How process remaining events before shutdown

ocgltd

My simple(ish) app outputs information using qDebug statements in destructors. I notice that when my app shuts down via exit(0), those qDebug statements don't run - or at least I don't see the output.

Is there a statement I can place as the last line of my main.cpp to perform any remaining outputs, handle any remaining events in the event loop, etc. ?

JonB

@ocgltd
Basically once you are inside QCoreApplication::exec() that is when you should always use QCoreApplication::quit(). This does not actually exit the application, rather it exits the a.exec() you have. Code then executes anything after that in your call, which is usually nothing, as in your return a.exec(); being the final statement in your main() function.

Using QCoreApplication::quit() (or more rarely QCoreApplication::exit()) exits the Qt application event loop "cleanly", and allows proper clean up. Using the non-Qt system exit() directly exits the C++ application, as it would whether Qt or not, and it not recommended for Qt applications.

Christian Ehrlicher

@ocgltd Properly clean up and delete your objects before you leave main()

ocgltd

@Christian-Ehrlicher My class is created on the stack, so I don't think there is more I can do to clean up. I tried putting my class creation inside another nested block as per below, but I still don't see the qDebug messages upon exit.

int main(int argc, char *argv[])
{
    QCoreApplication a(argc, argv);
    {
        MyClass myclass;
        QTimer::singleShot(20000, []() {
            exit(0);
        } );
        return a.exec();
    }
}

Christian Ehrlicher

You call the systems exit() function so there is no cleanup done as described in the c++ documentation: https://en.cppreference.com/w/cpp/utility/program/exit

Properly close the Qt app with QCoreApplication::quit()

ocgltd

@Christian-Ehrlicher That worked as expected. I checked the quit docs and it's not clear how/when I should use this.

Is this a replacement for exit() only? Should this be the last line of every Qt program? What's the right way to use this method?

Christian Ehrlicher

@ocgltd said in How process remaining events before shutdown:

hat's the right way to use this method?

You should use it when you want to properly shut down your application.

JonB

@ocgltd
Basically once you are inside QCoreApplication::exec() that is when you should always use QCoreApplication::quit(). This does not actually exit the application, rather it exits the a.exec() you have. Code then executes anything after that in your call, which is usually nothing, as in your return a.exec(); being the final statement in your main() function.

Using QCoreApplication::quit() (or more rarely QCoreApplication::exit()) exits the Qt application event loop "cleanly", and allows proper clean up. Using the non-Qt system exit() directly exits the C++ application, as it would whether Qt or not, and it not recommended for Qt applications.

ocgltd

@JonB Is there any benefit to callint quit() in a simple app which doesn't use an event loop?

JonB

@ocgltd
No, it does nothing other than exiting the event loop, and if that is not running I don't know what it does/does nothing.