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std::condition_variable_any::wait

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template< class Lock >
void wait( Lock& lock );
(1) (since C++11)
template< class Lock, class Predicate >
void wait( Lock& lock, Predicate pred );
(2) (since C++11)

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.

1) Atomically releases 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. If this function exits via exception, lock is also reacquired. (until C++14)
2) Equivalent to
while (!pred()) {
    wait(lock);
}
This overload may be used to ignore spurious awakenings while waiting for a specific condition to become true. Note that before enter to this method lock must be acquired, after wait(lock) exits it is also reacquired, i.e. lock can be used as a guard to pred() access.
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, (since C++14)

Contents

[edit] Parameters

lock - an object of type Lock that meets the BasicLockable requirements, which must be locked by the current thread
pred - predicate which returns ​false if the waiting should be continued.

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

 bool pred();

[edit] Return value

(none)

[edit] Exceptions

May throw std::system_error, may also propagate exceptions thrown by lock.lock() or lock.unlock().

(until C++14)

Does not throw

(since C++14)



[edit] Example

#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

[edit] See also

blocks the current thread until the condition variable is woken up or after the specified timeout duration
(public member function) [edit]
blocks the current thread until the condition variable is woken up or until specified time point has been reached
(public member function) [edit]