Getting Parent Layout in Qt

Question

quick question. Is there any way to (easily) retrieve the parent layout of a widget in Qt?

PS: QObject::parent() won't work, for logical reasons.

EDIT: I'm positive the widget has a parent layout, because I added it to a layout earlier in the code. Now, I have many other layouts in the window and while it is possible for me to keep track of them, I just want to know if there is an easy and clean way to get the parent layout.

EDIT2: Sorry, "easy and clean" was probably not the best way of putting. I meant using the Qt API.

EDIT3: I'm adding the widget to the layout like this:

QHBoxLayout* layout = new QHBoxLayout;

layout->addWidget(button);

Answer 1

(Updated answer)

I guess it is not easily possible then. Since a Widget can be technically contained in multiple layouts (a horizontal layout which is aligned inside a vertical layout, for instance).

Just remember that a QWidget's parent does not change if it is aligned in a layout.

You possibly have to keep track of that yourself, then.

Answer 2

SOLVED! Usage: QLayout* parentLayout = findParentLayout(addedWidget)

QLayout* findParentLayout(QWidget* w, QLayout* topLevelLayout)
{
  for (QObject* qo: topLevelLayout->children())
  {
     QLayout* layout = qobject_cast<QLayout*>(qo);
     if (layout != nullptr)
     {
        if (layout->indexOf(w) > -1)
          return layout;
        else if (!layout->children().isEmpty())
        {
          layout = findParentLayout(w, layout);
          if (layout != nullptr)
            return layout;
        }
     }
  }
  return nullptr;
}

QLayout* findParentLayout(QWidget* w)
{
    if (w->parentWidget() != nullptr)
        if (w->parentWidget()->layout() != nullptr)
            return findParentLayout(w, w->parentWidget()->layout());
    return nullptr;
}

Answer 3

Simply use:

QHBoxLayout* parentLayout = button->parentWidget()->layout();

I assume button is a child of the widget which contains the layout which contains button. button->parentWidget() returns a pointer to the widget of the button's parent and ->layout() returns the pointer to the layout of the parent.

Answer 4

use widget.parent().layout() and search brute force (recursion included) is my only advice. Maybe you can search be "name".

Answer 5

After some exploration, I found a "partial" solution to the problem.

If you are creating the layout and managing a widget with it, it is possible to retrieve this layout later in the code by using Qt's dynamic properties. Now, to use QWidget::setProperty(), the object you are going to store needs to be a registered meta type. A pointer to QHBoxLayout is not a registered meta type, but there are two workarounds. The simplest workaround is to register the object by adding this anywhere in your code:

Q_DECLARE_METATYPE(QHBoxLayout*)

The second workaround is to wrap the object:

struct Layout {
    QHBoxLayout* layout;
};
Q_DECLARE_METATYPE(Layout)

Once the object is a registered meta type, you can save it this way:

QHBoxLayout* layout = new QHBoxLayout;
QWidget* widget = new QWidget;
widget->setProperty("managingLayout", QVariant::fromValue(layout));
layout->addWidget(widget);

Or this way if you used the second workaround:

QHBoxLayout* layout = new QHBoxLayout;
QWidget* widget = new QWidget;
Layout l;
l.layout = layout;
widget->setProperty("managingLayout", QVariant::fromValue(l));
layout->addWidget(widget);

Later when you need to retrieve the layout, you can retrieve it this way:

QHBoxLayout* layout = widget->property("managingLayout").value<QHBoxLayout*>();

Or like this:

Layout l = widget->property("managingLayout").value<Layout>();
QHBoxLayout* layout = l.layout;

This approach is applicable only when you created the layout. If you did not create the layout and set it, then there is not a simple way of retrieving it later. Also you will have to keep track of the layout and update the managingLayout property when necessary.

Answer 6

Have you tried this? Don't forget to check for NULL.

QLayout *parent_layout = qobject_cast< QLayout* >( parent() );

If parent_layout equals NULL, then the parent widget is not a layout.

Answer 7

Have you tried QWidget::layout() ?