Pure Virtual Functions in C++
A Pure Virtual Function is a virtual function that is initialised to zero and has no definition in the base class. Such functions are merely included in the base class, but do not perform any task. A pure virtual function is also referred to as a do-nothing function. Such a function is defined separately in all derived classes and its definition in the base class is not required. The compiler, in case of a pure virtual function, requires each derived class to either define the function or re-declare it as a pure virtual function. Such a class which contains a pure virtual function cannot be instantiated, is referred to as an abstract class.
Consider the following code segment:
virtual void show_output() = 0;
//a pure virtual function declared
//and initialized to zero.
class derived : public base
//function re-defined in derived class.
cout<<”Inside show_output function.”;
As the presence of a pure virtual function in a class requires that it should be implemented in derived classes, in the above code segment show_output() function in class ‘base’ is defined as virtual and is initialized with zero, so it is a pure virtual function. Class ‘derived’ is publicly inherited from class ‘base’. Class ‘base’ is an abstract base class, as it includes a pure virtual function.