Defined in header <mutex> template< class Callable, class... Args >
void call_once( std::once_flag& flag, Callable&& f, Args&&... args ); (since C++11)
Executes the Callable object f exactly once, even if called from several threads.
Each group of call_once invocations that receives the same std::once_flag object will meet the following requirements:
Exactly one execution of exactly one of the functions (passed as f to the invocations in the group