单例模式

//实现单例的步骤
//1，构造函数私有化
//2，增加静态私有的当前类的指针变量
//3，提供静态对外接口，可以让用户获得单利对象


class A{
private:
    static A* a;

    A(){
        a = new A;
    }

public:
    static A* getInstance(){
        return a;
    }

}

A* A::a = NULL;


//单例分为懒式
class Singleton_lazy{
private:
    Singleton_lazy(){}
    static Singleton_lazy* getInstance(){
        if(pSingleton == NULL)
            pSingleton = new Single_lazy;
        return pSingleton; 
    }
};

Singleton_lazy* Singleton_lazy::pSingleton = NULL;


//饿汉式
class Singleton_hungry{
private:
    Singleton_hungry(){
    cout << "我是饿汉构造！" << endl;
    }
public:
    static Singleton_hungry* getInstance(){
        return pSingleton;
    }
private:
    static Singleton_hungry* pSingleton;
};
Singleton_hungry* Singleton_hungry::pSingleton = new Singleton_hungry;