When do we make a virtual function pure?

Answer 1

We make a virtual function pure whenever we wish to make our class an abstract base class (an abstract data type). Unlike a virtual function, pure virtual functions must be overridden by a derived class or by one of its derivatives (the function remains pure virtual until it is overridden, at which point it becomes virtual). Derived classes that do not provide a complete implementation for all the pure virtual functions it inherits become abstract themselves. You cannot instantiate an abstract base class other than through derivation.

Answer 2

When the class in which it is declared is intended to be abstract (meaning it must be derived from, it cannot be instantiated by itself), and where the derived class is expected to provide an implementation for the method even if the base class has its own implementation. Derived classes that do not fully implement all the pure-virtual methods of their base class become abstract themselves. However derivatives inherit the sum of all pure-virtual implementations from their base classes, other than from those that initially declared the pure-virtual methods.

Answer 3

Declare the function as you would a virtual function, but add "=0" before the semi-colon or function body. The following example demonstrates overloaded constant and volatile pure virtual functions:

struct foo

{

virtual const int bar( void ) const = 0;

virtual int bar( void ) = 0;

};

Note that unlike a virtual function which is expected to be overridden, a pure virtual function must be overridden. As such, a generic implementation is optional. If a generic implementation is required, it may be defined inside or outside the class declaration as per any other function. However, as with all class methods, all implementations should ideally be defined outside of the class declaration in order to maintain separation from the interface.