// abstract base class

// In an abstract base classes we could leave that area() member function
// without implementation at all. This is done by appending = 0 (equal to zero)
// to the function declaration. 


#include <iostream>
using namespace std;

class Polygon {
  protected:
    int width, height;
  public:
    void set_values (int a, int b)
      { width=a; height=b; }
    virtual int area (void) = 0;
  };

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

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

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