ANSWER: to give meaning to base class function
When we use virtual keyword our base class function becomes meaningless as it has no use. It only helps in calling all derived class functions of same name. If base class function is initialized with =0, it is known that function has no body and thus the virtual function of base class becomes pure virtual function.
virtual void show( )=0;
class rect: public geom
void show( )
cout << " the area is : y" << endl;