单例设计模式

class MyCAS {
private:
	MyCAS() {}
	
private:
	static MyCAS *instance;

public:
	static MyCAS *GetInstance()
	{
		if (instance == NULL)
		{
			instance = new MyCAS();
			static MyCAS_Recover cl;
		}

		return instance;
	}

	class MyCAS_Recover //释放内存
	{
	public:
		~MyCAS_Recover()
		{
			if (MyCAS::instance)
			{
				delete MyCAS::instance;
				MyCAS::instance = NULL;
			}
		}
	};

	void fun()
	{
		cout << &instance << endl;
	}
};

//静态成员初始化
MyCAS *MyCAS::instance = NULL;

int main()
{
	MyCAS *p_a=MyCAS::GetInstance();
	MyCAS *p_b = MyCAS::GetInstance();
	p_a->fun();
	p_b->fun();
	return 0;
}

引入线程的概念

static mutex resource_mutex;
class MyCAS {
private:
	MyCAS() {}
	
private:
	static MyCAS *instance;
	

public:
	static MyCAS *GetInstance()
	{
		//提高效率
		if (instance == NULL)
		{
			unique_lock<mutex> mymutex(resource_mutex);
			if (instance == NULL)
			{
				instance = new MyCAS();
				static MyCAS_Recover cl;
			}

		}
		
		return instance;
	}

	class MyCAS_Recover //释放内存
	{
	public:
		~MyCAS_Recover()
		{
			if (MyCAS::instance)
			{
				delete MyCAS::instance;
				MyCAS::instance = NULL;
			}
		}
	};

	void fun()
	{
		cout << &instance << endl;
	}
};

//静态成员初始化
MyCAS *MyCAS::instance = NULL;

void mythread()
{
	MyCAS *p_a = MyCAS::GetInstance();
}

int main()
{
	std::thread thread_1(mythread);
	std::thread thread_2(mythread);

	thread_1.join();
	thread_2.join();
	return 0;
}

std::call_once():能够保证函数A只被调用一次。

static mutex resource_mutex;
once_flag o_f;
class MyCAS {
private:
	MyCAS() {}
	
private:
	static MyCAS *instance;
	static void CreateInstance()
	{
		instance = new MyCAS();
		static MyCAS_Recover cl;
	}

public:
	static MyCAS *GetInstance()
	{
		call_once(o_f, CreateInstance);
		return instance;
	}

	class MyCAS_Recover //释放内存
	{
	public:
		~MyCAS_Recover()
		{
			if (MyCAS::instance)
			{
				delete MyCAS::instance;
				MyCAS::instance = NULL;
			}
		}
	};

	void fun()
	{
		cout << &instance << endl;
	}
};