std::once_flag(3)

NAME
std::once_flag - std::once_flag

Synopsis
   Defined in header <mutex>
   class once_flag; (since C++11)
   The class 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
std::once_flag::once_flag
   constexpr once_flag() noexcept;
   Constructs an once_flag object. The internal state is set to indicate that no
   function has been called yet.

Parameters
   (none)

See also
   call_once invokes a function only once even if called from multiple threads
   (C++11) (function template)
   C documentation for
   once_flag