Ownership in a QWidget

[tuli]

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:

class test : public QWidget
	{
	public:
		test()
		{
			QHBoxLayout* l = new QHBoxLayout();
			l->addWidget(&bar);
			setLayout(l);
		}
		QScrollBar bar;
	};

class test : public QWidget
	{
	public:
		test()
		{
			QHBoxLayout* l = new QHBoxLayout();
			l->addWidget(&bar);
			setLayout(l);
		}
		QScrollBar bar;
	};

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Why doesn't a double free occur, once when the layout discards the scrollbar and once when the test dtor?

[wysota]

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.

[tuli]

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

[Radek]

Well, it "works": On destroying test you get orphaned QHBoxLayout with an invalid item bar (now deleted). The QHBoxLayout needs to specify a parent

QHBoxLayout *l = new QHBoxLayout(this);

QHBoxLayout *l = new QHBoxLayout(this);

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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.

[wysota]

Well, it "works": On destroying test you get orphaned QHBoxLayout with an invalid item bar (now deleted). The QHBoxLayout needs to specify a parent

QHBoxLayout *l = new QHBoxLayout(this);

QHBoxLayout *l = new QHBoxLayout(this);

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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.

[anda_skoa]

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,
_