单例模式--代码

#include <iostream>
#include <stdio.h>
using namespace std;

//此版本不能保证线程安全
class Singleton1{
public:
    static Singleton1* GetInstance()
    {
        if (_instance == nullptr)
        {
            _instance = new Singleton1();
            atexit(Destructor);
        }
        return _instance;
    }

private:
    static void Destructor()
    {
        if (_instance != nullptr)
        {
            delete _instance;
            _instance = nullptr;
            cout << "清理资源" << endl;
        }
    }
    Singleton1(){cout << "构造函数" << endl;};
    ~Singleton1(){cout << "析构函数" << endl;}
    Singleton1(const Singleton1& other){cout << "拷贝构造函数" << endl;}
    Singleton1& operator=(const Singleton1&){cout << "赋值函数" << endl;}
    static Singleton1* _instance;
};
Singleton1* Singleton1::_instance = nullptr;

// c++11 magic static 特性：如果当变量在初始化的时候，并发同时进⼊声明语句，并发线程将会阻塞等待初始化结束。

/*该版本具备 所有优点：
1. 利⽤静态局部变量特性，延迟加载；
2. 利⽤静态局部变量特性，系统⾃动回收内存，⾃动调⽤析构函数；
3. 静态局部变量初始化时，没有 new 操作带来的cpu指令reorder操作；
4. c++11 静态局部变量初始化时，具备线程安全；
*/

class Singleton2{
public:
    static Singleton2& GetInstance()
    {
        static Singleton2 singleIns;
        return singleIns;
    }
private:
    Singleton2(){cout << "构造函数" << endl;};
    ~Singleton2(){cout << "析构函数" << endl;}
    Singleton2(const Singleton2& other){cout << "拷贝构造函数" << endl;}
    Singleton2& operator=(const Singleton2&){cout << "赋值函数" << endl;}
};

int main()
{
    Singleton2 &singleIns1 = Singleton2::GetInstance();
    cout << "singleIns1:" << &singleIns1 << endl;

    Singleton2 &singleIns2 = Singleton2::GetInstance();
    cout << "singleIns2:" << &singleIns2 << endl;


    return 0;
}