# std::condition_variable::wait

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< cpp‎ | thread‎ | condition variable

void wait( std::unique_lock < std::mutex >& lock ); | (1) | (since C++11)
template< class Predicate > void wait( std::unique_lock < std::mutex >& 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 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. 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 `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 `pred()`.

If these functions fail to meet the postconditions (lock.owns_lock()==true and lock.mutex() 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)

### Parameters

lock | - | an object of type std::unique_lock < std::mutex >, 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();​

### Return value

(none)

### Exceptions

1)

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)

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

### Notes

Calling this function if lock.mutex() is not locked by the current thread is undefined behavior.

Calling this function if lock.mutex() is not the same mutex as the one used by all other threads that are currently waiting on the same condition variable is undefined behavior.

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 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

### See also

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

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