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Answered 2013-06-02 22:50:47

If a class is intended to be used purely as a base class then it must have one or more pure-virtual functions (a function that may or may not have an implementation). Such a class is regarded as being an abstract base class because no instances of the class can be instantiated other than through derivation, and the derivative must provide an implementation or it too becomes an abstract base class. The abstract base class need not declare a pure-virtual method if it inherits one or more pure-virtual methods from another base class but does not provide a complete implementation. Only classes that provide a complete implementation can actually be instantiated. Implementations may be inherited from lower base classes other than the one that declared the function pure-virtual in the first place. Once a derivative implements a pure-virtual function, that implementation may be inherited by subsequent derivatives.

If the base class may be instantiated in its own right then it must not declare any pure-virtual methods, nor must it inherit any pure-virtual methods that have no implementations. In this case the base class may be derived from, but doesn't have to be derived from, whereas an abstract base class must be derived from.

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