Error in typecasting of QGraphicsItemGroup in Qt

tushu

I am trying to create a digital gates symbols ( like AND,NAND etc ) using line, arc, circle. To do this I am using QGraphicsItemGroup also.
For example. If a symbol contains 3 lines, 2 arc and 1 circle, then I add everything in object of QGraphicsItemGroup and then I add this object in scene.

myGroupItem.h
class myGroupItem: public QGraphicsItemGroup 
{
      virtual void paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
}

myGroupItem.cpp

void myGroupItem ::paint(QPainter *painter, const QStyleOptionGraphicsItem *option, QWidget *widget);
{
    auto copied_option = *option;
    copied_option.state &= ~QStyle::State_Selected;
    auto selected = option->state & QStyle::State_Selected;
    QGraphicsItemGroup::paint(painter, &copied_option, widget);
    if (selected) 
   {
      foreach (QGraphicsItem* item, _scene->selectedItems()) 
         {
            myGroupItem* inst = qgraphicsitem_cast<myGroupItem*>(item);
            if (!inst)
                continue;
            QList<QGraphicsItem*> childItems = inst->childItems();
            foreach (QGraphicsItem* chItem, childItems) 
            {
                if (myPoly* poly = dynamic_cast<myPoly*>(chItem)) 
                {
                    painter->save();
                    painter->setPen(QPen(option->palette.windowText(), 0, Qt::SolidLine));
                    painter->drawPath(poly->shape());
                    painter->restore();
                } 
              } else {
                    if (myLine* line = dynamic_cast<myLine*>(chItem)) {
                        painter->save();
                        painter->setPen(QPen(option->palette.windowText(), 0, Qt::SolidLine));
                        painter->drawPath(line->shape());
                        painter->restore();
                    } else {
                        if (myEllipse* ell = dynamic_cast<myEllipse*>(chItem)) {
                            painter->save();
                            painter->setPen(QPen(option->palette.windowText(), 0, Qt::SolidLine));
                            painter->drawPath(ell->shape());
                            painter->restore();
                        }
                    }
                }
            }
        }
    }
}

myLine.h
class myLine : public QGraphicsLineItem 
{
  
}

myEllipse.h
class myEllipse : public QGraphicsEllipseItem
{
  
}

class BuildDesign()
{
    myGroupItem* groupItem = new myGroupItem();
    // accessing all lines
    myLine* line = new myLine(co-ordinate of lines);
    groupItem ->addToGroup(line);      
 
    // accessing arc
    groupItem ->addToGroup(arc); 
 
    // accessing circle 
    groupItem ->addToGroup(ellipse);                         
     
    scene->addItem(groupItem);    
}

This way I am creating a symbol and it is creating propely. Issue is when I want to select that symbol and process on it.

// This function will check is there any item under mouse click. If yes then call another
    // function to highlight it and if not then Refresh the scene. 
   
 void BuildDesign::SelectObject(QEvent* event)
    {
        QMouseEvent* mouseEvent = static_cast<QMouseEvent*>(event);
        QPointF mousePoint = _view->mapToScene(mouseEvent->pos());
        QGraphicsItem* itemUnderClick = _scene->itemAt(mousePoint, QTransform());
    
        if (!itemUnderClick)
            RefreshScene();
        else
            HighlightSelectedObject(itemUnderClick);
    }
     
 void BuildDesign::HighlightSelectedObject(QGraphicsItem* itemUnderClick)
 {  
    // QGraphicsItemGroup* gItem = qgraphicsitem_cast<QGraphicsItemGroup*>(itemUnderClick);
    myGroupItem* gItem = qgraphicsitem_cast<myGroupItem*>(itemUnderClick);
    if (gItem) {
       gItem->setSelected(true);   // this never gets executed 
    }
 }

Now after debugging I found that, itemUnderClick has some address but after type casting to gItem always shows address as 0x0.
I want to execute

gItem->setSelected(true);

( which never execute ) and further I want to use

foreach (QGraphicsItem* currentItem, _scene->selectedItems())

and currentItem should be that selected symbol. But it is not happening.

Christian Ehrlicher

I don't see that you re-implement QGraphicsItem::type() so I doubt qgraphicsitem_cast<> will work.

tushu

@Christian-Ehrlicher Can you tell me when to use qgraphicsitem_cast<> and dynamic_cast ? ( difference between them )
Because after your comment, I tried dynamic_cast and it worked.

Axel Spoerl

qgraphicsitem_cast is a guarded static_cast, i.e. it checks if QGraphicsItem::type() returns the Type you want to cast to. If it does, it returns a static_cast - nullptr otherwise. This cast is safe and cheap. You have to re-implement the virtual Type() function to make it work.

dynamic_cast will ferry you across the same river by means of runtime RTTI traversal. Also safe but more expensive.