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);
    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]


  • Lifetime Qt Champion

    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



  • 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



  • 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);

    _mainTabControl->setTabText(currentIndex,title);
    @


  • Lifetime Qt Champion

    Do you plan to change the title several times ?


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