QPOint pt_click;
QGraphicsRectItem *rubber_rect_item;
...

rubber_rect_item = new QGraphicsRectItem();
rubber_rect_item->setVisible(false);

QPen rubber_pen;
rubber_pen.setColor(QColor(Qt::red));
rubber_pen.setWidth(2);
rubber_pen.setCapStyle(Qt::RoundCap);
rubber_pen.setStyle(Qt::DotLine);

rubber_rect_item->setRect(0,0,1,1);
rubber_rect_item->setVisible(false);
rubber_rect_item->setPen(rubber_pen);

void CScene::mouseMoveEvent ( QGraphicsSceneMouseEvent * mouseEvent )
{

pt_click = mouseEvent->scenePos();
rubber_rect_item->setVisible(true);
}

void CScene::mouseMoveEvent ( QGraphicsSceneMouseEvent * mouseEvent )
{
QRectF rectF;
rectF.setBottomLeft(pt_click);
rectF.setTopRight(mouseEvent->scenePos());
QRectF norm_rect;
norm_rect = rectF.normalized();
rubber_rect_item->setRect(norm_rect);
}