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─────C/C++ Language─────────────────────────────────────────────────────────
 
     An abstract class is a class that contains at least one pure
     virtual function. Specify a virtual function as pure by placing
     = 0 at the end of its declaration. You don't have to supply a
     definition for a pure virtual function.
 
     You cannot declare an instance of an abstract base class; you
     can use it only as a base class when declaring other classes.
 
     In the following program, draw() is a pure virtual function
     defined in the abstract class Shape. You cannot declare Shape
     objects. Shape acts as a base class for Rectangle and Circle.
     Rectangle and Circle provide definitions for draw(), so you can
     declare instances of those classes and call draw() for them.
 
         #include <iostream.h>
 
         class Shape
         {
         public:
             virtual void draw() = 0;
         };
 
         class Rectangle: public Shape
         {
         public:
             void draw();
         };
 
         class Circle : public Shape
         {
         public:
             void draw();
         };
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