std::condition_variable_any::wait

Since C++11

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 spurious wakeup.

# Declarations

template< class Lock >
void wait( Lock& lock );

(since C++11)

template< class Lock, class Predicate >
void wait( Lock& lock, Predicate pred );

(since C++11)

template< class Lock, class Predicate >
bool wait( Lock& lock, std::stop_token stoken, Predicate pred );

(since C++20)

# 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

# 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

#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();
}

# Defect reports

DR	Applied to	Behavior as published	Correct behavior
LWG 2114(P2167R3)	C++11	convertibility to bool was too weak to reflect the expectation of implementations	requirements strengthened
LWG 2135	C++11	the behavior was unclear if lock.lock() throws an exception	calls std::terminate in this case