Abstract Factory in C++

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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; }

};

 

int 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

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Abstract Factory in C++

Why read if you can watch?