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

Singleton* Singleton::getInstance() {
    Singleton* tmp = m_instance.load();
    if (tmp == nullptr) {
        std::lock_guard<std::mutex> lock(m_mutex);
        tmp = m_instance.load();
        if (tmp == nullptr) {
            tmp = new Singleton;
            m_instance.store(tmp);
        }
    }
    return tmp;
}
 

4.call_once(最簡單的實現)
     在多執行緒程式設計中，有一個常見的情景是某個任務只需要執行一次。在C++11中提供了很方便的輔助類once_flag，call_once。
     以下是對std::call_once的原文說明：
         rom:std::[email protected] 
         Calls fn passing args as arguments, unless another thread has already executed (or is currently executing) a call to call_once with the same flag. 
        If another thread is already actively executing a call to call_once with the same flag, it causes a passive execution: Passive executions do not call fn but do not return until the active execution itself has returned, and all visible side effects are synchronized at that point among all concurrent calls to this function with the same flag. 
If an active call to call_once ends by throwing an exception (which is propagated to its calling thread) and passive executions exist, one is selected among these passive executions, and called to be the new active call instead. 
         Note that once an active execution has returned, all current passive executions and future calls to call_once (with the same flag) also return without becoming active executions. 
       The active execution uses decay copies of the lvalue or rvalue references of fn and args, ignoring the value returned by fn.
also see: 


大意就是:
     call_one保證函式fn只被執行一次，如果有多個執行緒同時執行函式fn呼叫，則只有一個活動執行緒(active call)會執行函式，其他的執行緒在這個執行緒執行返回之前會處於”passive execution”(被動執行狀態)—不會直接返回，直到活動執行緒對fn呼叫結束才返回。對於所有呼叫函式fn的併發執行緒的資料可見性都是同步的(一致的)。 
     如果活動執行緒在執行fn時丟擲異常，則會從處於”passive execution”狀態的執行緒中挑一個執行緒成為活動執行緒繼續執行fn,依此類推。 
    一旦活動執行緒返回,所有”passive execution”狀態的執行緒也返回,不會成為活動執行緒。
     由上面的說明，我們可以確信call_once完全滿足對多執行緒狀態下對資料可見性的要求。
    所以利用call_once再結合lambda表示式,前面幾節那麼多複雜程式碼，在這裡千言萬語凝聚為一句話：

Singleton* Singleton::m_instance;
Singleton* Singleton::getInstance() {
    static std::once_flag oc;//用於call_once的區域性靜態變數
    std::call_once(oc, [&] {  m_instance = new Singleton();});
    return m_instance;
}
 

總結
本文中提到的幾種方法都是安全可用的方案,具體用哪種,我個人覺得還是call_once最簡單，我肯定選call_one。但不代表前面的那麼多都白寫了，其實學習每種方法過程中讓我對c++11記憶體模型有了更深入的理解，這才是最大的收穫。