This header has been present since C++14. It adds shared locking functionality to the timed mutex with these functions:

• lock_shared()
• try_lock_shared()
• try_lock_shared_for()
• try_lock_shared_until()
• unlock_shared()

This class is essentially an amalgamation of the shared mutex and timed mutex, as the name suggests. The interesting thing here is that it was added to the standard before the more basic shared mutex.