1 #include <iostream> 2 #include <mutex> 3  4 std::mutex mtx; 5 class Singleton { 6 private: 7     Singleton() {} 8     Singleton(const Singleton& a); 9     Singleton& operator=(const Singleton&);10     static Singleton * instance;11 public:12     static Singleton* getInstance() {13         if (instance == nullptr) {14             mtx.lock();15             if((instance == nullptr)){16                 instance =  new Singleton();17             }18             mtx.unlock();19         }20         return instance;21     }22 };23 Singleton* Singleton::instance = nullptr;24 25 int main(){26     Singleton* instance1 = Singleton::getInstance();27     Singleton* instance2 = Singleton::getInstance();28     if(instance1 == instance2){29         std::cout << "singleton" << std::endl;30     }31     return 0;32 }