prog: pure virtual function and abstract class

http://en.wikipedia.org/wiki/Virtual_function


A pure virtual function or pure virtual method is a virtual function that is required to be implemented by a derived class, if that class is not abstract. Classes containing pure virtual methods are termed "abstract"; they cannot be instantiated directly, and a subclass of an abstract class can only be instantiated directly, if all inherited pure virtual methods have been implemented by that class or a parent class. Pure virtual methods typically have a declaration (signature) and no definition (implementation).