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?