強調工廠方法的理由是 可以使不同型別的工廠派生自基本型別的工廠 工廠方法模式事實上是多型工廠模式的一個特例

//: C10:ShapeFactory2.cpp
// From "Thinking in C++, Volume 2", by Bruce Eckel & Chuck Allison.
// (c) 1995-2004 MindView, Inc. All Rights Reserved.
// See source code use permissions stated in the file 'License.txt',
// distributed with the code package available at www.MindView.net.
// Polymorphic Factory Methods.
#include <iostream>
#include <map>
#include <string>
#include <vector>
#include <stdexcept>
#include <cstddef>
#include "../purge.h"
using namespace std;

class Shape {
public:
  virtual void draw() = 0;
  virtual void erase() = 0;
  virtual ~Shape() {}
};

class ShapeFactory {
  virtual Shape* create() = 0;
  static map<string, ShapeFactory*> factories;
public:
  virtual ~ShapeFactory() {}
  friend class ShapeFactoryInitializer;
  class BadShapeCreation : public logic_error {
  public:
    BadShapeCreation(string type)
    : logic_error("Cannot create type " + type) {}
  };
  static Shape*
  createShape(const string& id) throw(BadShapeCreation) {
    if(factories.find(id) != factories.end())
      return factories[id]->create();
    else
      throw BadShapeCreation(id);
  }
};

// Define the static object:
map<string, ShapeFactory*> ShapeFactory::factories;

class Circle : public Shape {
  Circle() {} // Private constructor
  friend class ShapeFactoryInitializer;
  class Factory;
  friend class Factory;
  class Factory : public ShapeFactory {
  public:
    Shape* create() { return new Circle; }
    friend class ShapeFactoryInitializer;
  };
public:
  void draw() { cout << "Circle::draw" << endl; }
  void erase() { cout << "Circle::erase" << endl; }
  ~Circle() { cout << "Circle::~Circle" << endl; }
};

class Square : public Shape {
  Square() {}
  friend class ShapeFactoryInitializer;
  class Factory;
  friend class Factory;
  class Factory : public ShapeFactory {
  public:
    Shape* create() { return new Square; }
    friend class ShapeFactoryInitializer;
  };
public:
  void draw() { cout << "Square::draw" << endl; }
  void erase() { cout << "Square::erase" << endl; }
  ~Square() { cout << "Square::~Square" << endl; }
};

// Singleton to initialize the ShapeFactory:
class ShapeFactoryInitializer {
  static ShapeFactoryInitializer si;
  ShapeFactoryInitializer() {
    ShapeFactory::factories["Circle"]= new Circle::Factory;
    ShapeFactory::factories["Square"]= new Square::Factory;
  }
  ~ShapeFactoryInitializer() {
    map<string, ShapeFactory*>::iterator it =
      ShapeFactory::factories.begin();
    while(it != ShapeFactory::factories.end())
      delete it++->second;
  }
};

// Static member definition:
ShapeFactoryInitializer ShapeFactoryInitializer::si;

char* sl[] = { "Circle", "Square", "Square",
  "Circle", "Circle", "Circle", "Square" };

int main() {
  vector<Shape*> shapes;
  try {
    for(size_t i = 0; i < sizeof sl / sizeof sl[0]; i++)
      shapes.push_back(ShapeFactory::createShape(sl[i]));
  } catch(ShapeFactory::BadShapeCreation e) {
    cout << e.what() << endl;
    return EXIT_FAILURE;
  }
  for(size_t i = 0; i < shapes.size(); i++) {
    shapes[i]->draw();
    shapes[i]->erase();
  }
  purge(shapes);
  getchar();
} ///:~
 

輸出 Circle::draw Circle::erase Square::draw Square::erase Square::draw Square::erase Circle::draw Circle::erase Circle::draw Circle::erase Circle::draw Circle::erase Square::draw Square::erase Circle::~Circle Square::~Square Square::~Square Circle::~Circle Circle::~Circle Circle::~Circle Square::~Square

工廠方法作為virtual create()出現在它自己的ShapeFactory類中 這是一個私有成員函式 意味著不能直接呼叫它 但可以被覆蓋