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Casting Parent Object Not Working QT 5.2.1

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  • C Offline
    C Offline
    creeveshft
    wrote on last edited by
    #1

    Hello, I am trying to cast the parent widget in the child widget.

    this is how I create the ControlBar object inside a member function of class MainTabs

    @
    ControlBar * cb = new ControlBar(this);
    ui->mainTabControl->addTab(cb,"t");
    @

    I then cast the parentWidget to type MainTabs

    @
    MainTabs* mt = qobject_cast<MainTabs*>(parentWidget());
    if (mt ==0)
    {
    qDebug() << "error with the cast";
    }
    @

    The cast fails.

    not sure why?

    [edit: added missing coding tags @ SGaist]

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    • SGaistS Offline
      SGaistS Offline
      SGaist
      Lifetime Qt Champion
      wrote on last edited by
      #2

      Hi,

      @ControlBar * cb = new ControlBar(this);@

      I suppose that MainTabs is the "this" here, right ?

      If so:

      @ui->mainTabControl->addTab(cb,"t");@

      now there's a change of parent thus your cast will fail.

      Anyway, it's always a Bad Idea (™) to start casting the parent widget inside a child widget. If the child widget needs to know its parent for some reason then there's generally a design flow somewhere. e.g. You need the value of a property of the parent to set something in your child widget then you should rather give that child a setter or a constructor with a parameter that the parent will call with the right value.

      Think of this: you want to change your parent widget for something else, right now you also have to alter the code of your child widget.

      Hope it helps

      Interested in AI ? www.idiap.ch
      Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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      • C Offline
        C Offline
        creeveshft
        wrote on last edited by
        #3

        What I am trying to do is change the text of the tabs from within the ControlBar class.

        I can not think of a way to get a reference to the QTabWidget from ControlBar. I guess I could pass the QTabWidget to the constructor.

        I need to know the index of the tab in order to change its text, so I planned to use QTabWidget.indexOf( ControlBar*); and then I have the index.

        is the preferred way to pass an instance of the parent to the child?

        thanks

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        • C Offline
          C Offline
          creeveshft
          wrote on last edited by
          #4

          I got it to work by passing ui->mainTabControl to the constructor of my ControlBar class.

          I guess passing an instance of what you need to the constructor is the preferred way?

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          • C Offline
            C Offline
            creeveshft
            wrote on last edited by
            #5

            this is what is in my MainTabs class
            @
            void MainTabs::AddNewTab()
            {

            ControlBar * cb = new ControlBar(this,ui->mainTabControl);
            

            }

            @

            This is what is in my ControlBar class

            @
            int currentIndex = _mainTabControl->indexOf(this);

            _mainTabControl->setTabText(currentIndex,title);
            @

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            • SGaistS Offline
              SGaistS Offline
              SGaist
              Lifetime Qt Champion
              wrote on last edited by
              #6

              Do you plan to change the title several times ?

              Interested in AI ? www.idiap.ch
              Please read the Qt Code of Conduct - https://forum.qt.io/topic/113070/qt-code-of-conduct

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