例子--迷宫
假定我们现在来实现一个迷宫，迷宫由一个个房间组成，而房间之间由墙和 门来连接。所以迷宫就有几个基础组件：房间，门，墙。假设他们都有一个共同的动作，进入（无论能否进入）。

class MapSite {
    public:
      virtual void enter() = 0;
};

enum Direction { North, South, East, West };    // 房子4边

class Room : public MapSite {
    public:
        Room(int roomNo)  {  }
        vois setSide(Direction, MapSite*) {   }     // 设置房子4边。
        virtual void enter()  {  }

    private:
        MapSite* _sides[4];    // 房子四边由墙或者门组成。
        int _roomNo;
};

class Wall：public MapSite {
    public:
      Wall()  {  }
      virtual void enter()  {  }
};

class Door {
    public:
        Door(Room* , Room*)  {  }
        virtual enter()  {  }

    private:
        Room* _room1;
        Room* _room2;
        bool _isOpen;
};

而迷宫本身就可以看成是一系列房间的集合。

class Maze {
    public:
        Maze()  {  }
        void AddRoom(Room*)  {  }
        Room* RoomNo(int)  const;
  private:
        // 或用线性表，hash表，树存储一系列房间。
};

现在来定义一个类MazeGame，由它来创建迷宫

class MazeGame {
    public:
        Maze* CreateMaze();
    private:
        Maze* aMaze;
};

Maze* MazeGame::CreateMaze()  {
    Maze* aMaze = new Maze();

    Room* r1 = new Room(1);
    Room* r2 = new Room(2);
    Door* theDoor = new Door(r1, r2);
    
    aMaze->addRoom(r1);
    aMaze->addRoom(r2);
    
    r1->setSide(North, new Wall());
    ......
    return aMaze;
  }

这个函数的问题是在于它缺乏可扩展性，假设现在要增加一种新的游戏迷宫，施了魔法的迷宫，新增加2个组件，EnchantedRoom和NeedSpellDoor。需要咒语或者钥匙才能打开的门，和有宝物的房间（咒语，钥匙）。那么在MazeGame中是不是得另外新建一个函数CreateMagicMaze()?那假如我以后要新增n个新的游戏迷宫呢？

抽象工厂模式

试试这样写代码：

  class MazeFactory {
      public:
          MazeFactory();

          virtual Maze* makeMaze()  const  {  return new Maze();  }
          virtual Wall* makeWall()  const  {  return new Wall();  }
          virtual Room* makeRoom(int n)  const   {  return new Room(n);  }
          virtual Door* makeDoor(Room* r1, Room* r2)  const {  
              return new Door(r1, r2);  
          }
  };
  
  class EnchantedMazeFactory : public MazeFactory {
      public:
          EnchantedMazeFactory();

          virtual Room* makeRoom(int n)  const   {  
              return new EnchantedRoom(n, CastSpell()); 
          }
          virtual Door* makeDoor(Room* r1, Room* r2)  const {  
              return new NeedSpellDoor(r1, r2);  
          }
  };

  Maze* MazeGame::CreateMaze(MazeFactory& factory) {
      Maze* aMaze = new Maze();

      Room* r1 = factory.makeRoom(1);
      Room* r2 = factory.makeRoom(2);
      Door* theDoor = factory.makeDoor(r1, r2);
    
      aMaze->addRoom(r1);
      aMaze->addRoom(r2);
    
      r1->setSide(North, factory.makeWall());
      ......
      return aMaze;
  }