Transparency propagation question

[gianpatt]

Hello all,
in my application i'd like to set opacity only to the top level window but not for childs widget.

Reading the qt manual at http://doc.trolltech.com/4.5/qwidget.html seems that just add setAutoFillBackground to central widget to avoid propagation. It's correct?

This is the example image from qt page:


I wrote a test but not work:

#include <QtGui>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
 
    QWidget widget;
    widget.setWindowOpacity(0.7);
    QGroupBox box("Hello Sample", &widget);
    box.setAutoFillBackground(true);
 
    widget.show();
 
    return a.exec();    
}

#include <QtGui>
int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QWidget widget;
    widget.setWindowOpacity(0.7);
    QGroupBox box("Hello Sample", &widget);
    box.setAutoFillBackground(true);
    
    widget.show();

    return a.exec();    
}

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where am I wrong?
Thank you