How to exit a Qt console app from an inner class? [ SOLVED ]



  • Hi!
    This is my situation: I am running a TCP service from my main method. This service will be running forever unless a critical exception happen. Now, if an exception occurs I want to quit the whole application. Understanding that I can't use the Object::connect() method from the main() to call QCoreApplication::quit(), is there any accurate way to notify the app to finish the entire application in this context? I really appreciate any hint. Thanks.

      int main(int argc, char *argv[])
      {
          QCoreApplication app(argc, argv);
          InternetService service();
          return app.exec();
      }
    

  • Lifetime Qt Champion

    Hi,

    Why can't you use QObject::connect ? There are static variants of that method



  • As @SGaist said, you can use any of the static connect functions. Read this: http://doc.qt.io/qt-5/qobject.html#connect-4
    For example, something like QObject::connect(pointerToYourObj, SIGNAL(someFinishedSignal()), qApp, SLOT(quit())); should work perfectly.

    And BTW, what is this InternetService service();? Are you not declaring a function that returns an InternetService?



  • Following your suggestions finally I could make it work:

      int main(int argc, char *argv[])
      {
          QCoreApplication *app = new QCoreApplication(argc, argv);
          InternetService *service = new InternetService();
          QObject::connect(service, SIGNAL(finished()), app, SLOT(quit()));
          return app->exec();
      }
    

    What was my silly mistake? Both arguments in the QObject::connect() method must be POINTERS!

    Anyway, thank you for your help! :)


  • Lifetime Qt Champion

    But now you have to delete these two, in your case you can use:

    int main(int argc, char *argv[])
      {
          QCoreApplication app(argc, argv);
          InternetService service;
          QObject::connect(&service, SIGNAL(finished()), &app, SLOT(quit()));
          return app.exec();
      }
    


  • @SGaist Even better... thanks! :)



  • @SGaist I made a little modification in my code but now it is not working and I can't figure out why?
    This is my situation:

    int main(int argc, char *argv[])
      {
          QCoreApplication app(argc, argv);
          InternetService service;
          QObject::connect(&service, SIGNAL(finished()), &app, SLOT(quit()));
          service.verifyEnv();
          return app.exec();
      }
    

    Just for testing, inside the verifyEnv() (public) method of the InternetService class, I only added this line:

    void InternetService::verifyEnv()
    {
        emit finished();
    }
    

    Nevertheless, the QCoreApplication object never quits. Why?


  • Lifetime Qt Champion

    Got any message when starting the application in your terminal ?


  • Moderators

    @xtingray In your new code that isn't working you are sending the signal emit finished() before you are in your event loop. So AFAIK that signal isn't caught once the loop starts.

    The loop i.e. app.exec() needs to be running before you call service.verifyEnv() and it's resulting emit finished().

    Since you are trying to exit right away by calling verifyEnv there is no need for the app.exec in that case, just return 0 from your main and never enter the loop.



  • @xtingray

    in any case you you can call quit() from anywhere from your code

    just include
    #include <QCoreApplication>

    and call
    qApp->quit();



  • First at all, thank you for all your suggestions. My comments:

    1. I tried to replace return app.exec() with return 0. The problem is that the program quits as far as I run it, even if I remove the signal instruction. As it is a service, I need it running all the time. I mean, the finished() signal is required but only for specific exceptions.
    2. I tried using the method qApp->quit(); too and surprisingly the application keeps running :O

    I decided to write a "Hello World" example, in case some of you want to help me to resolve this mystery:
    http://maefloresta.com/portal/files/test.zip

    The challenge is quite simple: If I run a first instance of the example, the service must keep running indefinitely. If I run a (concurrent) second instance, it has to quit as soon as it discovers that the port is busy.

    Anyone interested in to try it? Thanks.



  • @xtingray Make InternetService inherit from QObject and verifyEnv a slot. Then invoke the method as follows:

    int main(int argc, char *argv[])
    {
    QCoreApplication app(argc, argv);
    InternetService service;
    QObject::connect(&service, SIGNAL(finished()), &app, SLOT(quit()));
    QMetaObject::invokeMethod(&service, "verifyEnv", Qt::QueuedConnection);
    return app.exec();
    }

    As an alternative, you could use the QtSingleApplication solution to restrict your application to a single instance only.


  • Moderators

    I use QtSingleApplication for that stuff. It's probably better than just checking a port as you may want to do something if the port is busy. Just because a port is busy doesn't mean your service is actually running and is the one using it.

    It could be another app. You could have permission issues and be unable to bind to that port. Or a host of other things.



  • Finally, the best/clean way I found to deal with my requirement was this:

    • Inside the main.cpp and before I start my service, I make all the verifications I need... if something goes wrong, then I call "return 0;" and that's it, a clean exit. If everything goes well, then I start my service. End of the story.

    Thank you for your advices guys! :)


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