std::condition_variable_any::wait

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

wait causes the current thread to block until the condition variable is notified or a spurious wakeup occurs, optionally looping until some predicate is satisfied (bool(stop_waiting()) == true).

1) Atomically unlocks lock, blocks the current executing thread, and adds it to the list of threads waiting on *this. The thread will be unblocked when notify_all() or notify_one() is executed. It may also be unblocked spuriously. When unblocked, regardless of the reason, lock is reacquired and wait exits.

2) Equivalent to

while (!stop_waiting()) {
    wait(lock);
}

This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true.

Note that lock must be acquired before entering this method, and it is reacquired after wait(lock) exits, which means that lock can be used to guard access to stop_waiting().

3) An interruptible wait: registers the condition_variable_any for the duration of wait(), to be notified if a stop request is made on the given stoken's associated stop-state; it is then equivalent to

while (!stoken.stop_requested()) {
    if (stop_waiting()) return true;
    wait(lock);
}
return stop_waiting();

Note that the returned value indicates whether stop_waiting evaluated to true, regardless of whether there was a stop requested or not.

If these functions fail to meet the postconditions (lock is locked by the calling thread), std::terminate is called. For example, this could happen if relocking the mutex throws an exception.

Parameters

lock - an object of type Lock that meets the BasicLockable requirements, which must be locked by the current thread

stoken - a std::stop_token to register interruption for

stop_waiting

-

predicate which returns false if the waiting should be continued (bool(stop_waiting()) == false).

The signature of the predicate function should be equivalent to the following:

bool pred();

Return value

1-2) (none)

3) stop_waiting()

Exceptions

1) Does not throw

2) Same as (1) but may also propagate exceptions thrown by stop_waiting

3) Same as (2)

Notes

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 <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
 
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 retroactively to previously published C++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 2135	C++11	`wait` threw an exception on unlocking/relocking failure	calls std::terminate

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Member functions
condition_variable_any::condition_variable_any
condition_variable_any::~condition_variable_any
Notification
condition_variable_any::notify_one
condition_variable_any::notify_all
Waiting
condition_variable_any::wait
condition_variable_any::wait_for
condition_variable_any::wait_until