Impossible to get the scene position of an Item in a QGraphicsGridLayout - c++

Question

i've got a problem for my school projet... I just can't get the scene position of any object in a QGraphicsGridLayout, it always give me (0,0). Maybe i subclass badly QGraphicsWidget ?
This is a example of how i use it :
main:
int main(int argc, char *argv[])
{
QApplication app(argc, argv);
QGraphicsScene *scene = new QGraphicsScene();
myCustomWidget *a = new myCustomWidget("a");
myCustomWidget *b = new myCustomWidget("b");
myCustomWidget *c = new myCustomWidget("c");
QGraphicsGridLayout *layout = new QGraphicsGridLayout();
layout->addItem(a,0,0,1,2, Qt::AlignCenter);
layout->addItem(b,1,0,1,1);
layout->addItem(c,1,1,1,1);
QGraphicsWidget *w = new QGraphicsWidget();
w->setLayout(layout);
scene->addItem(w);
QWidget* Window = new QWidget();
Window->setMinimumSize(640,380);
QVBoxLayout* layoutWindow = new QVBoxLayout();
Window->setLayout(layoutWindow);
QGraphicsView *view = new QGraphicsView(Window);
view->setScene(scene);
view->show();
layoutWindow->addWidget(view);
Window->show();
////////////////////////////////
// Always return QPointF(0,0) //
////////////////////////////////
qDebug() << c->scenePos();
return app.exec();
}
myCustomWidget:
class myCustomWidget : public QGraphicsWidget
{
private:
QString caption;
public:
myCustomWidget(QString caption, QGraphicsWidget* parent = 0) : QGraphicsWidget(parent)
{
this->caption = caption;
}
QSizeF sizeHint(Qt::SizeHint which, const QSizeF &constraint) const
{
Q_UNUSED(which);
Q_UNUSED(constraint);
return QSizeF(100,100);
}
QRectF boundingRect() const
{
return QRectF(QPointF(0,0), QSizeF(100,100));
}
void paint(QPainter *painter, const QStyleOptionGraphicsItem *, QWidget * = 0) Q_DECL_OVERRIDE
{
QFont font;
font.setPixelSize(0.75 * qMin(boundingRect().width(), boundingRect().height()));
painter->fillRect(boundingRect(), Qt::black);
painter->setFont(font);
painter->setPen(QPen(Qt::white));
painter->drawText(boundingRect(), Qt::AlignCenter, caption);
}
};