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parent() not returning the "parent" QWidget given to constructor, why?



  • I have a problem, the parent() function does not return the same object that I passed into the constructor. Thus, when I ((MainWindow*)parent())->function(); I get a segfault... not nice...

    so I checked why this happens with a bunch of qDebug()s... this is what I got...

    MainWindow Contructor, this                      == 0x7ffc53ce97f0
    FileList Constructor, parent()                   == 0x7ffc53ce97f0
    FileList Function, parent()                      == 0x557b8c8c9170
    parent()->parent()->parent()->parent()->parent() == 0x7ffc53ce97f0
    

    so... where do 4 parent objects come from, that I never created? And it seems they only spawn into existance AFTER my constructor ran... so it's not like it's the superclass constructor that causes this...

    I mean I guess I could just save the parent I give to the constructor as a member variable (or just call parent() five times...), but doesn't that kind of defeat the purpose of parent()?

    I'd like to know, WHY does this happen? And what can I do about it?


  • Moderators

    @DasCapschen
    this depends on your code.
    If you call QMainWindow::setCentralWidget() for example your widget is reparented. No matter what parent you passed at construction time



  • @DasCapschen said in parent() not returning the "parent" QWidget given to constructor, why?:

    WHY does this happen?

    Layouts and container widgets re-parent anything that is passed to them.

    And what can I do about it?

    Good OOP design works as a 1 way tree, the parent knows about the child but the child assumes and knows nothing about the parent so ((MainWindow*)parent())->function(); is very bad design. the child should emit a signal and the parent should connect to that signal to run the function() slot


  • Lifetime Qt Champion

    Hi
    As the others says, this should be done via signal and slots for a much better design.

    However, the cast is also suboptimal and dangerous. ( as your segfault tells you :)
    This will not crash if parent is not what you think.

    MainWindow* mywin = qobject_cast<MainWindow* > ( parent() ) ;
    if (mywin) mywin->function();
    

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