Copying classes derived from QGraphicsItem using memcpy does not creat new object

Question

I have several different classes derived from QGraphicsItem or its children (like QGraphicsRectItem). I am at the point when I need to copy selected objects of those classes while not knowing exactly which one I copy.

Since QGraphicsScene::selectedItems() return a list of selected items I decided to use it, however I cannot copy the QGraphicsItem since its an abstract class. To address this I am trying to copy the object using malloc and memcpy.

1. MainWindow.cpp

   MainWindow::MainWindow(QWidget *parent) :
   QMainWindow(parent),
   ui(new Ui::MainWindow)
   {
       ui->setupUi(this);
       scene = new QGraphicsScene();
       item = new QGraphicsRectItem(50,50,50,50);
       item->setFlag(QGraphicsItem::ItemIsSelectable);
       scene->addItem(item);
       item->setSelected(true);
       ui->graphicsView->setScene(scene);
   }
   
   MainWindow::~MainWindow()
   {
       delete ui;
   }
   
   void MainWindow::on_pushButton_clicked()
   {
       for(QGraphicsItem *item : scene->selectedItems())
       {
           QGraphicsItem *copiedItem=(QGraphicsItem *)malloc(sizeof(*item));  
           memcpy(copiedItem, item, sizeof(*copiedItem));   
           copiedItem->moveBy(50, 50);
           scene->addItem(copiedItem);
   
           qDebug() << item;
           qDebug() << copiedItem;
       }
   }
   

2. MainWindow.h

   namespace Ui {
   class MainWindow;
   }
   
   class MainWindow : public QMainWindow
   {
       Q_OBJECT
   
       public:
       explicit MainWindow(QWidget *parent = 0);
       ~MainWindow();
   
       QGraphicsScene *scene;
       QGraphicsRectItem *item;
   
       private slots:
           void on_pushButton_clicked();
   
       private:
           Ui::MainWindow *ui;
   };
   

GUI consisting of QGraphicsView and QPushButton is sufficient for this example.

This code seem to work, item and copiedItem have different addresses but the same properties as qDebug() returns.

The scene however return the following error:

QGraphicsScene::addItem: item has already been added to this scene

I don't quite understand why the scene thinks the items are the same while they have different addresses. Is there any way to solve this issue?

EDIT: If possible I would like to do this without modifying the code of classes derived from QGraphicsItem, since this is a group work and I would not like to bug other functionalities.

Answer 1

If you look at the class definition inside qgraphicsitem.h, you'll see the following:

private:
    Q_DISABLE_COPY(QGraphicsItem)
    Q_DECLARE_PRIVATE(QGraphicsItem)

What this means is that, by design, you are not supposed to copy a QGraphicsItem, each object is supposed to be unique.

EDIT: I would imagine that the reason that you are denied the copying ability is because QGraphicsItem follows the Composite Design Pattern. Creating a copy of a single object that has child items would result in child items having more than one parent. To get round that, you'd have to not only copy the item you're interested in but every child in the child hierarchy as well. For very large hierarchies, this can become a very time-consuming operation.

If you really feel that you need to make copies, you can create a cloning factory function/class that creates a clone of the QGraphicsItem and all it's children by going through all the object's properties and transferring them to a newly created QGraphicsItem.

If this is not feasible, perhaps think about accomplishing your goal in a different way.

Answer 2

You really want to be using the copy constructor or the assignment operator, not malloc and memcpy. This is how you copy objects in C++:

QGraphicsItem copiedItem = *item; 

Answer 3

The problem of copying a class by block copying its memory with malloc is that if the class contains pointers to objects or arrays, then only a shallow copy will occur.

In your case of getting pointers to a QGraphicsItem, you'll need to identify the type of item you're copying (its actual child class, not base class) and use the copy constructor. QGraphicsItem includes a function called type(), which returns an int that indicates which item it is. You can also add to this in your own derived classes by implementing the type() function. For example, from the Qt docs: -

class CustomItem : public QGraphicsItem
{
   ...
   enum { Type = UserType + 1 };

   int type() const
   {
       // Enable the use of qgraphicsitem_cast with this item.
       return Type;
   }
   ...
};

Alternatively, if all the classes are your own type, you could use your own system. Once you know the type, you can then copy the item with its copy constructor: -

// Example, assuming type denotes a QGraphicsItemRect    
QGraphicsItemRect rect = (*originalRect);

Note that if you have inherited from QGraphicsItem and added members that are pointers, you'll need to add your own copy constructor to ensure a deep copy occurs, instead of a shallow copy.