C++实现单例模式

单线程：

class Singleton
{
private:
    static Singleton* _instance;
    Singleton()
    {

    }
public:
    static Singleton* Instance()
    {
        if (nullptr == _instance)
            _instance = new Singleton();
        return _instance;
    }
    static void DestoryInstance()
    {
        if (nullptr != _instance)
        {
            delete _instance;
            _instance = nullptr;
        }
    }
    void fun()
    {
        cout << "fun" << endl;
    }
};

Singleton* Singleton::_instance = nullptr;

C++11多线程：

class Singleton
{
private:
    Singleton()
    {

    }
public:
    static Singleton* Instance()
    {
        static Singleton m_instance;
        return &m_instance;
    }

    void fun()
    {
        cout << "fun" << endl;
    }
};

注：C++11标准才支持线程安全的局部静态变量初始化。
C++memory model中定义static local variable：The initialization of such a variable is defined to occur the first time control passes through its declaration; for multiple threads calling the function, this means there’s the potential for a race condition to define first.

C++11另一种方式：

class Singleton
{
private:
    static Singleton* _instance;
    static once_flag flag;

    Singleton()
    {

    }
public:
    static Singleton* Instance()
    {
        call_once(flag, []() { _instance = new Singleton();});
        return _instance;
    }

    void fun()
    {
        cout << "fun" << endl;
    }
};

Singleton* Singleton::_instance = nullptr;
once_flag Singleton::flag;



int main()
{
    vector<thread> vt;
    for (int i = 0; i < 100; ++i)
    {
        vt.push_back(thread([]() {
            Singleton* single = Singleton::Instance();
            single->fun();
        }));
    }
    for_each(vt.begin(), vt.end(), mem_fn(&thread::join));

    system("pause");
    return 0;
}