NAME¶

std::condition_variable - std::condition_variable

Synopsis¶

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

std::condition_variable is a synchronization primitive used with a std::mutex to
block one or more threads until another thread both modifies a shared variable (the
condition) and notifies the std::condition_variable.

The thread that intends to modify the shared variable must:

1. Acquire a std::mutex (typically via std::lock_guard).
2. Modify the shared variable while the lock is owned.
3. Call notify_one or notify_all on the std::condition_variable (can be done after
releasing the lock).

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

Any thread that intends to wait on a std::condition_variable must:

1. Acquire a std::unique_lock<std::mutex> on the mutex used to protect the shared
variable.
2. Do one of the following:

1. Check the condition, in case it was already updated and notified.
2. Call wait, wait_for, or wait_until on the std::condition_variable (atomically
releases the mutex and suspends thread execution until the condition variable is
notified, a timeout expires, or a spurious wakeup occurs, then atomically
acquires the mutex before returning).
3. Check the condition and resume waiting if not satisfied.

or:
1. Use the predicated overload of wait, wait_for, and wait_until, which performs
the same three steps.

std::condition_variable works only with std::unique_lock<std::mutex>, which 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, or MoveAssignable.

Member functions¶

constructor constructs the object
(public member function)
destructor destructs the object
(public member function)
operator= not copy-assignable
[deleted] (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 awakened
(public member function)
blocks the current thread until the condition variable is awakened or
wait_for after the specified timeout duration
(public member function)
blocks the current thread until the condition variable is awakened or
wait_until until specified time point has been reached
(public member function)

Native handle¶

native_handle returns the native handle
(public member function)

Example¶

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

// Run this code

#include <condition_variable>
#include <iostream>
#include <mutex>
#include <string>
#include <thread>

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 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 lk(m);
ready = true;
std::cout << "main() signals data ready for processing\n";
}
cv.notify_one();

// wait for the worker
{
std::unique_lock 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

See also¶

condition_variable_any provides a condition variable associated with any lock type
(C++11) (class)
mutex provides basic mutual exclusion facility
(C++11) (class)
lock_guard implements a strictly scope-based mutex ownership wrapper
(C++11) (class template)
unique_lock implements movable mutex ownership wrapper
(C++11) (class template)