Condition Variables

Condition variables enable threads to be synchronized via messages. They need the <condition_variable> header, one thread to act as a sender, and the other as the receiver of the message; the receiver waits for the notification from the sender. Typical use cases for condition variables are sender-receiver or producer-consumer workflows.

A condition variable can be the sender but also the receiver of the message.

Method	Description
cv.notify_one()	Notifies a waiting thread.
cv.notify_all()	Notifies all waiting threads.
cv.wait(lock, ...)	Waits for the notification while holding a std::unique_lock.
cv.wait_for(lock, relTime, ...)	Waits for a time duration for the notification while holding a std::unique_lock.
cv.wait_until(lock, absTime, ...)	Waits until a time point for the notification while holding a std::unique_lock.

The subtle difference between cv.notify_one and cv.notify_all is that cv.notify_all will notify all waiting threads. In contrast, cv.notify_one will notify only one of the waiting threads while the other threads remain in the wait state. Before we cover the gory details of condition variables - which are the three dots in the wait operations - here is an example.