// virtual members

#include <iostream>
using namespace std;

class Polygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
      
// A member of a class that can be redefined in its derived classes is known as
// a virtual member. In order to declare a member of a class as virtual,
// we must precede its declaration with the keyword virtual.
      
    virtual int area ()
      { return (0); }
  };

class Rectangle: public Polygon {
  public:
    int area ()
      { return (width * height); }
  };

class Triangle: public Polygon {
  public:
    int area ()
      { return (width * height / 2); }
  };

int main () {
  Rectangle rect;
  Triangle trgl;
  Polygon poly;
  Polygon *poly1 = &rect;
  Polygon *poly2 = &trgl;
  Polygon *poly3 = &poly;
  poly1->set_values (4,5);
  poly2->set_values (4,5);
  poly3->set_values (4,5);
  cout << "The area of rectangel is " << poly1->area() << endl;
  cout << "The area of triangle is " << poly2->area() << endl;
  cout << "The area of polygon is " << poly3->area() << endl;
  getchar();
  
  return 0;
}