QPromise/QFuture and QMetaObject::invokeMethod

Dariusz

Hey

I'm trying to do this >

    QFuture<int> icServerHTTP::startServer(const quint16 port) noexcept {
        QPromise<int> promise;
        if (thread() != QThread::currentThread()) {
            QFuture<int> future = promise.future();
            QMetaObject::invokeMethod(this, [this, port, promise = std::move(promise)]() mutable {
                try {
                    int result = _startServer(port);
                    promise.addResult(result);
                    promise.finish();
                } catch (...) {
                    promise.setException(std::current_exception());
                }
            }, Qt::QueuedConnection);
            return future;
        }
        promise.addResult(_startServer(port));
        promise.finish();
        return promise.future();
    }

I tried wrapping it in smart pointer, but that cause >

Due to memory issues and smart pointer cleaning the obj instead of the future/etc doing it... Ahh tricky...

Any idea how to make it work?
Thanks!

SGaist

Hi,

What exactly are you trying to put in a smart pointer ?
Which one is it ?
How are you trying to do it ?

Dariusz

I tried to put promise in smart pointer.
In general I cant pass promise to invokeMethod. The concept is that if I run in correct thread, I want it to just run normally, but if not, I need it to execute in correct thread, but I still want to be able to wait for it to finish/get correct result out of it. Atm this crashes.
I keep hitting those issues more and more, asking it to execute in another thread/etc/etc... :/

You can run the example above and just instead of putting _startServer(port) just set int to 20 or something like that. But u need to simulate InvokeMethod, I guess you can just run invoke method regardless/ignore thread check to reproduce the problem... Like this >

        QPromise<int> promise;
        QFuture<int> future = promise.future();
        QMetaObject::invokeMethod(this, [this, port, promise]() mutable {
            try {
                // Call startServer on the correct thread
                int result = 10;
                // Set the result to the promise
                promise.addResult(result);
                promise.finish();
            } catch (...) {
                // Handle any exceptions and set as failure in promise
                promise.setException(std::current_exception());
            }
        }, Qt::QueuedConnection);

        return future;

Dariusz

Ehh ok I got it working.

Turns out I can't call future.result(), before it actually is finished... and I need to fo isFinished/waitForFinished before checking it sigh.

SGaist

Isn't QFuture::unwrap what you are looking for ?

Dariusz

@SGaist No, in this case the QFuture was crashing when I called for result before it was finished. I expected it to have safeguard internally to check if its done/finished instead of trying to access invalid data/crash.

SGaist

In that case, shouldn't you be using the then method to call your custom function ?