Promise and Future: Returning a Notification

If we use promises and futures to synchronize threads, they have a lot in common with condition variables. Most of the time, promises and futures are the better choices. Before we present an example, here is the big picture.

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Criteria	Condition Variables	Tasks
Multiple synchronizations	Yes	No
Critical section	Yes	No
Error handling in receiver	No	Yes
Spurious wakeup	Yes	No
Lost wakeup	Yes	No

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The advantage of using a condition variable over a promise and future is that we can use condition variables to synchronize threads multiple times. In contrast to that, a promise can send its notification only once, so we have to use more promise and future pairs to get the functionality of a condition variable. If we use the condition variable for only one synchronization, the condition variable is a lot more difficult to use in the right way. A promise and future pair needs no shared variable and, therefore, it doesn’t have a lock, and isn’t prone to spurious or lost wakeups. In addition to that, tasks can handle exceptions. There are lots of reasons to prefer tasks to condition variables.

Remember how difficult it was to use condition variables? If not, here are the key parts required to synchronize two threads.