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Defined in header <mutex>
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 eachother such that only one of the calls will actually run to completion.
std::once_flag is noncopyable.
once_flag object. The internal state is set to indicate that no function has been called yet.