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# std::condition_variable

From cppreference.com
< cpp‎ | thread

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.

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`.

### [edit] Member types

Member type | Definition
`native_handle_type` | implementation-defined

### [edit] Member functions

(constructor) | constructs the object (public member function) [edit]
(destructor) | destructs the object (public member function) [edit]
operator= [deleted] | not copy-assignable (public member function) [edit]
##### Notification
notify_one | notifies one waiting thread (public member function) [edit]
notify_all | notifies all waiting threads (public member function) [edit]
##### Waiting
wait | blocks the current thread until the condition variable is woken up (public member function) [edit]
wait_for | blocks the current thread until the condition variable is woken up or after the specified timeout duration (public member function) [edit]
wait_until | blocks the current thread until the condition variable is woken up or until specified time point has been reached (public member function) [edit]
##### Native handle
native_handle | returns the native handle (public member function) [edit]

### [edit] 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

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