// Purpose.  Abstract Factory design pattern demo.
// 
// Discussion.  "Think of constructors as factories that churn out objects".
// Here we are allocating the constructor responsibility to a factory object,
// and then using inheritance and virtual member functions to provide a
// "virtual constructor" capability.  So there are two dimensions of
// decoupling occurring.  The client uses the factory object instead of "new"
// to request instances; and, the client "hard-wires" the family, or class, of
// that factory only once, and throughout the remainder of the application
// only relies on the abstract base class.

#include <iostream.h>

class Shape {
public:
   Shape()              { id_ = total_++; }
   virtual void draw()  = 0;
protected:
   int         id_;
   static int  total_;
};
int Shape::total_ = 0;

class Circle : public Shape { public:
   void draw() { cout << "circle " << id_ << ": draw" << endl; } };
class Square : public Shape { public:
   void draw() { cout << "square " << id_ << ": draw" << endl; } };
class Ellipse : public Shape { public:
   void draw() { cout << "ellipse " << id_ << ": draw" << endl; } };
class Rectangle : public Shape { public:
   void draw() { cout << "rectangle " << id_ << ": draw" << endl; } };

class Factory { public:
   virtual Shape* createCurvedInstance()   = 0;
   virtual Shape* createStraightInstance() = 0;
};
class SimpleShapeFactory : public Factory { public:
   Shape* createCurvedInstance()   { return new Circle; }
   Shape* createStraightInstance() { return new Square; }
};
class RobustShapeFactory : public Factory { public:
   Shape* createCurvedInstance()   { return new Ellipse; }
   Shape* createStraightInstance() { return new Rectangle; }
};

main()
{
#ifdef SIMPLE
   Factory*  factory = new SimpleShapeFactory;
#elif ROBUST
   Factory*  factory = new RobustShapeFactory;
#endif
   Shape*    shapes[3];

   shapes[0] = factory->createCurvedInstance();   // shapes[0] = new Ellipse;
   shapes[1] = factory->createStraightInstance(); // shapes[1] = new Rectangle;
   shapes[2] = factory->createCurvedInstance();   // shapes[2] = new Ellipse;

   for (int i=0; i < 3; i++)
      shapes[i]->draw();
}

// ellipse 0: draw
// rectangle 1: draw
// ellipse 2: draw