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std::condit...le_any::wait(3)  C++ Standard Libary std::condit...le_any::wait(3)

NAME
     std::condition_variable_any::wait - std::condition_variable_any::wait

Synopsis
	template<  class  Lock	>					     (1)
     (since C++11)
	void wait( Lock& lock );
	template< class Lock,  class  Predicate  >			     (2)
     (since C++11)
	void wait( Lock& lock, Predicate pred );
	template<  class  Lock,  class	Predicate >			     (3)
     (since C++20)
	bool wait( Lock& lock, std::stop_token stoken, Predicate pred );

	wait causes the current thread to block until the condition variable  is
     notified or
	a spurious wakeup occurs. pred can be optionally provided to detect spu-
     rious wakeup.

	1) Atomically calls lock.unlock() and blocks on *this.
	The  thread  will be unblocked when notify_all() or notify_one() is exe-
     cuted. It may
	also be unblocked spuriously.
	When unblocked, calls lock.lock() (possibly blocking on the lock),  then
     returns.
	2,3) Waiting for a specific condition to become true, can be used to ig-
     nore spurious
	awakenings.
	2) Equivalent to
	while (!pred())
	    wait(lock);
	3)  Registers  *this  for the duration of this call, to be notified if a
     stop request
	is made on stoken's associated stop-state; it is then equivalent to
	while (!stoken.stop_requested())
	{
	    if (pred())
		return true;
	    wait(lock);
	}
	return pred();
	If pred() is ill-formed, or
	the return value of pred() is not convertible to bool
	(until C++20)
	decltype(pred()) does not model boolean-testable
	(since C++20), the program is ill-formed.

	Right after wait returns, lock is locked by the calling thread. If this
	postcondition cannot be satisfied^[1], calls std::terminate.

	 1. a This can happen if the re-locking of the mutex  throws  an  excep-
     tion.

Parameters
	lock   - an lock which must be locked by the calling thread
	stoken - a stop token to register interruption for
	pred   - the predicate to check whether the waiting can be completed

Type requirements
	-
	Lock must meet the requirements of BasicLockable.
	-
	Predicate must meet the requirements of FunctionObject.

Return value
	1,2) (none)
	3) The latest result of pred() before returning to the caller.

Exceptions
	1) Does not throw.
	2,3) Any exception thrown by pred.

Notes
	The  returned  value of overload (3) indicates whether pred evaluated to
     true,
	regardless of whether there was a stop requested or not.

	The effects of notify_one()/notify_all() and each of  the  three  atomic
     parts of
	wait()/wait_for()/wait_until()	(unlock+wait,  wakeup,	and  lock)  take
     place in a
	single total order that can be viewed as modification order of an atomic
     variable:
	the order is specific to this individual condition variable. This  makes
     it
	impossible  for  notify_one()  to, for example, be delayed and unblock a
     thread that
	started waiting just after the call to notify_one() was made.

Example
     // Run this code

      #include <chrono>
      #include <condition_variable>
      #include <iostream>
      #include <thread>

      std::condition_variable_any cv;
      std::mutex cv_m; // This mutex is used for three purposes:
		       // 1) to synchronize accesses to i
		       // 2) to synchronize accesses to std::cerr
		       // 3) for the condition variable cv
      int i = 0;

      void waits()
      {
	  std::unique_lock<std::mutex> lk(cv_m);
	  std::cerr << "Waiting... \n";
	  cv.wait(lk, []{ return i == 1; });
	  std::cerr << "...finished waiting. i == 1\n";
      }

      void signals()
      {
	  std::this_thread::sleep_for(std::chrono::seconds(1));
	  {
	      std::lock_guard<std::mutex> lk(cv_m);
	      std::cerr << "Notifying...\n";
	  }
	  cv.notify_all();

	  std::this_thread::sleep_for(std::chrono::seconds(1));

	  {
	      std::lock_guard<std::mutex> lk(cv_m);
	      i = 1;
	      std::cerr << "Notifying again...\n";
	  }
	  cv.notify_all();
      }

      int main()
      {
	  std::thread t1(waits), t2(waits), t3(waits), t4(signals);
	  t1.join();
	  t2.join();
	  t3.join();
	  t4.join();
      }

Possible output:
      Waiting...
      Waiting...
      Waiting...
      Notifying...
      Notifying again...
      ...finished waiting. i == 1
      ...finished waiting. i == 1
      ...finished waiting. i == 1

	Defect reports

	The following behavior-changing defect	reports  were  applied	retroac-
     tively to
	previously published C++ standards.

	   DR	  Applied  to	      Behavior as published		 Correct
     behavior
	LWG 2135 C++11	    the behavior was unclear if 	calls  std::ter-
     minate in this
			    lock.lock() throws an exception	case

See also
		   blocks  the	current  thread  until the condition variable is
     awakened or
	wait_for   after the specified timeout duration
		   (public member function)
		   blocks the current thread until  the  condition  variable  is
     awakened or
	wait_until until specified time point has been reached
		   (public member function)
	C documentation for
	cnd_wait

External links
	1.   The  Old  New  Thing  article: Spurious wake-ups in Win32 condition
     variables.

Hidden category:
	  * Pages with unreviewed LWG DR marker

http://cppreference.com 	   2024.06.10	   std::condit...le_any::wait(3)
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