The following code example is taken from the book
 
  C++ Templates - The Complete Guide
 by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002
 
  © Copyright David Vandevoorde and Nicolai M. Josuttis 2002
#include "coord.hpp"
// common abstract base class GeoObj for geometric objects
class GeoObj {
  public:
    // draw geometric object:
    virtual void draw() const = 0;
    // return center of gravity of geometric object:
    virtual Coord center_of_gravity() const = 0;
    //...
};
// concrete geometric object class Circle
// - derived from GeoObj
class Circle : public GeoObj {
  public:
    virtual void draw() const;
    virtual Coord center_of_gravity() const;
    //...
};
// concrete geometric object class Line
// - derived from GeoObj
class Line : public GeoObj {
  public:
    virtual void draw() const;
    virtual Coord center_of_gravity() const;
    //...
};
//...