c++11工厂子类实现自注册的两种方法

文章目录

一、产品类构建

1. 猫基类与各品种猫子类

class Cat {
    
    
public:
    virtual void Printer() = 0;
};

class Persian : public Cat {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a cat, persian."<<std::endl;}
};

class Birman : public Cat {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a cat, Birman."<<std::endl;}
};

class NorwegianForestCat : public Cat {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Norwegian Forest Cat."<<std::endl;}
};

class Ragdoll : public Cat {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Cat, Ragdoll."<<std::endl;}
};

class Himalayan : public Cat {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Cat, Himalayan."<<std::endl;}
};

2.狗基类与各品种狗子类

class Dog {
    
    
public:
    virtual void Printer() = 0;
};

class Affenpinscher : public Dog {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Dog, Affenpinscher"<<std::endl;;}
};

class AfghanHound  : public Dog {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Dog, Afghan Hound "<<std::endl;;}
};

class AiredaleTerrier  : public Dog {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Dog, Airedale Terrier "<<std::endl;;}
};

class Akita  : public Dog {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Dog, Akita "<<std::endl;;}
};

class AlaskanMalamute  : public Dog {
    
    
public:
    virtual void Printer() {
    
    std::cout<<"I am a Dog, Alaskan Malamute "<<std::endl;;}
};

二、工厂类构建

现在假设有五家动物繁育工厂，Animal1Factory，Animal2Factory，Animal3Factory，Animal4Factory，Animal5Factory。

Animal1Factoryt负责繁育的品种猫与狗分别为：Persian与Affenpinscher；
Animal2Factory负责繁育的品种猫与狗分别为：Birman与AfghanHound；
Animal3Factory负责繁育的品种猫与狗分别为：NorwegianForestCat与AiredaleTerrier；
Animal4Factory负责繁育的品种猫与狗分别为：Ragdoll与Akita；
Animal5Factory负责繁育的品种猫与狗分别为：Himalayan与AlaskanMalamute。

可构建动物繁育工厂基类：

class AnimalFactory {
    
    
public:
    virtual Cat *produceCat() = 0;
	virtual Dog *produceDog() = 0;
};

其他五个动物繁育工厂子类构建如下：

class Animal1Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Persian();
	}
	Dog *produceDog(){
    
    
		return new Affenpinscher();
	}
}; 

class Animal2Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Birman();
	}
	Dog *produceDog(){
    
    
		return new AfghanHound();
	}   
};

class Animal3Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Ragdoll();
	}
	Dog *produceDog(){
    
    
		return new AiredaleTerrier();
	}   
};

class Animal4Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new NorwegianForestCat();
	}
	Dog *produceDog(){
    
    
		return new Akita();
	}   
};
class Animal5Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Himalayan();
	}
	Dog *produceDog(){
    
    
		return new AlaskanMalamute();
	}   
};

三、客户端使用switch-case实现调用不同工厂子类

对以上构建的各产品类与工厂类，可在客户端使用switch-case根据不同的key去创建不同的派生工厂类对象，实现产生该工厂繁育的品种猫与狗。

int main(int argc, char **argv)
{
    
    

    for (size_t i = 0; i < 100; i++)
    {
    
    
        AnimalFactory* factory = 0;
        int factory_name;
        std::cout<<"please input the factory index: ";
        std::cin>>factory_name;

        switch (factory_name) 
        {
    
    
        case 1:
            factory = new Animal1Factory();
            break;
        case 2:
            factory = new Animal2Factory();
            break;
        case 3:
            factory = new Animal3Factory();
            break;
        case 4:
            factory = new Animal4Factory();
            break;
        case 5:
            factory = new Animal5Factory();
            break;
        default:
            break;
        }

        if (factory != 0)
        {
    
    
            Cat* cat = factory->produceCat();
            Dog* dog = factory->produceDog();

            cat->Printer();
            dog->Printer();
        }        
    }
    return 0;
}

输出如下：
在这里插入图片描述

如果工厂子类个数已经确定，个数也不多的时候，用switch-case来调用工厂子类没啥问题。但是，随着工厂子类数越来越多，这个switch-case结构就显得有点傻了。例如，一个具有将近100多个工厂子类库，维护这样一个switch-case简直是肉疼。并且，switch-case调用工厂子类方式对于需要突然新增工厂子类的项目非常不友好。下面将要介绍的工厂子类自注册方法，便是解决该问题的一个很好的途径。

四、自注册方法一：公开注册函数显式注册

这是介绍的两种自注册方法之一，利用类内的公开注册函数显示注册工厂子类。另一种是在构造函数中隐式注册自己。我个人更加偏爱于后者。

自注册的方法，将各子类共用一个注册表，在构造完后与实例化前，将子类注册进全局的注册表。

对工厂基类新增一些功能函数，用来注册新的工厂子类，并从注册表中获取某一子类：

class AnimalFactory {
    
    
public:
    virtual Cat *produceCat() = 0;
	virtual Dog *produceDog() = 0;
    static AnimalFactory* GetInstance(std::string name)
    {
    
    
        _instance = LookUp(name);
        return _instance;
    }
    static void Register(const std::string name, AnimalFactory* func)
    {
    
    
        AnimalFactory* regist = LookUp(name); // 防止重复注册，保证各子类的唯一性
        if (regist != NULL)
        {
    
    
            return;
        }
        else
        {
    
    
            registry_.insert(std::make_pair(name, func));
        }
    }
protected: 
    static AnimalFactory* LookUp(std::string name)
    {
    
    
        std::map<std::string, AnimalFactory*>::iterator iter = registry_.find(name);
        if (iter != registry_.end())
            return iter->second;
        else
            return NULL;        
    }
private:
    static std::map<std::string, AnimalFactory*> registry_; 
    static AnimalFactory* _instance;
}; 

