What is Pure Virtual Function? Why and when it is used ?

A virtual function that is initialized to zero (0) is
referred to as pure virtual function.It has no body and
hence also known as do-nothing or the dummy function.
Example: virtual void show()=0;

A class containing one or more pure virtual functions is
called an Abstract class, which means an instance of such
class can't be created (but pointer to that class can be
created).We should use pure virtual function if we do not
want to instantiate a class but make it act as a base class
for all the classes that derive from it.An important thing
to note about pure virtual functions is that these
functions must be overridden in all the derived classes
otherwise the compile would flag out an error.

Sample program:

class alpha
{
public:virtual void show()=0; //pure virtual function
};
class beta:public alpha
{
public:void show() //overriding
{
cout<<"OOP in C++"; } }; void main() { alpha *p; beta b; p=&b; p->show();
}

Output: OOP in C++