从代码看设计模式之----工厂模式（C++）
设计模式依照本人理解，就是真对一类问题一个更好的标准的解决方案，按照这个解决方案做出来的东西，后续方便扩展，方便维护，等等一系列方便。不按照这个方案行不行？当然也可以，杀猪杀屁股，各有各的杀法。这是我对设计模式的两点认识：第一，设计模式只是针对一类问题较好的一个解决方案，不唯一，不一定最好；第二，不按照设计模式也能解决问题。
遵照这个思想，我从四人帮的设计模式的书中扣了一些代码出来，调试通过，想以此让自己对设计模式的理解更为深入。毕竟四人帮书中的描述虽然经典，但是终归是没有看代码来的直接点。
下面就来看代码。首先，要看一下没有进行过优化的代码，这个代码是打算建立一个迷宫（Maze），迷宫的元素是墙、房间、门。它们都是MapSite的子类。同时一个房间有四个方向，有可能是墙也有可能是门。
#include <iostream>
using namespace std;
enum Direction {
North,
East,
South,
West,
};
class MapSite {
public:
virtual void Enter() = 0;
};
class Room : public MapSite {
public:
Room(int roomNo):_roomNumber(roomNo) {};
MapSite *GetSide(Direction) const;
virtual void SetSide(Direction direction, MapSite *) {
cout << "Set the "
<< _roomNumber
<< " room "
<< direction
<< " side."
<< endl;
};
int GetRoomNo() {
return _roomNumber;
};
virtual void Enter() {
cout << "Enter the Room." << endl;
};
private:
MapSite *_sides[4];
int _roomNumber;
};
class Wall : public MapSite {
public:
Wall() {};
virtual void Enter() {
cout << "Enter the wall." << endl;
};
};
class Door : public MapSite {
public:
Door(Room* = 0, Room* = 0) {};
virtual void Enter() {
cout << "Enter the door." << endl;
};
Room *OtherSideFrom(Room*) {};
private:
Room *_room1;
Room *_room2;
bool _isOpen;
};
class Maze {
public:
Maze() {};
void AddRoom(Room* room) {
cout << "Add the room" << room->GetRoomNo() << "." << endl;
};
Room* RoomNo(int) const{};
};
class MazeGame {
public:
Maze *CreateMaze();
};
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);
r1->SetSide(East, theDoor);
r1->SetSide(South, new Wall);
r1->SetSide(West, new Wall);
r2->SetSide(North, new Wall);
r2->SetSide(East, new Wall);
r2->SetSide(South, new Wall);
r2->SetSide(West, theDoor);
return aMaze;
}
int main() {
MazeGame mygame;
mygame.CreateMaze();
return 0;
}
这个源码就是普通的实现方式。按照工厂模式进行修改之后的代码如下：
#include <iostream>
using namespace std;
enum Direction {
North,
East,
South,
West,
};
class MapSite {
public:
virtual void Enter() = 0;
};
class Room : public MapSite {
public:
Room(int roomNo):_roomNumber(roomNo) {};
MapSite *GetSide(Direction) const;
virtual void SetSide(Direction direction, MapSite *) {
cout << "Set the "
<< _roomNumber
<< " room "
<< direction
<< " side."
<< endl;
};
int GetRoomNo() {
return _roomNumber;
};
virtual void Enter() {
cout << "Enter the Room." << endl;
};
private:
MapSite *_sides[4];
int _roomNumber;
};
class Wall : public MapSite {
public:
Wall() {};
virtual void Enter() {
cout << "Enter the wall." << endl;
};
};
class Door : public MapSite {
public:
Door(Room* = 0, Room* = 0) {};
virtual void Enter() {
cout << "Enter the door." << endl;
};
Room *OtherSideFrom(Room*) {};
private:
Room *_room1;
Room *_room2;
bool _isOpen;
};
class Maze {
public:
Maze() {};
void AddRoom(Room* room) {
cout << "Add the room" << room->GetRoomNo() << "." << endl;
};
Room* RoomNo(int) const{};
};
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 MazeGame {
public:
Maze* CreateMaze(MazeFactory &factory) {
Maze* aMaze = factory.MakeMaze();
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());
r1->SetSide(East, theDoor);
r1->SetSide(South, factory.MakeWall());
r1->SetSide(West, factory.MakeWall());
r2->SetSide(North, factory.MakeWall());
r2->SetSide(East, factory.MakeWall());
r2->SetSide(South, factory.MakeWall());
r2->SetSide(West, theDoor);
return aMaze;
};
};
int main() {
MazeGame mygame;
MazeFactory factory;
mygame.CreateMaze(factory);
return 0;
}
对比这两个源码就会发现，实现的功能是完全相同的，只不过第二种实现采用了工厂模式。设计模式既然好，肯定有它的好处，那我们接下来看看，如果是对这个程序进行扩展，要修改迷宫为房间里可以有炸弹的迷宫的时候，我们在采用了工厂模式之后，需要进行哪些修改。
#include <iostream>
using namespace std;
enum Direction {
North,
East,
South,
West,
};
class MapSite {
public:
virtual void Enter() = 0;
};
class Room : public MapSite {
public:
Room(int roomNo):_roomNumber(roomNo) {};
MapSite *GetSide(Direction) const;
virtual void SetSide(Direction direction, MapSite *) {
cout << "Set the "
<< _roomNumber
<< " room "
<< direction
<< " side."
<< endl;
};
int GetRoomNo() {
return _roomNumber;
};
virtual void Enter() {
cout << "Enter the Room." << endl;
};
private:
MapSite *_sides[4];
int _roomNumber;
};
class BombRoom : public Room {
public:
BombRoom(int num):Room(num) {
_roomNum = num;
};
virtual void SetSide(Direction direction, MapSite *) {
cout << "Set the "
<< _roomNum
<< " bomb room "
<< direction
<< " side."
<< endl;
};
private:
int _roomNum;
};
class Wall : public MapSite {
public:
Wall() {};
virtual void Enter() {
cout << "Enter the wall." << endl;
};
};
class Door : public MapSite {
public:
Door(Room* = 0, Room* = 0) {};
virtual void Enter() {
cout << "Enter the door." << endl;
};
Room *OtherSideFrom(Room*) {};
private:
Room *_room1;
Room *_room2;
bool _isOpen;
};
class Maze {
public:
Maze() {};
void AddRoom(Room* room) {
cout << "Add the room" << room->GetRoomNo() << "." << endl;
};
Room* RoomNo(int) const{};
};
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 BombMazeFactory : public MazeFactory {
public:
virtual Room* MakeRoom(int n) const {
return new BombRoom(n);
};
};
class MazeGame {
public:
Maze* CreateMaze(MazeFactory &factory) {
Maze* aMaze = factory.MakeMaze();
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());
r1->SetSide(East, theDoor);
r1->SetSide(South, factory.MakeWall());
r1->SetSide(West, factory.MakeWall());
r2->SetSide(North, factory.MakeWall());
r2->SetSide(East, factory.MakeWall());
r2->SetSide(South, factory.MakeWall());
r2->SetSide(West, theDoor);
return aMaze;
};
};
int main() {
MazeGame mygame;
// MazeFactory factory;
BombMazeFactory factory;
mygame.CreateMaze(factory);
return 0;
}
可以看出改动量很小，而且可以随时把代码改回原来的生成普通迷宫的情况。如果是在最初没有采用工厂模式的代码上进行修改呢？那工作量是显而易见的了，有兴趣的可以自己试试，在这里不再累述。
后续会继续分析工厂方法模式。