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Multiple Base Classes

As described in Multiple Inheritance, a class can be derived from more than one base class. In a multiple-inheritance model (where classes are derived from more than one base class), the base classes are specified using the base-list grammar element (see the Grammar section in Overview). For example, the class declaration for CollectionOfBook, derived from Collection and Book, can be specified:

// deriv_MultipleBaseClasses.cpp
// compile with: /LD
class Collection {
};
class Book {};
class CollectionOfBook : public Book, public Collection {
    // New members
};

The order in which base classes are specified is not significant except in certain cases where constructors and destructors are invoked. In these cases, the order in which base classes are specified affects the following:

  • The order in which initialization by constructor takes place. If your code relies on the Book portion of CollectionOfBook to be initialized before the Collection part, the order of specification is significant. Initialization takes place in the order the classes are specified in the base-list.

  • The order in which destructors are invoked to clean up. Again, if a particular "part" of the class must be present when the other part is being destroyed, the order is significant. Destructors are called in the reverse order of the classes specified in the base-list.

    NoteNote

    The order of specification of base classes can affect the memory layout of the class. Do not make any programming decisions based on the order of base members in memory.

When specifying the base-list, you cannot specify the same class name more than once. However, it is possible for a class to be an indirect base to a derived class more than once.

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