Factory Method

Intent, Motivation, Applicability, Structure, Participants, Collaborations, Consequences, Implementation, Sample Code, Known Uses, Related Patterns

Sample Code

The function CreateMaze builds and returns a maze. One problem with this function is that it hard-codes the classes of maze, rooms, doors, and walls. We'll introduce factory methods to let subclasses choose these components.

First we'll define factory methods in MazeGame for creating the maze, room, wall, and door objects:

class MazeGame { public: Maze* CreateMaze(); // factory methods: virtual Maze* MakeMaze() const { return new Maze; } virtual Room* MakeRoom(int n) const { return new Room(n); } virtual Wall* MakeWall() const { return new Wall; } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new Door(r1, r2); } };

Each factory method returns a maze component of a given type. MazeGame provides default implementations that return the simplest kinds of maze, rooms, walls, and doors.

Now we can rewrite CreateMaze to use these factory methods:

Maze* MazeGame::CreateMaze () { Maze* aMaze = MakeMaze(); Room* r1 = MakeRoom(1); Room* r2 = MakeRoom(2); Door* theDoor = MakeDoor(r1, r2); aMaze->AddRoom(r1); aMaze->AddRoom(r2); r1->SetSide(North, MakeWall()); r1->SetSide(East, theDoor); r1->SetSide(South, MakeWall()); r1->SetSide(West, MakeWall()); r2->SetSide(North, MakeWall()); r2->SetSide(East, MakeWall()); r2->SetSide(South, MakeWall()); r2->SetSide(West, theDoor); return aMaze; }

Different games can subclass MazeGame to specialize parts of the maze. MazeGame subclasses can redefine some or all of the factory methods to specify variations in products. For example, a BombedMazeGame can redefine the Room and Wall products to return the bombed varieties:

class BombedMazeGame : public MazeGame { public: BombedMazeGame(); virtual Wall* MakeWall() const { return new BombedWall; } virtual Room* MakeRoom(int n) const { return new RoomWithABomb(n); } };

An EnchantedMazeGame variant might be defined like this:

class EnchantedMazeGame : public MazeGame { public: EnchantedMazeGame(); virtual Room* MakeRoom(int n) const { return new EnchantedRoom(n, CastSpell()); } virtual Door* MakeDoor(Room* r1, Room* r2) const { return new DoorNeedingSpell(r1, r2); } protected: Spell* CastSpell() const; };

Sample Code of Abstract Factory, Adapter, Bridge, Builder, Chain of Responsibility, Command, Composite, Decorator, Facade, Factory Method, Flyweight, Interpreter, Iterator, Mediator, Memento, Observer, Prototype, Proxy, Singleton, State, Strategy, Template Method, Visitor