#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 ;

 }