设计模式之工厂模式

特征:封装对象的创建.
1,给一个对象添加新类型时,类的创建必然要发生改变,导致了问题的出现.
2,解决办法:强制用一个通用的工厂(factory)来创建对象,而不允许将创建对象的代码遍布于整个程序.
3,如果创建对象的代码都装到这个工厂执行,那么在增加新对象时,所要做的全部工作就是只需修改工厂.
4,每个面向对象的应用程序都需要创建对象,并且因为人们可能添加新类来扩展应用程序,因此工厂模式可能是所有设计模式中最有用的模式之一.

5,工程模式的实现方法之一:在基类中定义一个静态成员函数.
实例代码:

#include <iostream>
#include <stdexcept>
#include <vector>
using namespace std;

class Shape
{
public:
    virtual void draw()=0;
    virtual void erase()=0;
    virtual ~Shape(){}
    class shape_error : public logic_error
    {
    public:
        shape_error(string type):logic_error("Cann't create shape: "+type){};
    };
    static Shape* factory(const string& type) throw(shape_error);
};

class Circle : public Shape
{
    Circle(){}//构造函数设为私有,保证只有静态工厂方法可以创建对象
    friend Shape* Shape::factory(const string& type);
public:
    void draw(){ cout<<"Circle:Draw"<<endl; }
    void erase(){ cout<<"Circle:Erase"<<endl; }
    ~Circle(){ cout<<"Circle:~Circle"<<endl; }
};

class Square : public Shape
{
    Square(){}//构造函数设为私有,保证只有静态工厂方法可以创建对象
    friend Shape* Shape::factory(const string& type);
public:
    void draw(){ cout<<"Square:Draw"<<endl; }
    void erase(){ cout<<"Square:Erase"<<endl; }
    ~Square(){ cout<<"Square:~Circle"<<endl; }
};


Shape* Shape::factory(const string& type) throw(shape_error)
{
    if(type=="Circle") return new Circle;
    if(type=="Square") return new Square;
    throw shape_error(type);
}

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(Shape::factory(sl[i]));
    } catch(Shape::shape_error& e)
    {
        cout << e.what() << endl;
        for(vector<Shape*>::iterator ite=shapes.begin();ite!=shapes.end();ite++)
            delete *ite;
        return EXIT_FAILURE;
    }
    for(size_t i = 0; i < shapes.size(); i++)
    {
        shapes[i]->draw();
        shapes[i]->erase();
    }
    for(vector<Shape*>::iterator ite=shapes.begin();ite!=shapes.end();ite++)
        delete *ite;
    return 0;
}

6,多态工厂模式:工厂方法作为单独的类中的虚函数出现.
实例代码:

#include <iostream>
#include <stdexcept>
#include <vector>
#include <map>
using namespace std;

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

class ShapeFactory //类工厂
{
    virtual Shape* create()=0; //虚函数
    static map<string,ShapeFactory*> factories;
public:
    virtual ~ShapeFactory(){}
    friend class ShapeFactoryInitializer;
    //核心方法
    class shape_error : public logic_error
    {
    public:
        shape_error(string type):logic_error("Cann't create shape: "+type){};
    };
    static Shape* createShape(const string& id) throw(shape_error)
    {
        if(factories.find(id)!=factories.end())
            return factories[id]->create();
        else
            throw shape_error(id);
    }
};
map<string,ShapeFactory*> ShapeFactory::factories;

class Circle : public Shape
{
    Circle(){}//构造函数设为私有,保证只有静态工厂方法可以创建对象
    friend class ShapeFactoryInitializer; //友元
    //注意私有友元类的使用
    class Factory;
    friend class Factory;
    class Factory : public ShapeFactory
    {
    public:
        Shape* create(){ return new Circle; }
        friend class ShapeFactoryInitializer; //友元
    };
public:
    void draw(){ cout<<"Circle:Draw"<<endl; }
    void erase(){ cout<<"Circle:Erase"<<endl; }
    ~Circle(){ cout<<"Circle:~Circle"<<endl; }
};

class Square : public Shape
{
    Square(){}//构造函数设为私有,保证只有静态工厂方法可以创建对象
    friend class ShapeFactoryInitializer; //友元
    //注意私有友元类的使用
    class Factory;
    friend class Factory;
    class Factory : public ShapeFactory
    {
    public:
        Shape* create(){ return new Square; }
        friend class ShapeFactoryInitializer; //友元
    };
public:
    void draw(){ cout<<"Square:Draw"<<endl; }
    void erase(){ cout<<"Square:Erase"<<endl; }
    ~Square(){ cout<<"Square:~Circle"<<endl; }
};

class ShapeFactoryInitializer
{
    static ShapeFactoryInitializer si;
    ShapeFactoryInitializer()
    {
        ShapeFactory::factories["Circle"]=new Circle::Factory; //这里
        ShapeFactory::factories["Square"]=new Square::Factory; //这里
    }
    ~ShapeFactoryInitializer()
    {
        for(map<string,ShapeFactory*>::iterator ite=ShapeFactory::factories.begin();ite!=ShapeFactory::factories.end();ite++)
            delete ite->second;
    }
};
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::shape_error& e)
    {
        cout << e.what() << endl;
        for(vector<Shape*>::iterator ite=shapes.begin();ite!=shapes.end();ite++)
            delete *ite;
        return EXIT_FAILURE;
    }
    for(size_t i = 0; i < shapes.size(); i++)
    {
        shapes[i]->draw();
        shapes[i]->erase();
    }
    for(vector<Shape*>::iterator ite=shapes.begin();ite!=shapes.end();ite++)
        delete *ite;
    return 0;
}

7,抽象工厂模式:
使用若干工厂方法模式,每个工厂方法模式创建一个不同类型的对象.

#include <iostream>
using namespace std;

class Obstacle
{
public:
    virtual void action()=0;
};

class Fire : public Obstacle
{
    void action(){ cout<<"Fire."<<endl; }
};

class Water : public Obstacle
{
    void action(){ cout<<"Water."<<endl; }
};

class Player
{
public:
    virtual void interactWith(Obstacle*)=0;
};

class MM : public Player
{
public:
    void interactWith(Obstacle* ob)
    {
        cout<<"MM:";
        ob->action();
    }
};

class GG : public Player
{
public:
    void interactWith(Obstacle* ob)
    {
        cout<<"GG:";
        ob->action();
    }
};

class GameFactory
{
public:
    virtual Player* makePlayer()=0;
    virtual Obstacle* makeObstacle()=0;
};

class GGandWater : public GameFactory
{
    virtual Player* makePlayer(){ return new GG; }
    virtual Obstacle* makeObstacle(){ return new Water; }
};

class MMandFire : public GameFactory
{
    virtual Player* makePlayer(){ return new MM; }
    virtual Obstacle* makeObstacle(){ return new Fire; }
};

class CreateGame //综合管理
{
    GameFactory* gef;
    Player* p;
    Obstacle* ob;
public:
    CreateGame(GameFactory* factory) : gef(factory),p(factory->makePlayer()),ob(factory->makeObstacle()){}
    void player(){ p->interactWith(ob); }
    ~CreateGame()
    {
        delete gef;
        delete p;
        delete ob;
    }
};

int main()
{
    CreateGame g1(new GGandWater);
    g1.player();
    CreateGame g2(new MMandFire);
    g2.player();
    return 0;
}