std::condition_variable

 

C++

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Technical Specifications

 

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condition_variable (C++11)
condition_variable_any (C++11)
notify_all_at_thread_exit (C++11)
cv_status (C++11)
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std::condition_variable

Member functions

condition_variable::condition_variable

condition_variable::~condition_variable

Notification

condition_variable::notify_one

condition_variable::notify_all

Waiting

condition_variable::wait

condition_variable::wait_for

condition_variable::wait_until

Native handle

condition_variable::native handle

 

Defined in header <condition_variable>
class condition_variable;		(since C++11)

The condition_variable class is a synchronization primitive that can be used to block a thread, or multiple threads at the same time, until another thread both modifies a shared variable (the condition), and notifies the condition_variable.

The thread that intends to modify the variable has to

1. acquire a std::mutex (typically via std::lock_guard)
2. perform the modification while the lock is held
3. execute notify_one or notify_all on the std::condition_variable (the lock does not need to be held for notification)

Even if the shared variable is atomic, it must be modified under the mutex in order to correctly publish the modification to the waiting thread.

Any thread that intends to wait on std::condition_variable has to

1. acquire a std::unique_lock<std::mutex>, on the same mutex as used to protect the shared variable
2. execute wait, wait_for, or wait_until. The wait operations atomically release the mutex and suspend the execution of the thread.
3. When the condition variable is notified, a timeout expires, or a spurious wakeup occurs, the thread is awakened, and the mutex is atomically reacquired. The thread should then check the condition and resume waiting if the wake up was spurious.

std::condition_variable works only with std::unique_lock<std::mutex>; this restriction allows for maximal efficiency on some platforms. std::condition_variable_any provides a condition variable that works with any BasicLockable object, such as std::shared_lock.

Condition variables permit concurrent invocation of the wait, wait_for, wait_until, notify_one and notify_all member functions.

The class std::condition_variable is a StandardLayoutType. It is not CopyConstructible, MoveConstructible, CopyAssignable, MoveAssignable.

Member types

Member type	Definition
native_handle_type	implementation-defined

Member functions

(constructor)	constructs the object (public member function)
(destructor)	destructs the object (public member function)
operator= [deleted]	not copy-assignable (public member function)
Notification
notify_one	notifies one waiting thread (public member function)
notify_all	notifies all waiting threads (public member function)
Waiting
wait	blocks the current thread until the condition variable is woken up (public member function)
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)
Native handle
native_handle	returns the native handle (public member function)

Example

condition_variable is used in combination with a std::mutex to facilitate inter-thread communication.

Run this code

#include <iostream>
#include <string>
#include <thread>
#include <mutex>
#include <condition_variable>
 
std::mutex m;
std::condition_variable cv;
std::string data;
bool ready = false;
bool processed = false;
 
void worker_thread()
{
    // Wait until main() sends data
    std::unique_lock<std::mutex> lk(m);
    cv.wait(lk, []{return ready;});
 
    // after the wait, we own the lock.
    std::cout << "Worker thread is processing data\n";
    data += " after processing";
 
    // Send data back to main()
    processed = true;
    std::cout << "Worker thread signals data processing completed\n";
 
    // Manual unlocking is done before notifying, to avoid waking up
    // the waiting thread only to block again (see notify_one for details)
    lk.unlock();
    cv.notify_one();
}
 
int main()
{
    std::thread worker(worker_thread);
 
    data = "Example data";
    // send data to the worker thread
    {
        std::lock_guard<std::mutex> lk(m);
        ready = true;
        std::cout << "main() signals data ready for processing\n";
    }
    cv.notify_one();
 
    // wait for the worker
    {
        std::unique_lock<std::mutex> lk(m);
        cv.wait(lk, []{return processed;});
    }
    std::cout << "Back in main(), data = " << data << '\n';
 
    worker.join();
}

Output:

main() signals data ready for processing
Worker thread is processing data
Worker thread signals data processing completed
Back in main(), data = Example data after processing