Qt: How to force a hidden widget to calculate its layout?

Question

What I am trying to do is render a qwidget onto a different window (manually using a QPainter)

I have a QWidget (w) with a layout and a bunch of child controls. w i

Answer 1

Forcing a layout calculation on a widget without showing it on the screen:

widget->setAttribute(Qt::WA_DontShowOnScreen);
widget->show();

The show() call will force the layout calculation, and Qt::WA_DontShowOnScreen ensures that the widget is not explicitly shown.