What is Qt for boost::promise<T>? - c++

Question

I see that Qt has future class that is direct analog for boost::future but what is qt for boost::promise?

Answer 1

Constructing my own QFuture as shown in the accepted answer did not work for me. At first it seemed like it was working, but in my testing I realized it was not blocking the caller. Whoops! So I dug into the code a little further and found that QFutureInterface is what you want to use as your 'promise'. Like boost::promise, QFutureInterface is what you interact with in your worker thread, and it is a factory for QFutures.
So here's what I've been doing in Qt 4.8 (not sure if this is applicable to later versions).
QFutureInterface<QVariant> promise;
promise.reportStarted();
...
promise.reportResult(someVariant);
promise.reportFinished();
Then in the client thread, assuming you have access to the QFutureInterface 'promise'
QVariant result = promise.future().result();
The future() call is a factory method for creating a QFuture bound to your QFutureInterface. You should be able to get the QFuture and call result() on it later if you wanted.

Answer 2

The boost::promises are means of setting values in futures. In Qt, you can't set futures, you can only return them. That's the only way of "setting" data in a future.
So, in order to set data on a future, you have to return it from a function that was invoked by QtConcurrent::run. To do this, you'd use any of Qt's mechanisms for communicating between threads -- events, mutex-protected variables, etc. You have to tell the thread that runs the code that would return a future that given future is to be returned. That's the only way of achieving what a promise would do.
Alas, if you want to go into the undocumented territory, then the following code does what boost::promise::setValue would:
QFuture<int> f;
int i = 1;
...
f.d.reportResult(&i);
// or
f.d.reportFinished(&i);
I haven't bothered checking if it works (yet).

Answer 3

There is no official Qt analog but there are a few community libraries implementing promises (or similar patterns):
Ben Lau's AsyncFuture (benlau/asyncfuture)
Based on the undocumented QFutureInterface mentioned in one of the other answers.
And it's not exactly a promise pattern but rather an observer pattern.
Benoit Walter's QtPromise (bwalter/qt-promise)
Partially based on Ben Lau's AsyncFuture.
Simon Brunel's QtPromise (simonbrunel/qtpromise)
My QtPromise (julrich/QtPromise)
Disclaimer: I'm the author.

Answer 4

Promises for Qt are now also available with QML/JavaScript btw: https://v-play.net/updates/release-2-18-1-javascript-promises-for-rest-services-tinder-swipe-material-cards-qml-qsortfilterproxymodel-qml-youtube-player
Here is some example code:
import VPlayApps 1.0
import QtQuick 2.0
App {
Component.onCompleted: {
var p1 = Promise.resolve(3);
var p2 = 1337;
var p3 = HttpRequest
.get("http://httpbin.org/get")
.then(function(resp) {
return resp.body;
});
var p4 = Promise.all([p1, p2, p3]);
p4.then(function(values) {
console.log(values[0]); // 3
console.log(values[1]); // 1337
console.log(values[2]); // resp.body
});
}
}

Answer 5

I created this library that is highly integrated with Qt and implements javascript-like promises:
https://github.com/juangburgos/QDeferred
It allows to create a thread-safe async API as follows:
multiplyNumbersInThread(3, 4)
.fail([](int res) {
Q_UNUSED(res);
qDebug() << "multiplyPositiveNumbers failed!";
})
.done([](int res) {
qDebug() << "multiplyPositiveNumbers succeded! Result :" << res;
});
Hope you find it useful.