std::promise and std::future

In this lesson, we will see how you have full control over the task with std::promise and std::future.

Promise and future are a mighty pair. A promise can put a value, an exception, or simply a notification into the shared data channel. One promise can serve many std::shared_future futures. With C++23, we may get extended futures that are compose-able.

std::promise

std::promise enables us to set a value, a notification, or an exception. In addition, the promise can provide its result in a delayed fashion.

Method Description
prom.swap(prom2) and Swaps the promises.
std::swap(prom, prom2)
prom.get_future() Returns the future.
prom.set_value(val) Sets the value.
prom.set_exception(ex) Sets the exception.
prom.set_value_at_thread_exit(val) Stores the value and makes it ready if the promise exits.
prom.set_exception_at_thread_exit(ex) Stores the exception and makes it ready if the promise exits.

If the value or the exception is set by the promise more then once, a std::future_error exception is thrown.

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