/* The following code example is taken from the book * "C++ Templates - The Complete Guide" * by David Vandevoorde and Nicolai M. Josuttis, Addison-Wesley, 2002 * * (C) Copyright David Vandevoorde and Nicolai M. Josuttis 2002. * Permission to copy, use, modify, sell and distribute this software * is granted provided this copyright notice appears in all copies. * This software is provided "as is" without express or implied * warranty, and with no claim as to its suitability for any purpose. */#include "coord.hpp"// common abstract base class GeoObj for geometric objectsclass 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 GeoObjclass Circle : public GeoObj {  public:    virtual void draw() const;    virtual Coord center_of_gravity() const;    //...};// concrete geometric object class Line// - derived from GeoObjclass Line : public GeoObj {  public:    virtual void draw() const;    virtual Coord center_of_gravity() const;    //...};//...