32 Thread support library [thread]

32.5 Mutual exclusion [thread.mutex]

32.5.7 Call once [thread.once]

32.5.7.1 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; }; }
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The class once_­flag is an opaque data structure that call_­once uses to initialize data without causing a data race or deadlock.
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constexpr once_flag() noexcept;
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Synchronization: The construction of a once_­flag object is not synchronized.
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Postconditions: 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.