29. Pure virtual functions

• A pure virtual function is one which must be implemented in derived classes.

• A class with a pure virtual function is called an abstract base class.

• A class with a pure virtual function cannot be instantiated:

  
      class Shape {
      public:
          virtual void draw() = 0; // pure virtual
  
      };//class Shape
  
  
      int main()
      {
          Shape sh; // Error, cannot create instance
      }