the code short!

  • Use static_cast or dynamic_cast
  • Dynamic cast will check(at runtime) that the pointer really is to an object of that type
  • If you cast pointers or references when multiple inheritance is being used, then addresses may change
  • Normally, casting a pointer just changes the type, but leaves the address unchanged
  • If you go to or from a second base class, the address will change
  • If you go back to sub-class, the pointer value changes back again(use dynamic cast if necessary, to check the type)
Uses/Association
  • Implementation: Maintain a pointer or reference between them, to get to the other object

Multiple inheritance with common base classes

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Multiple inheritance with virtual base classes

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Dangers

  • Be aware of:
    • Inheriting the same names from multiple base classes
    • Inheriting the same base class twicem through two different intermediate classes
  • To resolve the problem:
    • Use scoping operator :: to dis-ambiguate
    • Or use virtual base classes, to keep one copy
    • Or ensure that only one base class has any data, or any non-abstract methods

Wait…What is abstract method in C++?(Pure-virtual functions)

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