AnimalFactory* AnimalFactory::_instance = NULL;
std::map<std::string, AnimalFactory*> AnimalFactory::registry_;

各工厂子类增加静态函数static void Regist()，具体代码如下所示：

class Animal1Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Persian();
	}
	Dog *produceDog(){
    
    
		return new Affenpinscher();
	}
    static void Regist(){
    
    AnimalFactory::Register("Animal1Factory", new Animal1Factory());}   
}; 

class Animal2Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Birman();
	}
	Dog *produceDog(){
    
    
		return new AfghanHound();
	}
    static void Regist(){
    
    AnimalFactory::Register("Animal2Factory", new Animal2Factory());}   
};

class Animal3Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Ragdoll();
	}
	Dog *produceDog(){
    
    
		return new AiredaleTerrier();
	}
    static void Regist(){
    
    AnimalFactory::Register("Animal3Factory", new Animal3Factory());}    
};

class Animal4Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new NorwegianForestCat();
	}
	Dog *produceDog(){
    
    
		return new Akita();
	}
    static void Regist(){
    
    AnimalFactory::Register("Animal4Factory", new Animal4Factory());}       
};

class Animal5Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Himalayan();
	}
	Dog *produceDog(){
    
    
		return new AlaskanMalamute();
	}
    static void Regist(){
    
    AnimalFactory::Register("Animal5Factory", new Animal5Factory());}       
};

客户端调用工厂子类如下：

static void before_main()
{
    
    
    std::cout<<"regist sub factory..."<<std::endl;
    Animal1Factory::Regist();
    Animal2Factory::Regist();
    Animal3Factory::Regist();
    Animal4Factory::Regist();
    Animal5Factory::Regist();
}

int main(int argc, char **argv)
{
    
    
    before_main();
    for (size_t i = 0; i < 100; i++)
    {
    
    
        AnimalFactory* factory = 0;
        std::string factory_name;
        std::cout<<"please input the factory name: ";
        std::cin>>factory_name;
        factory = AnimalFactory::GetInstance(factory_name);
        if (factory != 0)
        {
    
    
            Cat* cat = factory->produceCat();
            Dog* dog = factory->produceDog();

            cat->Printer();
            dog->Printer();
        }        
    }
    return 0;
}

输出如下：
在这里插入图片描述

五、自注册方法二：构造函数隐形注册

上面介绍的自注册方法，需要在使用工厂子类前，在客户端对各子类进行一个显式可见的注册。这样做对于使用它们的客户仍不是很友好。现在介绍一种在构造函数内隐形注册的方法，此方法不需要用户在客户端对各工厂子类进行注册。

工厂基类与第一种自注册方法一样
各子类工厂的构造函数中添加Animal1Factory(){AnimalFactory::Register("Animal1Factory", this);}，并在子类未尾添加该子类的一个静态实例，具体代码如下：

class Animal1Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Persian();
	}
	Dog *produceDog(){
    
    
		return new Affenpinscher();
	}
    Animal1Factory(){
    
    AnimalFactory::Register("Animal1Factory", this);}
}; 
static Animal1Factory animal1factory;

class Animal2Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Birman();
	}
	Dog *produceDog(){
    
    
		return new AfghanHound();
	}
    Animal2Factory(){
    
    AnimalFactory::Register("Animal2Factory", this);}      
};
static Animal2Factory animal2factory; // 添加一个静态实例，实现该子类的注册

class Animal3Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Ragdoll();
	}
	Dog *produceDog(){
    
    
		return new AiredaleTerrier();
	}
    Animal3Factory(){
    
    AnimalFactory::Register("Animal3Factory", this);}      
};
static Animal3Factory animal3factory;

class Animal4Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new NorwegianForestCat();
	}
	Dog *produceDog(){
    
    
		return new Akita();
	}
    Animal4Factory(){
    
    AnimalFactory::Register("Animal4Factory", this);}      
};
static Animal4Factory animal4factory;

class Animal5Factory : public AnimalFactory{
    
    
public:
	Cat *produceCat(){
    
    
		return new Himalayan();
	}
	Dog *produceDog(){
    
    
		return new AlaskanMalamute();
	}
    Animal5Factory(){
    
    AnimalFactory::Register("Animal5Factory", this);}       
};
static Animal5Factory animal5factory;

客户端调用工厂子类如下（注意，此时已经在客户端见不到注册的痕迹）：

int main(int argc, char **argv)
{
    
    
    for (size_t i = 0; i < 100; i++)
    {
    
    
        AnimalFactory* factory = 0;
        std::string factory_name;
        std::cout<<"please input the factory name: ";
        std::cin>>factory_name;
        factory = AnimalFactory::GetInstance(factory_name);


        if (factory != 0)
        {
    
    
            Cat* cat = factory->produceCat();
            Dog* dog = factory->produceDog();

            cat->Printer();
            dog->Printer();
        }        
    }
    return 0;
}

输出：
在这里插入图片描述

总结

利用C++工厂方法，可以灵活的实现丰富的功能。正如工厂方法里的名词：工厂-产品，工厂负责生产产品。不同的工厂会产生同样类型的产品，就像衣服有很多种品牌（工厂），然后，衣服也粗分为裤子、外套等（产品）。在工厂-产品模式下，自注册其实分为两层：1）新产品越来越多，需要对新产品进行注册；2）新增加的品牌也会越来越多，需要对新工厂进行注册。网上对前一种注册应用介绍较多，因此，本文在工厂子类自注册应用背景下对自注册方法进行介绍。自注册避免了客户端调用冗长的switch-case，其中利用公开注册函数仍需要在正式使用子类前，对各子类进行注册，构造函数注册法则将注册隐藏在类文件中，当新工厂出现时，客户代码具有最好的兼容性。