Exceptions and Qt

[Perdrix]

Qt rethrew the exception AFAICT as it was reported by the catch code at the end of main() before termination
D.

[Christian Ehrlicher]

@Perdrix said in Exceptions and Qt:

Qt rethrew the exception AFAICT

No, not even in Qt4:

https://github.com/qt/qt/blob/0a2f2382541424726168804be2c90b91381608c6/src/corelib/kernel/qeventloop.cpp#L198

[SamiV123]

Well, the situation is kinda clear. Officially Qt does not allow you to propagate exceptions from a slot or event handler or anything that Qt calls.

I expect that the code has been written as if there are no exceptions that means that stack unwinding that happens due to exception can a) leak resources b) cause unexpected state and behavior. Might work (or appear to work) once in a while just to cause silent corruption and issues later on.

[SamiV123]

@Perdrix said in Exceptions and Qt:

There are places where exceptions are thrown and caught locally when they are recoverable

Sounds like an incorrect use of exceptions tbh. You should not use exceptions for logical conditions that are part of the expected program execution and code flow.

[Perdrix]

Its not there as part of expected flow its there to catch a totally invalid condition.

And the exception if caught by Qt MUST have been rethrown - how else could it end up being caught by the catch clause at the end of main.

In fact the code to which you pointed me DOES VERY CLEARLY rethrow the exception:

catch (...) {
        qWarning("Qt has caught an exception thrown from an event handler. Throwing\n"
                 "exceptions from an event handler is not supported in Qt. You must\n"
                 "reimplement QApplication::notify() and catch all exceptions there.\n");

        // copied from below
        locker.relock();
        QEventLoop *eventLoop = d->threadData->eventLoops.pop();
        Q_ASSERT_X(eventLoop == this, "QEventLoop::exec()", "internal error");
        Q_UNUSED(eventLoop); // --release warning
        d->inExec = false;
        --d->threadData->loopLevel;

        throw;
    }

THIS IS INTENDED TO TERMINATE THE APPLICATION WITH PREJUDICE

David

[SimonSchroeder]

@Perdrix said in Exceptions and Qt:

Qt has recently started complaining about these

I don't think this is new. As far as I know this has always been there.

If you want to terminate the application, you should call quit() on the QApplication. This will exit the application somewhat orderly. No need to rethrow exceptions. However, I would just call exit(0) and let the operating system handle clean up. I am not sure what advantage you gain by exiting your app by throwing exceptions compared to other methods of exiting the app.

[SamiV123]

@Perdrix said in Exceptions and Qt:

Its not there as part of expected flow its there to catch a totally invalid condition.

If we're talking about a BUG then that should be an assert (controlled abort) then. But you also said "recoverable" so that makes very little sense.

[aha_1980]

@Perdrix There has been a thread about Exceptions on the mailing list half a year ago: https://lists.qt-project.org/pipermail/development/2024-May/045252.html

You might get more insights about the problematics of Exceptions especially on different architectures from there.

Regards

[Perdrix]

Sigh! This is an assertion. Some exceptions such as are thrown by std::filesystem are caught locally and never escape to Qt - those are the recoverable ones.

[Perdrix]

@aha_1980 Oh my that's one scary thread on the mailing list. That suggests we need to replace all assertions with local code that detects the error and then terminates. At least we have the technology setup to implement that..

Now I at least understand something about the problem. Did anyone ever test -funwind?

David