Qt - remove all widgets from layout?

Question

I had the same problem: I have a game app whose main window class inherits QMainWindow. Its constructor looks partly like this:

m_scene = new QGraphicsScene;
m_scene->setBackgroundBrush( Qt::black );
...
m_view = new QGraphicsView( m_scene );
...
setCentralWidget( m_view );

When I want to display a level of the game, I instantiate a QGridLayout, into which I add QLabels, and then set their pixmaps to certain pictures (pixmaps with transparent parts). The first level displays fine, but when switching to the second level, the pixmaps from the first level could still be seen behind the new ones (where the pixmap was transparent).

I tried several things to delete the old widgets. (a) I tried deleting the QGridLayout and instantiating a new one, but then learned that deleting a layout does not delete the widgets added to it. (b) I tried calling QLabel::clear() on the new pixmaps, but that of course had only an effect on the new ones, not the zombie ones. (c) I even tried deleting my m_view and m_scene, and reconstructing them every time I displayed a new level, but still no luck.

Then (d) I tried one of the solutions given above, namely

QLayoutItem *wItem;
while (wItem = widget->layout()->takeAt(0) != 0)
    delete wItem;

but that didn't work, either.

However, googling further, I found an answer that worked. What was missing from (d) was a call to delete item->widget(). The following now works for me:

// THIS IS THE SOLUTION!
// Delete all existing widgets, if any.
if ( m_view->layout() != NULL )
{
    QLayoutItem* item;
    while ( ( item = m_view->layout()->takeAt( 0 ) ) != NULL )
    {
        delete item->widget();
        delete item;
    }
    delete m_view->layout();
}

and then I instantiate a new QGridLayout as with the first level, add the new level's widgets to it, etc.

Qt is great in many ways, but I do think this problems shows that things could be a bit easier here.

Layout management page in Qt's help states:

The layout will automatically reparent the widgets (using QWidget::setParent()) so that they are children of the widget on which the layout is installed.

My conclusion: Widgets need to be destroyed manually or by destroying the parent WIDGET, not layout

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.

My conclusion: Same as above

To @Muelner for "contradiction" "The ownership of item is transferred to the layout, and it's the layout's responsibility to delete it." - this doesn't mean WIDGET, but ITEM that is reparented to the layout and will be deleted later by the layout. Widgets are still children of the widget the layout is installed on, and they need to be removed either manually or by deleting the whole parent widget.

If one really needs to remove all widgets and items from a layout, leaving it completely empty, he needs to make a recursive function like this:

// shallowly tested, seems to work, but apply the logic

void clearLayout(QLayout* layout, bool deleteWidgets = true)
{
    while (QLayoutItem* item = layout->takeAt(0))
    {
        if (deleteWidgets)
        {
            if (QWidget* widget = item->widget())
                widget->deleteLater();
        }
        if (QLayout* childLayout = item->layout())
            clearLayout(childLayout, deleteWidgets);
        delete item;
    }
}

This code deletes all its children. So everything inside the layout 'disappears'.

qDeleteAll(yourWidget->findChildren<QWidget *>(QString(), Qt::FindDirectChildrenOnly));

This deletes all direct widgets of the widget yourWidget. Using Qt::FindDirectChildrenOnly is essential as it prevents the deletion of widgets that are children of widgets that are also in the list and probably already deleted by the loop inside qDeleteAll.

Here is the description of qDeleteAll:

void qDeleteAll(ForwardIterator begin, ForwardIterator end)

Deletes all the items in the range [begin, end] using the C++ delete > operator. The item type must be a pointer type (for example, QWidget *).

Note that qDeleteAll needs to be called with a container from that widget (not the layout). And note that qDeleteAll does NOT delete yourWidget - just its children.

Untried: Why not create a new layout, swap it with the old layout and delete the old layout? This should delete all items that were owned by the layout and leave the others.

Edit: After studying the comments to my answer, the documentation and the Qt sources I found a better solution:

If you still have Qt3 support enabled, you can use QLayout::deleteAllItems() which is basically the same as hint in the documentation for QLayout::takeAt:

The following code fragment shows a safe way to remove all items from a layout:

QLayoutItem *child;
while ((child = layout->takeAt(0)) != 0) {
  ...
  delete child;
}

Edit: After further research it looks like both version above are equivalent: only sublayouts and widget without parents are deleted. Widgets with parent are treated in a special way. It looks like TeL's solution should work, you only should be careful not to delete any top-level widgets. Another way would be to use the widget hierarchy to delete widgets: Create a special widget without parent and create all your removeable widgets as child of this special widget. After cleaning the layout delete this special widget.

You also want to make sure that you remove spacers and things that are not QWidgets. If you are sure that the only things in your layout are QWidgets, the previous answer is fine. Otherwise you should do this:

QLayoutItem *wItem;
while (wItem = widget->layout()->takeAt(0) != 0)
      delete wItem;

It is important to know how to do this because if the layout you want to clear is part of a bigger layout, you don't want to destroy the layout. You want to ensure that your layout maintains it's place and relation to the rest of your window.

You should also be careful, you're are creating a load of objects each time you call this method, and they are not being cleaned up. Firstly, you probably should create the QWidget and QScrollArea somewhere else, and keep a member variable pointing to them for reference. Then your code could look something like this:

QLayout *_layout = WidgetMemberVariable->layout();

// If it is the first time and the layout has not been created
if (_layout == 0)
{
  _layout = new QVBoxLayout(this);
  WidgetMemberVariable->setLayout(_layout);
}

// Delete all the existing buttons in the layout
QLayoutItem *wItem;
while (wItem = widget->layout()->takeAt(0) != 0)
    delete wItem;

//  Add your new buttons here.
QPushButton button = new QPushButton(item);
_layout->addWidget(button);

QPushButton button = new QPushButton("button");
_layout->addWidget(button);

You do not write about going the other way, but you could also just use a QStackedWidget and add two views to this, one for each arrangement of buttons that you need. Flipping between the two of them is a non issue then and a lot less risk than juggling various instances of dynamically created buttons