C++ 单例模式实现

 #include <iostream>
 #include <mutex>

 std::mutex mtx;
 class Singleton {
 private:
     Singleton() {}
     Singleton(const Singleton& a);
     Singleton& operator=(const Singleton&);
     static Singleton * instance;
 public:
     static Singleton* getInstance() {
         if (instance == nullptr) {
             mtx.lock();
             if((instance == nullptr)){
                 instance =  new Singleton();
             }
             mtx.unlock();
         }
         return instance;
     }
 };
 Singleton* Singleton::instance = nullptr;

 int main(){
     Singleton* instance1 = Singleton::getInstance();
     Singleton* instance2 = Singleton::getInstance();
     if(instance1 == instance2){
         std::cout << "singleton" << std::endl;
     }
     return ;
 }