Styjun

I have my own QGraphicsPixmapItem which consists of a QGraphicsPixmapItem and some additional attributes (like std::string name)

class MyQGraphicsPixmapItem : public QGraphicsPixmapItem 
private:
 std::string name;
 ...
public:
 MyQGraphicsPixmapItem();
 explicit MyQGraphicsPixmapItem(const QPixmap &pixmap, std::string name, QGraphicsItem *parent = nullptr);
 std::string getname() const;
...
;

And the constructor is like this :

MyQGraphicsPixmapItem::MyQGraphicsPixmapItem(const QPixmap &pixmap, std::string name, QGraphicsItem *parent) :
QGraphicsPixmapItem(pixmap, parent), name(name)
...

Here is the problem : I have a bunch of MyQGraphicsPixmapItem that I add in a QGraphicsScene. But when I use the method QGraphicsScene::itemAt(const QPointF &position, const QTransform &deviceTransform) const, it returns QGraphicsItem* (not MyQGraphicsPixmapItem*). So I suppose that I have to use a down-casting right ? But even after using the down casting like this :

 MyQGraphicsPixmapItem* item = static_cast<MyQGraphicsPixmapItem*>(QGraphicsScene::itemAt(...));
 std::cout << item->getName() << std::endl;

it returns an empty string (like there was no this.name = name; as shown in the constructor).

In conclusion, I create a bunch of MyQGraphicsPixmapItem in a QGraphicsScene with the right name initialization (I tested it with std::cout during the creation of QGraphicsScene) but when I want to randomly select a QGraphicsItem and check what is his name, I use that QGraphicsScene::itemAt that sets back every time the std::string name to empty despite the down-casting.
Also, I'm extremely sure that I'm pointing at the right MyQGraphicsPixmapItem with the right arguments (I've done some tests).
I was also thinking about implementing the right "itemAt" in my class "MyScene" (which inherits, you guessed it, from "QGraphicsScene") but I would use type_casting again.

PS : Tell me if my question is well asked.

Sincerely yours

You should be able to use dynamic_cast but Qt provides also its own cast qgraphicsitem_cast which returns item casted to the given type if item is of that type or otherwise 0.

Note from the doc:

To make this function work correctly with custom items, reimplement
the type() function for each custom QGraphicsItem subclass.

Example class:

class MyQGraphicsPixmapItem : public QGraphicsPixmapItem

public:
 ...
 enum Type = UserType + 1 ;
 int type() const override return Type; 
 ...
;

Example test:

MyQGraphicsPixmapItem myItem;
qDebug() << &myItem;
QGraphicsItem *item = &myItem;
MyQGraphicsPixmapItem *castedItem = qgraphicsitem_cast<MyQGraphicsPixmapItem*>(item);
if (castedItem) 
 qDebug() << castedItem;
 else 
 qWarning() << "casting failed";

Update:

QGraphicsScene::itemAt returns the topmost visible item at the specified position, or 0 if there are no items at this position.

If you verified with qgraphicsitem_cast that you successfully casted i.e. it returned pointer and not 0 then you really received your custom item and not some other graphics item. Then it should have name defined and not an empty string if you defined a name for all of your custom items.

To debug this issue further you could use QGraphicsScene::items to list all items on the scene considered visible by QGraphicsScene::itemAt. Do casting in the loop and print out a name of all of your custom items.

One thing that comes to my mind is that this is related to coordinates you are using when calling itemAt. You are e.g. doing your casting when you click with a mouse and then query the scene in the item coordinates and not with the scene coordinates. QGraphicsSceneMouseEvent::scenePos returns the mouse cursor position in scene coordinates. Maybe you are not getting the custom item you think?