https://en.cppreference.com/w/cpp/thread/call_once std::call_once - cppreference.com template void call_once( std::once_flag& flag, Callable&& f, Args&&... args ); (since C++11) Executes the Callable object f exactly once, even if called concurrently, from several threads. In detail: If, by the time call_on en.cppreference.com 여러 쓰레드에서 동시에 호출하더라도 딱 한번 호출됨을 보장해준다...