33 Concurrency support library [thread]

33.6 Mutual exclusion [thread.mutex]

33.6.7 Call once [thread.once]

33.6.7.1 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.