A Object inheriting from QGraphicsLineItem

[wysota]

Ok, so when I create a new object, they will always have pos == 0,0;

Yes, by default origins of both items (the item and its parent) match.

and if I move the item around the pos will give me back the position relative to its starting 0,0 pos.

No, it will give you the position relative to the parent. If you move the parent, the child will be moved too so its scene position will change but the value returned by pos() will not.

But my objects have no parents at all, so they should give back the scene position...

They should return position relative to the scene, not the scene position

but, if so, I can transform into scene coordinates with mapToScene.

Yes, or you can use QGraphicsItem::scenePos() and QGraphicsItem::sceneBoundingRect().