#include <chrono>
 #include <iostream>
 #include <future>
 #include <mutex>

 constexpr auto tenMill = 10000000;

 std::mutex myMutex;

 class MySingleton{
 public:
   static MySingleton& getInstance(){
     std::lock_guard<std::mutex> myLock(myMutex);
     if ( !instance ){
         instance = new MySingleton();
     }
     volatile int dummy{};
     return *instance;
   }
 private:
   MySingleton() = default;
   ~MySingleton() = default;
   MySingleton(const MySingleton&) = delete;
   MySingleton& operator=(const MySingleton&) = delete;

   static MySingleton* instance;
 };


 MySingleton* MySingleton::instance = nullptr;

 std::chrono::duration<double> getTime(){

   auto begin = std::chrono::system_clock::now();
   for ( size_t i = 0; i <= tenMill; ++i){
        MySingleton::getInstance();
   }
   return std::chrono::system_clock::now() - begin;

 }

 int main(){

     auto fut1= std::async(std::launch::async, getTime);
     auto fut2= std::async(std::launch::async, getTime);
     auto fut3= std::async(std::launch::async, getTime);
     auto fut4= std::async(std::launch::async, getTime);

     auto total= fut1.get() + fut2.get() + fut3.get() + fut4.get();

     std::cout << total.count() << std::endl;
 }