看了一个实例中是这样写的:(example:diagramscene)
QPolygonF myPolygon;
myPolygon << QPointF(-100, -100) << QPointF(100, -100)<< QPointF(100, 100) << QPointF(-100, 100)<< QPointF(-100, -100);
setPolygon(myPolygon);
setFlag(QGraphicsItem::ItemIsMovable, true);
setFlag(QGraphicsItem::ItemIsSelectable, true);
QPolygonF 是什么?