工厂模式（c++描述）

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工厂模式的定义我也说不清，请同学们自己百度一下吧。我在这里用c++描述一下工厂模式的实现。

一个工厂通常包含一些接口类，而工厂就是“生产”这些接口类的具体对象的。工厂本身通常是一个单例模式的类。说的好像不明不白的。看例子吧。

先不建立工厂，这会儿先写一个接口类：

class IAnimal{public:    virtual void speak() = 0;    virtual void getLegs() = 0;    // it is important to add virtual destructor in interface class    virtual ~IAnimal(){}};

接口类很简单，一会儿会用这个接口类生成具体的类。工厂中包含的就是这个接口类。

还是先不忙写工厂，这时再写几个具体的类，继承这个接口：

class Cat : public IAnimal{public:    virtual void speak()    {        std::cout << "mow" << std::endl;    }    virtual void getLegs()    {        std::cout << "four legs" << std::endl;    }    static Cat* createSelf()    {        return new Cat;    }    ~Cat()    {        // free memory if this class allocate memory from heap    }};class Dog : public IAnimal{public:    virtual void speak()    {        std::cout << "wang" << std::endl;    }    virtual void getLegs()    {        std::cout << "four legs" << std::endl;    }    static Dog* createSelf()    {        return new Dog;    }    ~Dog()    {        // free memory if this class allocate memory from heap    }};

cat类和dog类实现IAnimal接口。

现在来写工厂类。这是一个单例（singleton类），意味着只有一个工厂实例。

class AnimalFactory{public://     this singleton is thread safe in c++11.    static AnimalFactory instance()    {        static AnimalFactory instance;        return instance;    }    IAnimal* create(string animal)    {        map<string, IAnimal*>::iterator it = factory_map.find(animal);        if(it != factory_map.end())        {            return it->second;        }        return NULL;    }private:    std::map<string, IAnimal*> factory_map;    void registe(string animal, IAnimal* ia)    {        factory_map[animal] = ia;    }    AnimalFactory()    {        registe("Cat", Cat::createSelf());        registe("dog", Dog::createSelf());        registe("jiji", Jiji::createSelf());        registe("monkey", Monkey::createSelf());    }};

工厂类中，使用静态方法instance（）实现单例，工厂类的构造函数是私有的（只有这样才能单例）。接下来使用create()方法取得接口类的指针，用来指向各式的子类对象，这样工厂就能“创造出”各种动物了（多态的体现啊）。

好了，就这样，已经完成了。在main中试试效果：

int main(){    IAnimal* an_animal = NULL;    string animal_name;    while(1)    {        cout << "input an animal:" << endl;        cin >> animal_name;        if(animal_name == "Q" || animal_name == "q") break;        an_animal = AnimalFactory::instance().create(animal_name);        if(!an_animal)        {            cout << "the factory can not create" + animal_name + ", you should registe it";        }        else        {            an_animal->speak();            an_animal->getLegs();        }    }    return 0;}

下面是完整的程序，如果想亲手试试可以直接copy。本人使用mingw4.8编译通过。

#include <map>#include <iostream>using namespace std; class IAnimal{public:    virtual void speak() = 0;    virtual void getLegs() = 0;    // it is important to add this virtual destructor    virtual ~IAnimal(){}};class Cat : public IAnimal{public:    virtual void speak()    {        std::cout << "mow" << std::endl;    }    virtual void getLegs()    {        std::cout << "four legs" << std::endl;    }    static Cat* createSelf()    {        return new Cat;    }    ~Cat()    {        // free memory if this class allocate memory from heap    }};class Dog : public IAnimal{public:    virtual void speak()    {        std::cout << "wang" << std::endl;    }    virtual void getLegs()    {        std::cout << "four legs" << std::endl;    }    static Dog* createSelf()    {        return new Dog;    }    ~Dog()    {        // free memory if this class allocate memory from heap    }};class Jiji : public IAnimal{public:    virtual void speak()    {        std::cout << "gege" << std::endl;    }    virtual void getLegs()    {        std::cout << "two legd" << std::endl;    }    static Jiji* createSelf()    {        return new Jiji;    }    ~Jiji()    {        // free memory if this class allocate memory from heap    }};class Monkey : public IAnimal{public:    virtual void speak()    {        std::cout << "owow" << std::endl;    }    virtual void getLegs()    {        std::cout << "two legs" << std::endl;    }    static Monkey* createSelf()    {        return new Monkey;    }    ~Monkey()    {        // free memory if this class allocate memory from heap    }};class AnimalFactory{public://     this singleton is thread safe in c++11.    static AnimalFactory instance()    {        static AnimalFactory instance;        return instance;    }    IAnimal* create(string animal)    {        map<string, IAnimal*>::iterator it = factory_map.find(animal);        if(it != factory_map.end())        {            return it->second;        }        return NULL;    }private:    std::map<string, IAnimal*> factory_map;    void registe(string animal, IAnimal* ia)    {        factory_map[animal] = ia;    }    AnimalFactory()    {        registe("Cat", Cat::createSelf());        registe("dog", Dog::createSelf());        registe("jiji", Jiji::createSelf());        registe("monkey", Monkey::createSelf());    }};int main(){    IAnimal* an_animal = NULL;    string animal_name;    while(1)    {        cout << "input an animal:" << endl;        cin >> animal_name;        if(animal_name == "Q" || animal_name == "q") break;        an_animal = AnimalFactory::instance().create(animal_name);        if(!an_animal)        {            cout << "the factory can not create" + animal_name + ", you should registe it";        }        else        {            an_animal->speak();            an_animal->getLegs();        }    }    return 0;}

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