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Showing content from https://cplusplus.com/reference/mutex/once_flag/ below:

class

<mutex>

std::once_flag

Flag argument type for call_once

Object of this type are used as arguments for call_once.

Using the same object on different calls to call_once in different threads causes a single execution if called concurrently.

It is a non-copyable, non-movable, default-constructible class, declared in <mutex> with the following prototype:

struct once_flag {
  constexpr once_flag() noexcept;
  once_flag (const once_flag&) = delete;
  once_flag& operator= (const once_flag&) = delete;
};