I'm fairly familiar with C++11's std::thread
, std::async
and std::future
components (e.g. see this answer), which are straight-forward.
However, I cannot quite grasp what std::promise
is, what it does and in which situations it is best used. The standard document itself doesn't contain a whole lot of information beyond its class synopsis, and neither does just::thread.
Could someone please give a brief, succinct example of a situation where an std::promise
is needed and where it is the most idiomatic solution?
In the words of [futures.state] a std::future
is an asynchronous return object ("an object that reads results from a shared state") and a std::promise
is an asynchronous provider ("an object that provides a result to a shared state") i.e. a promise is the thing that you set a result on, so that you can get it from the associated future.
The asynchronous provider is what initially creates the shared state that a future refers to. std::promise
is one type of asynchronous provider, std::packaged_task
is another, and the internal detail of std::async
is another. Each of those can create a shared state and give you a std::future
that shares that state, and can make the state ready.
std::async
is a higher-level convenience utility that gives you an asynchronous result object and internally takes care of creating the asynchronous provider and making the shared state ready when the task completes. You could emulate it with a std::packaged_task
(or std::bind
and a std::promise
) and a std::thread
but it's safer and easier to use std::async
.
std::promise
is a bit lower-level, for when you want to pass an asynchronous result to the future, but the code that makes the result ready cannot be wrapped up in a single function suitable for passing to std::async
. For example, you might have an array of several promise
s and associated future
s and have a single thread which does several calculations and sets a result on each promise. async
would only allow you to return a single result, to return several you would need to call async
several times, which might waste resources.
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