Editing Abstract factory pattern (section)

== Example ==
This [[C++23]] implementation is based on the pre-C++98 implementation in the book.

<syntaxhighlight lang="c++">
import std;

enum class Direction {North, South, East, West};

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

class Room: public MapSite {
public:
  Room(): roomNumber(0) {}
  Room(int n): roomNumber(n) {}
  void setSide(Direction d, MapSite* ms) {
    std::println("Room::setSide {} ms", d);
  }
  virtual void enter() {}
  Room(const Room&) = delete; // rule of three
  Room& operator=(const Room&) = delete;
private:
  int roomNumber;
};

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

class Door: public MapSite {
public:
  Door(Room* r1 = nullptr, Room* r2 = nullptr): room1(r1), room2(r2) {}
  virtual void enter() {}
  Door(const Door&) = delete; // rule of three
  Door& operator=(const Door&) = delete;
private:
  Room* room1;
  Room* room2;
};

class Maze {
public:
  void addRoom(Room* r) {
    std::println("Maze::addRoom {}", r);
  }
  Room* roomNo(int) const {
    return nullptr;
  }
};

class MazeFactory {
public:
  MazeFactory() = default;
  virtual ~MazeFactory() = default;

  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);
  }
};

// If createMaze is passed an object as a parameter to use to create rooms, walls, and doors, then you can change the classes of rooms, walls, and doors by passing a different parameter. This is an example of the Abstract Factory (99) pattern.

class MazeGame {
public:
  Maze* createMaze(MazeFactory& factory) {
    Maze* aMaze = factory.makeMaze();
    Room* r1 = factory.makeRoom(1);
    Room* r2 = factory.makeRoom(2);
    Door* aDoor = factory.makeDoor(r1, r2);
    aMaze->addRoom(r1);
    aMaze->addRoom(r2);
    r1->setSide(Direction::North, factory.makeWall());
    r1->setSide(Direction::East, aDoor);
    r1->setSide(Direction::South, factory.makeWall());
    r1->setSide(Direction::West, factory.makeWall());
    r2->setSide(Direction::North, factory.makeWall());
    r2->setSide(Direction::East, factory.makeWall());
    r2->setSide(Direction::South, factory.makeWall());
    r2->setSide(Direction::West, aDoor);
    return aMaze;
  }
};

int main() {
  MazeGame game;
  MazeFactory factory;
  game.createMaze(factory);
}
</syntaxhighlight>

The program output is:

<syntaxhighlight lang="output">
Maze::addRoom 0x1317ed0
Maze::addRoom 0x1317ef0
Room::setSide 0 0x1318340
Room::setSide 2 0x1317f10
Room::setSide 1 0x1318360
Room::setSide 3 0x1318380
Room::setSide 0 0x13183a0
Room::setSide 2 0x13183c0
Room::setSide 1 0x13183e0
Room::setSide 3 0x1317f10
</syntaxhighlight>