Catching exceptions with Qt

[The_Fallen]

Hi,

in my Qt app I use exceptions for the error handling. Everything works fine, except for those exceptions that I forgot to catch directly. In those cases the app crashes and has a console output like this:

Invalid parameter passed to C runtime function.
Invalid parameter passed to C runtime function.

So I assume the problem is that the exception is going through some Qt code and something goes wrong there. Now I thought: Ok, then I'll just catch all exceptions at the top-most level, i.e. the main function, like this.

try {
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}
catch (...) {
    qDebug() << "CRASH!";
}

try {
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}
catch (...) {
    qDebug() << "CRASH!";
}

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Unfortunately this doesn't seem to make a difference, the app still crashes. What am I doing wrong?

Cheers,
fallen

[squidge]

If an inner exception handler catches the error, your top-level (outer) exception handler will not receive it unless the inner exception handler rethrows the exception.

But really, its more typical to be an assert or debug() message.

[The_Fallen]

Yes, but does Qt catch exceptions anywhere? I didn't think so, since Qt doesn't use exceptions for error handling... And if Qt does catch exceptions, it's quite freaky that it doesn't rethrow it and just crashes...

[GreenScape]

look at your code again:

try {
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}
    catch (...) {
    qDebug() << "CRASH!";
}

try {
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}
    catch (...) {
    qDebug() << "CRASH!";
}

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when execution leaves try block all local variables automatically destroyed, maybe that leads to another exception to be thrown. i think we all know what happens when exception is thrown while there is another unresolved exception... so, try this:

QApplication a(argc, argv);
    MainWindow w;
    w.show();
try {
    return a.exec();
}
    catch (...) {
    qDebug() << "CRASH!";
}

QApplication a(argc, argv);
    MainWindow w;
    w.show();
try {
    return a.exec();
}
    catch (...) {
    qDebug() << "CRASH!";
}

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[The_Fallen]

Thanks, but unfortunately that doesn't help...

[Zlatomir]

Disclaimer: I'm not good with exceptions but i think you will need (to exclude the main return from the try block) something like this:

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    //...
    int ret_val;
    try {
        ret_val = app.exec();
    }
    catch (...){
        qDebug() << "Exception";
    }
    return ret_val;
}

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);
    //...
    int ret_val;
    try {
        ret_val = app.exec();
    }
    catch (...){
        qDebug() << "Exception";
    }
    return ret_val;
}

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But remember that Qt doesn't throw exceptions, you will catch only yours.

[The_Fallen]

Thanks, but it's still crashing.
But I think I'm getting closer to the problem, since now I'm getting a segmentation fault in QApplicationPrivate::notify_helper():

// deliver the event
bool consumed = receiver->event(e);
e->spont = false;
return consumed;

// deliver the event
bool consumed = receiver->event(e);
e->spont = false;
return consumed;

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receiver is just not valid anymore here... But why is this code still executed, when an execution has been thrown?