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Passing another variable to a SLOT in QObject::connect

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  • D Offline
    D Offline
    dangelog
    wrote on last edited by
    #2

    Both statements are invalid.

    In short:

    • connect() wants only the SIGNATURE of the signal/slot (without the return type), not the argument names, nor anything else;
    • a slot cannot have more arguments than the ones carried by the signal you're connecting to it (where should they come from?);
    • passing values inside connect() is an error, although it's pretty clear what you'd like to do;
    • simply use a "proxy" slot that calls the target slot with the arguments, or read the docs about QSignalMapper.

    Software Engineer
    KDAB (UK) Ltd., a KDAB Group company

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    • M Offline
      M Offline
      manasij7479
      wrote on last edited by
      #3

      bq. simply use a “proxy” slot that calls the target slot with the arguments....

      How do I do that?

      "Error, no keyboard — press F1 to continue."

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      • F Offline
        F Offline
        Franzk
        wrote on last edited by
        #4

        @
        //...
        connect(button, SIGNAL(pressed)), SLOT(handleButtonPress()));
        //...

        void MyClass::handleButtonPress()
        {
        someLabel->setText("My String");
        }@

        "Horse sense is the thing a horse has which keeps it from betting on people." -- W.C. Fields

        http://www.catb.org/~esr/faqs/smart-questions.html

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        • D Offline
          D Offline
          dangelog
          wrote on last edited by
          #5

          In your (very simple) case, if it's applicable, you can just provide a default argument for the slot. Otherwise, simply create another slot that calls the setText one with the string you want.

          Software Engineer
          KDAB (UK) Ltd., a KDAB Group company

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          • F Offline
            F Offline
            Franzk
            wrote on last edited by
            #6

            [quote author="peppe" date="1297240228"]In your (very simple) case, if it's applicable, you can just provide a default argument for the slot. Otherwise, simply create another slot that calls the setText one with the string you want.[/quote]Indeed. With a slot called setText(), providing a default argument can be misleading:

            @myLabel->setText();@

            What does that do?

            Oh well. It's a style issue.

            "Horse sense is the thing a horse has which keeps it from betting on people." -- W.C. Fields

            http://www.catb.org/~esr/faqs/smart-questions.html

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            • M Offline
              M Offline
              manasij7479
              wrote on last edited by
              #7

              bq. Otherwise, simply create another slot that calls the setText one with the string you want.

              Any way without creating whole new classes?

              "Error, no keyboard — press F1 to continue."

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              • T Offline
                T Offline
                tobias.hunger
                wrote on last edited by
                #8

                "QSignalMapper":http://doc.qt.nokia.com/4.7/qsignalmapper.html might also be an option.

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                • L Offline
                  L Offline
                  luca
                  wrote on last edited by
                  #9

                  to solve simple problem like yours I use this solution:
                  @
                  connect(button1, SIGNAL(clicked()), SLOT(buttonClicked());
                  connect(button2, SIGNAL(clicked()), SLOT(buttonClicked());
                  button1->setProperty("name", "button1");
                  button2->setProperty("name", "button2");
                  ...
                  ...
                  MyClass::buttonClicked()
                  {
                  QPushButton *button = sender();
                  if(button->property("name").toString() == "button1")
                  {
                  .....
                  .....
                  }
                  else if(button->property("name").toString() == "button2")
                  {
                  ....
                  ....
                  }
                  }
                  @

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                  • A Offline
                    A Offline
                    andre
                    wrote on last edited by
                    #10

                    Luca, that is unsave code.

                    You don't know what kind of object sender() refers to, or if it is non-0 at all (direct call to buttonClicked()). So, you should at least check that before assuming this. Using a QSignalMapper is the safe way to go, and it still allows other ways to trigger the slot and still work correctly.

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                    • F Offline
                      F Offline
                      Franzk
                      wrote on last edited by
                      #11

                      Safe or not safe, it's arguably not even simpler than the QSignalMapper approach. Likewise, if a slot is going to make (significant) behavioral changes based on the sender(), then it is probably time to review the design anyway.

                      "Horse sense is the thing a horse has which keeps it from betting on people." -- W.C. Fields

                      http://www.catb.org/~esr/faqs/smart-questions.html

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                      • L Offline
                        L Offline
                        luca
                        wrote on last edited by
                        #12

                        [quote author="Andre" date="1297288071"]Luca, that is unsave code.

                        You don't know what kind of object sender() refers to, or if it is non-0 at all (direct call to buttonClicked()). So, you should at least check that before assuming this. Using a QSignalMapper is the safe way to go, and it still allows other ways to trigger the slot and still work correctly.
                        [/quote]
                        Yes, I usually check if pointer is a push button with a dynamic_cast:
                        @
                        QPushButton button = dynamic_cast<QPushButton>(sender());
                        if(button==NULL)
                        {
                        return;
                        }
                        @

                        Mine is only a fast example...

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                        • F Offline
                          F Offline
                          Franzk
                          wrote on last edited by
                          #13

                          qobject_cast might be more thorough.

                          "Horse sense is the thing a horse has which keeps it from betting on people." -- W.C. Fields

                          http://www.catb.org/~esr/faqs/smart-questions.html

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                          • M Offline
                            M Offline
                            MarekR22
                            wrote on last edited by
                            #14

                            Hi there is a tool which do exactly what you need.
                            See "QSignalMapper":http://doc.trolltech.com/latest/qsignalmapper.html
                            There is good example how to use it.

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                            • G Offline
                              G Offline
                              goetz
                              wrote on last edited by
                              #15

                              If you're in the same class (or have access to the possible sender objects pointers) you simply can compare pointers:

                              @
                              void MyClass::buttonClicked()
                              {
                              if(sender() == button1) {
                              // ....
                              } else if(sender() == button2) {
                              // ....
                              }
                              }
                              @

                              http://www.catb.org/~esr/faqs/smart-questions.html

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