How to set the QComboBox's default index when it working under the model/view ?

here is my codes:

Code:

#include <QtGui>
 
int main(int argc, char **argv)
{
    QApplication app(argc,argv);
    QDirModel* model = new QDirModel();
    QTreeView* view = new QTreeView();
    view->setModel(model);
    QComboBox* combo =  new QComboBox();
    combo->setView(view);
    combo->setModel(model);
    combo->show();
    QString root = "D:/";/*QDir::rootPath();*/
    combo->setRootModelIndex((static_cast<QDirModel*>(combo->model()))->index(root)); 
    app.exec();
}

the ComboBox always show the root as "C:",how can I set it to the "D:"?

I'm using Qt 4.4.0.If you use the old version such as 4.3.1,there will be a few other bugs.

thanks a lot.