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std::condition_variable::notify_one

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void notify_one();
(since C++11)

If any threads are waiting on *this, calling notify_one unblocks one of the waiting threads.

Contents

[edit] Parameters

(none)

[edit] Return value

(none)

[edit] Exceptions

noexcept specification:  
noexcept
  

[edit] Notes

The effects of notify_one()/notify_all() and wait()/wait_for()/wait_until() take place in a single total order, so it's 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.

The notifying thread does not need to hold the lock on the same mutex as the one held by the waiting thread(s); in fact doing so is a pessimization, since the notified thread would immediately block again, waiting for the notifying thread to release the lock.

[edit] Example

#include <iostream>
#include <condition_variable>
#include <thread>
#include <chrono>
 
std::condition_variable cv;
std::mutex cv_m;
int i = 0;
bool done = false;
 
void waits()
{
    std::unique_lock<std::mutex> lk(cv_m);
    std::cout << "Waiting... \n";
    cv.wait(lk, []{return i == 1;});
    std::cout << "...finished waiting. i == 1\n";
    done = true;
}
 
void signals()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    std::cout << "Notifying falsely...\n";
    cv.notify_one();
   //Most probably, the waiting thread is awaken unnecessarily.
 
    std::unique_lock<std::mutex> lk(cv_m);
    i = 1;
    while (!done) 
    {
        std::cout << "Notifying true change...\n";
        lk.unlock(); //It is a good idea unlock before notifying.
        cv.notify_one();
        std::this_thread::sleep_for(std::chrono::seconds(1));
        lk.lock();
    }
}
 
int main()
{
    std::thread t1(waits), t2(signals);
    t1.join(); 
    t2.join();
}

Possible output:

Waiting... 
Notifying falsely...
Notifying true change...
...finished waiting. i == 1

[edit] See also

notifies all waiting threads
(public member function) [edit]