std::once_flag - Man Page

once_flag (const once_flag &)=delete
Deleted copy constructor.
once_flag & operator= (const once_flag &)=delete
Deleted assignment operator.

template<typename _Callable , typename... _Args> void call_once (once_flag &__once, _Callable &&__f, _Args &&... __args)
Invoke a callable and synchronize with other calls using the same flag.