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Casting Parent Object Not Working QT 5.2.1

  • 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);

    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]

  • Lifetime Qt Champion


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

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

    If so:


    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

  • 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?


  • 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?

  • 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);


  • Lifetime Qt Champion

    Do you plan to change the title several times ?

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