Defined in header
std::once_flag is a helper structure for std::call_once.
An object of type
std::once_flag that is passed to multiple calls to std::call_once allows those calls to coordinate with each other such that only one of the calls will actually run to completion.
std::once_flag is neither copyable nor movable.
 Member functions
constexpr once_flag() noexcept;
once_flag object. The internal state is set to indicate that no function has been called yet.
 See also
| invokes a function only once even if called from multiple threads |
C documentation for once_flag