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;
  };
}
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;
Effects: Constructs an object of type once_­flag.
Synchronization: The construction of a once_­flag object is not synchronized.
Ensures: 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.