FAQ 21.10 What is a pure virtual member function?

A pure virtual member function is a member function that the base class forces derived classes to provide. Normally these member functions have no implementation; but see FAQ 21.11.

A pure virtual member function specifies that a member function will exist on every object of a concrete derived class even though the member function is not (normally) defined in the base class. This is because the syntax for specifying a pure virtual member function forces derived classes to implement the member function if the derived classes intend to be instantiated (that is, if they intend to be concrete).

For example, all objects of classes derived from Shape will have the member function draw(). However, because Shape is an abstract concept, it does not contain enough information to implement draw(). Thus draw() should be a pure virtual member function in Shape.

 class Shape { public:   virtual void draw() = 0; }; 

This pure virtual function makes Shape an abstract base class (ABC). Imagine that the "= 0" is like saying "the code for this function is at the NULL pointer."

Pure virtual member functions allow users to write code against an interface for which there are several functionally different variants. This means that semantically different objects can be passed to a function if these objects are all under the umbrella of the same abstract base class.