一、单例模式要点

1、构造函数设置为私有函数，不允许类外创建
2、提供静态接口供客户获取单例
3、将类指针设为静态，因为静态函数只能操作静态变量
4、静态类指针类外初始化

二、懒汉模式

1. 实例创建时机：第一次访问实例的时候创建
2. 模式类别：线程不安全、线程安全1、线程安全2

//线程不安全之懒汉模式 
class singlelazy {
public:
    static singlelazy* getInstance() {
        if (lazy == nullptr)
            lazy = new singlelazy();
        return lazy;
    }
private:
    singlelazy() {};
    static singlelazy* lazy;
};
singlelazy* singlelazy::lazy = nullptr;

//线程安全之懒汉模式1
mutex mt;
class singlelazy {
public:
    static singlelazy* getInstance() {
        mt.lock();
        if (lazy == nullptr)
            lazy = new singlelazy();
        mt.unlock();
        return lazy;
    }
private:
    singlelazy(){};
    static singlelazy* lazy;
};
singlelazy* singlelazy::lazy = nullptr;

//线程安全之懒汉模式2
mutex mt;
class singlelazy {
public:
    static singlelazy* getInstance() {
        
        if (lazy == nullptr) {
            mt.lock();
            if (lazy == nullptr)
                lazy = new singlelazy();
            mt.unlock();
        }
        return lazy;
    }
private:
    singlelazy(){};
    static singlelazy* lazy;
};
singlelazy* singlelazy::lazy = nullptr;

三、饿汉模式

1. 实例创建时机：第一次访问实例的时候创建
2. 模式类别：线程安全

//饿汉模式
class singlehungry {
public:
    static singlehungry* getInstance() {
        return hungry;
    }
private:
    singlehungry() {};
    static singlehungry* hungry;
};
singlehungry* singlehungry::hungry = new singlehungry();

四、单例模式的调用

int main()
{   //懒汉模式
    singlelazy* lazy1 = singlelazy::getInstance();
    singlelazy* lazy2 = singlelazy::getInstance();
    if (lazy1 == lazy2)
        cout << "singlelazy is single_example" << endl;
    else
        cout<<  "singlelazy is not single_example" << endl;
    //饿汉模式
    singlehungry* hungry1 = singlehungry::getInstance();
    singlehungry* hungry2 = singlehungry::getInstance();
    if(hungry1==hungry2)
        cout<< "singlehungry is single_example" << endl;
    else
        cout << "singlehungry is not single_example" << endl;
}