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<condition_variable>

Defines the classes condition_variable and condition_variable_any that are used to create objects that wait for a condition to become true.

This header uses Concurrency Runtime (ConcRT) so that you can use it together with other ConcRT mechanisms. For more information about ConcRT, see Concurrency Runtime.

#include <condition_variable>
Note Note

In code that is compiled by using /clr or /clr:pure, this header is blocked.

Code that waits for a condition variable must also use a mutex. A calling thread must lock the mutex before it calls the functions that wait for the condition variable. The mutex is then locked when the called function returns. The mutex is not locked while the thread waits for the condition to become true. So that there are no unpredictable results, each thread that waits for a condition variable must use the same mutex object.

Objects of type condition_variable_any can be used with a mutex of any type. The type of the mutex that is used does not have to provide the try_lock method. Objects of type condition_variable can only be used with a mutex of type unique_lock<mutex>. Objects of this type may be faster than objects of type condition_variable_any<unique_lock<mutex>>.

To wait for an event, first lock the mutex, and then call one of the wait methods on the condition variable. The wait call blocks until another thread signals the condition variable.

Spurious wakeups occur when threads that are waiting for condition variables become unblocked without appropriate notifications. To recognize such spurious wakeups, code that waits for a condition to become true should explicitly check that condition when the code returns from a wait function. This is usually done by using a loop; you can use wait(unique_lock<mutex>& lock, Predicate pred) to perform this loop for you.

while (condition is false)
    wait for condition variable;

The condition_variable_any and condition_variable classes each have three methods that wait for a condition.

  • wait waits for an unbounded time period.

  • wait_until waits until a specified time.

  • wait_for waits for a specified time interval.

Each of these methods has two overloaded versions. One just waits and can wake up spuriously. The other takes an additional template argument that defines a predicate. The method does not return until the predicate is true.

Each class also has two methods that are used to notify a condition variable that its condition is true.

  • notify_one wakes up one of the threads that is waiting for the condition variable.

  • notify_all wakes up all of the threads that are waiting for the condition variable.

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