32 Concurrency support library [thread]

Condition variables provide synchronization primitives used to block a thread until notified by some other thread that some condition is met or until a system time is reached.

Class condition_variable provides a condition variable that can only wait on an object of type unique_lock<mutex>, allowing the implementation to be more efficient.

Class condition_variable_any provides a general condition variable that can wait on objects of user-supplied lock types.

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

The executions of notify_one and notify_all are atomic.

The executions of wait, wait_for, and wait_until are performed in three atomic parts:

1.	the release of the mutex and entry into the waiting state;
2.	the unblocking of the wait; and
3.	the reacquisition of the lock.

The implementation behaves as if all executions of notify_one, notify_all, and each part of the wait, wait_for, and wait_until executions are executed in a single unspecified total order consistent with the “happens before” order.

Condition variable construction and destruction need not be synchronized.