30 Thread support library [thread]

30.4 Mutual exclusion [thread.mutex]

30.4.4 Call once [thread.once] Struct once_flag [thread.once.onceflag]

constexpr once_flag() noexcept;

Effects: Constructs an object of type once_flag.

Synchronization: The construction of a once_flag object is not synchronized.

Postcondition: The object's internal state is set to indicate to an invocation of call_once with the object as its initial argument that no function has been called.