> Sent: Friday, February 22, 2019 at 4:29 PM > From: "Giuseppe D'Angelo" <giuseppe.dang...@kdab.com> > To: "Jason H" <jh...@gmx.com> > Cc: interest@qt-project.org > Subject: Re: [Interest] Taking back a widget from a QBoxLayout? > > Il 22/02/19 20:42, Jason H ha scritto: > > ''' > > When you use a layout, you do not need to pass a parent when constructing > > the child widgets. The layout will automatically reparent the widgets > > (using QWidget::setParent()) so that they are children of the widget on > > which the layout is installed. > > > > Note: Widgets in a layout are children of the widget on which the layout is > > installed, not of the layout itself. Widgets can only have other widgets as > > parent, not layouts. > > ''' > > > This is the bulk of the story, and ultimately, what's interesting for you. > > > > However: > > ''' > > void QLayout::addItem(QLayoutItem *item) > > Implemented in subclasses to add an item. How it is added is specific to > > each subclass. > > > > This function is not usually called in application code. To add a widget to > > a layout, use the addWidget() function; to add a child layout, use the > > addLayout() function provided by the relevant QLayout subclass. > > > > Note: The ownership of item is transferred to the layout, and it's the > > layout's responsibility to delete it. > > > > void QLayout::addWidget(QWidget *w) > > Adds widget w to this layout in a manner specific to the layout. This > > function uses addItem(). > > ''' > > > > So the docs are clear as mud. > > The docs are indeed unclear, but look at what you're quoting -- a > function to add QLayoutItems to a layout. A QWidget is NOT a QLayoutItem > (but it is possibly managed by one). QLayout::addItem will add a > QLayoutItem to a layout, passing ownership of the _item_ to the layout > (but not of the widget the layout item is managing). > > > In other words, if you have a toplevel widget with two children (A and > B), managed by a layout, what you're going to build looks like this from > an ownership point of view: > > > * QWidget (top) > | > +—— QWidget A > | > +—— QWidget B > | > \—— QBoxLayout > ¦ > +-- QWidgetItem (managing A) > ¦ > \-- QWidgetItem (managing B) >
I've been using Qt for 14 years and never noticed this. Wow! I always just thought it could manage the widget directly. > Solid lines represent ownership via QObject parent/child. Dashed lines > represent ownership via other means. > > > > QLayout mentions "ownership" but the tips mention "parent". FWIW, I knew > > QWidgets parented QWidgets, but I don't know what it means by "ownership". > > They refer to the same thing. > > A parent QObject owns (... has ownership of) its children QObjects, > meaning that deleting the parent will deleting the children as well. > > Some object Foo acquiring ownership of Bar means that deleting Foo will > also delete Bar. This can happen through QObject parent/child, or some > other mechanism. Take Aways: 1. The docs should be changed to use "parent" rather than ownership. Ownership is its own thing ( https://doc.qt.io/qt-5/qtqml-cppintegration-data.html#data-ownership ) 2. The docs should elaborate on the implicit creation of the QLayoutItems that manage widgets. 3. I learned something today. Thanks for that clarification Giuseppe! _______________________________________________ Interest mailing list Interest@qt-project.org https://lists.qt-project.org/listinfo/interest
