- Solution

// singletonCallOnce.cpp
#include <iostream>
#include <mutex>
class MySingleton{
  private:
    static std::once_flag initInstanceFlag;
    static MySingleton* instance;
    MySingleton()= default;
    ~MySingleton()= default;
  public:
    MySingleton(const MySingleton&)= delete;
    MySingleton& operator=(const MySingleton&)= delete;
    static MySingleton* getInstance(){
      std::call_once(initInstanceFlag,MySingleton::initSingleton);
      return instance;
    }
    static void initSingleton(){
      instance= new MySingleton();
    }
};
MySingleton* MySingleton::instance= nullptr;
std::once_flag MySingleton::initInstanceFlag;
int main(){
  std::cout << std::endl;
  std::cout << "MySingleton::getInstance(): "<< MySingleton::getInstance() << std::endl;
  std::cout << "MySingleton::getInstance(): "<< MySingleton::getInstance() << std::endl;
  std::cout << std::endl;
}