class Singleton{

private:

Singleton();

Singleton(const Singleton& other);

public:

static Singleton* getInstance();

static Singleton* m_instance;

};

Singleton* Singleton::m_instance=nullptr;

//线程非安全版本

Singleton* Singleton::getInstance() {

if (m_instance == nullptr) {

m_instance = new Singleton();

}

return m_instance;

}

//线程安全版本，但锁的代价过高

Singleton* Singleton::getInstance() {

Lock lock;

if (m_instance == nullptr) {

m_instance = new Singleton();

}

return m_instance;

}

//双检查锁，但由于内存读写reorder不安全

Singleton* Singleton::getInstance() {

if(m_instance==nullptr){

Lock lock;

if (m_instance == nullptr) {

m_instance = new Singleton();

}

}

return m_instance;

}

//C++ 11版本之后的跨平台实现 (volatile)

std::atomic<Singleton*> Singleton::m_instance;

std::mutex Singleton::m_mutex;

Singleton* Singleton::getInstance() {

Singleton* tmp = m_instance.load(std::memory_order_relaxed);

std::atomic_thread_fence(std::memory_order_acquire);//获取内存fence

if (tmp == nullptr) {

std::lock_guard<std::mutex> lock(m_mutex);

tmp = m_instance.load(std::memory_order_relaxed);

if (tmp == nullptr) {

tmp = new Singleton;

std::atomic_thread_fence(std::memory_order_release);//释放内存fence

m_instance.store(tmp, std::memory_order_relaxed);

}

}

return tmp;

}