QFuture: failed vs cancelled

Nikita Kobzev

Imagine the code working with QFuture:

class Example : public QObject {
public:
    Example(): QObject() {
        connect(&watcher_, &QFutureWatcher<int>::finished, this, &Example::jobFinished);
    }

    QFuture<int> runJobInAnotherThread() {
        QFuture<int> future = QtConcurrent::run([](QPromise<int>& promise) {
            while (!promise.isCanceled() /*&& job is not done*/) {
                //do job
                //!!!!MIGHT THROW QException HERE!!!!
            }
            promise.addResult(123);
        });

        watcher_.setFuture(future);
        return future;
    }

private:
    QFutureWatcher<int> watcher_;

    void jobFinished() {

        //"normal" and "exception" flow are processed here
        //but how to process the "cancel" flow? watcher_.isCanceled() can't be
        //used, as "exception" flow also makes isCanceled() return "true"

        try {
            int result = watcher_.result();
            //process the result
        } catch (QException& ex) {
            //process the exception
        }
    }
};

auto example = new Example;
QFuture<int> future = example.runJobInAnotherThread();
//future might be cancelled by the user here
//future.cancel()

Here we have a QtConcurrent::run and a QFutureWatcher, waiting for the result on its finished signal.

Inside the finished handler, I need to check for three different cases:

normal
background thread exception
future canceled by the user

I can not find a way to check "exception" vs "cancellation", as they both do mark the future as isCanceled()=true, and they both fail to retrieve any results from the future.

Can anyone suggest a way?

Nikita Kobzev

Seems I found a way. QFuture::isValid() can be used for what I need.

void jobFinished() {
        if (!watcher_.future().isValid()) {
           //future was cancelled
        }

        try {
            int result = watcher_.result();
            //process the result
        } catch (QException& ex) {
            //process the exception
        }
    }

SGaist

Hi,

Based on the setException doc, you should check the result. It shall contain the exception.

Nikita Kobzev

The problem is not to catch an exception. The problem is to distinguish exception from cancellation.

SGaist

Shouldn't you handle that using the QFuture::onFailed and QFuture::onCanceled methods ?

Nikita Kobzev

Well.. Lots of code already written via QFutureWatcher. So I was expecting to achieve the goal without rewriting much.

Nikita Kobzev

Seems I found a way. QFuture::isValid() can be used for what I need.

void jobFinished() {
        if (!watcher_.future().isValid()) {
           //future was cancelled
        }

        try {
            int result = watcher_.result();
            //process the result
        } catch (QException& ex) {
            //process the exception
        }
    }