33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

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

namespace std { struct once_flag { constexpr once_flag() noexcept; once_flag(const once_flag&) = delete; once_flag& operator=(const once_flag&) = delete; }; }
The class once_flag is an opaque data structure that call_once uses to initialize data without causing a data race or deadlock.
constexpr once_flag() noexcept;
Synchronization: The construction of a once_flag object is not synchronized.
Postconditions: 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.