Example - Abstract Factory in C ++

cpp, шаблоны проектирования, abstract, new

Discussion. "Think of object creation as a factory that spawns those same objects."

Here we assign object creation responsibilities to the factory object, and then use inheritance functions and virtual members to provide a "virtual constructor" capability. So there are two aspects. The client uses a factory object instead of the "new" operator to request instantiation; and the client is "hard-wired" to the family or class of that factory, and relies on the abstract base class for the duration of the application.

#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();
  }
}

Conclusion

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