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Thread: Ownership in a QWidget

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    Default Ownership in a QWidget

    A quick question:

    QWidget::setLayout takes ownership of the layout and QLayout::addWidget takes ownership of the passed widgets. How does it know when to delete an object and when not? Or: why does this work:

    Qt Code:
    1. class test : public QWidget
    2. {
    3. public:
    4. test()
    5. {
    6. l->addWidget(&bar);
    7. setLayout(l);
    8. }
    9. };
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    Why doesn't a double free occur, once when the layout discards the scrollbar and once when the test dtor?

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    Default Re: Ownership in a QWidget

    The destructor of test is executed before the destructor of QWidget so bar gets deleted (and removed from the layout) before QWidget destructor has a chance to delete the layout.
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    Default Re: Ownership in a QWidget

    Ah I understand now. Thank you. Is this a common pattern or is it better to allocate one at runtime with "new QScrollBar()" ?

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    Default Re: Ownership in a QWidget

    Well, it "works": On destroying test you get orphaned QHBoxLayout with an invalid item bar (now deleted). The QHBoxLayout needs to specify a parent
    Qt Code:
    1. QHBoxLayout *l = new QHBoxLayout(this);
    To copy to clipboard, switch view to plain text mode 
    or l needs to be a data item of test and test needs a dtor which will delete l. Naturally, specifying the parent is a way simpler and cleaner.

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    Default Re: Ownership in a QWidget

    Quote Originally Posted by Radek View Post
    Well, it "works": On destroying test you get orphaned QHBoxLayout with an invalid item bar (now deleted). The QHBoxLayout needs to specify a parent
    Qt Code:
    1. QHBoxLayout *l = new QHBoxLayout(this);
    To copy to clipboard, switch view to plain text mode 
    or l needs to be a data item of test and test needs a dtor which will delete l.
    setLayout() does an implicit setParent() of the layout to the widget so the layout will be destroyed when the widget it manages is destroyed.
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    Default Re: Ownership in a QWidget

    Quote Originally Posted by tuli View Post
    Ah I understand now. Thank you. Is this a common pattern or is it better to allocate one at runtime with "new QScrollBar()" ?
    Child widgets are usually allocated on the heap.
    Keeps the headers clean from includes that are internals to the parent widget.

    Cheers,
    _

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