5 Singleton

class Singleton {
public:
	static Singleton* GetInstance();
	static Singleton* m_Instance;
private:
	Singleton();
	Singleton( const Singleton& singleton );
}；

// 线程不安全
Singleton* Singleton::GetInstance(){
	if ( m_Instance != nullptr )
		return m_Instance;
	m_Instance = new Singleton();
}

// 线程安全 无脑锁
Singleton* Singleton::GetInstance(){
	Lock lock;
	...
}

// 线程不安全 双检查锁  内存指令重排下有问题（执行顺序与假象的不一致，可能没有构造就返回了）
Singleton* Singleton::GetInstance(){
	if ( m_Instance != nullptr )
		return m_Instance;
	Lock lock;
	if ( m_Instance != nullptr )
		return m_Instance;
	m_Instance = new Singleton();
	return m_Instance;
}

// 线程安全 双检查锁  volatile (强制从内存中读取一次、禁止指令重排)
std::atomic<Sigleton*> Singleton::m_Instance;
std::mutex Singleton:m_Mutex;

Singleton* Singleton::GetInstance(){
	singleton *tmp = m_Instance.load(memory_order_relaxed);
	// 用于确保执行顺序
	std::atomic_thread_fence(std::memory_order_acquire);
	if ( tmp != nullptr )
		return m_Instance;
	std::lock_guard<std::mutex> guard(m_Mutex);
	tmp = m_Instance.load(memory_order_relaxed);
	if ( m_Instance != nullptr )
		return m_Instance;
	tmp = new Singleton();
	atomic_thread_fence(memory_order_release);
	m_Instance.store(tmp, memory_order_relaxed);
	return m_Instance;
}