32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.6 Call once [thread.once]

32.5.6.1 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;
  };
}