std::condition_variable::wait

性病：情况[医]变量：：等待

void wait( std::unique_lock& lock	(1)	(since C++11)
template< class Predicate > void wait( std::unique_lock<std::mutex>& lock, Predicate pred	(2)	(since C++11)

wait导致当前线程阻塞，直到通知条件变量或出现虚假唤醒为止，还可以选择循环直到满足某个谓词。

1%29原子释放lock，阻止当前正在执行的线程，并将其添加到等待执行的线程列表中。*this.线程将在notify_all()或notify_one()被处决了。它也可能是伪造的。不管原因是什么，lock重新获得和wait出口。如果此函数通过异常退出，lock也被重新获得。%28直到C++14%29

2%29相当于

二次

while (!pred()) {
    wait(lock
}

二次

这种过载可能被用来忽略虚假的唤醒，而等待特定的条件成为现实。请注意，在输入此方法之前lock必须获得，之后wait(lock)退出也会被重新获取，即lock可以用作警卫pred()进入。

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)

参数

lock	-	an object of type std::unique_lock, 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(

返回值

%280%29

例外

1%29

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%29与%281%29相同，但也可能传播由pred

注记

调用此函数如果lock.mutex()不被当前线程锁定是未定义的行为。

调用此函数如果lock.mutex()与当前正在等待相同条件变量的所有其他线程使用的互斥对象不同，这是未定义的行为。

对…的影响notify_one()/notify_all()三个原子部分wait()/wait_for()/wait_until()%28解锁+等待、唤醒和锁定%29按一个总顺序进行，可视为修改顺序原子变量的顺序是特定于这个单独的条件的。[医]变量。这使得不可能notify_one()，例如，延迟并取消阻塞在调用之后才开始等待的线程。notify_one()被制造出来了。

例

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#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(
}

二次

可能的产出：

二次

Waiting...
Waiting...
Waiting...
Notifying...
Notifying again...
...finished waiting. i == 1
...finished waiting. i == 1
...finished waiting. i == 1

二次

另见

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)

c CND文件[医]等待

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