How to access the contents of the data property of my custom QGraphicsItem after a cast

I have a custom class which inherits from QGraphicsRectItem (basically QGraphicsItem). At the beginning I had attributes as well as the corresponding getters and setters. Everything is fine until I get the items from the scene and try to call the setters (or getters): The properties no longer exist. So I made a cast on the elements retrieved from the scene and as expected .. the program crashes (because the elements no longer existed ... well I think). To solve the problem I used the data property offered by the base class (QGraphicsItem) I thought that since it is an already implemented property I should not have any problems even after a cast. Yet the problem persists. Why and how to fix it? There are others solutions in my mind but i want to understand why Here is a simplified code that represent my problem

MyItem.h

#ifndef MYITEM_H
#define MYITEM_H

#include <QGraphicsRectItem>
#include <QGraphicsTextItem>
#include <QObject>
#include <QDebug>

class MyItem :public QObject, public QGraphicsRectItem
{
    Q_OBJECT
public:
    enum {Type = UserType + 200};
    MyItem();
    QString getTestProperty() const;
    void setTestProperty(QString p_newValue);
private:
    QGraphicsTextItem* test_txt;
};

#endif // MYITEM_H

MyItem.cpp

#include "myitem.h"

MyItem::MyItem(): QGraphicsRectItem(0,0,100,100)
{
    setFlag(QGraphicsItem::ItemIsMovable);
    test_txt = new QGraphicsTextItem(this);
    setData(0,QVariant("hi there"));
}

void MyItem::setTestProperty(QString p_newValue) {
    setData(0,p_newValue);
    qDebug()<<data(0).toString()<<endl;
    test_txt->setPlainText(data(0).toString());
}
QString MyItem::getTestProperty() const {
    return  data(0).toString();
}

MainWindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QGraphicsScene>
#include <QGraphicsView>
#include <QtDebug>

#include <myitem.h>

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();
private:
    QGraphicsScene* m_scene;
    QGraphicsView* m_view;
};
#endif // MAINWINDOW_H

MainWindow.cpp

#include "mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
    setMinimumSize(800,600);
    m_scene = new QGraphicsScene;
    m_view = new QGraphicsView;
    m_view->setScene(m_scene);

    MyItem* item1 = new MyItem();
    m_scene->addItem(item1);
    setCentralWidget(m_view);
    item1->setTestProperty("hello there :)");

    MyItem* supposedPointerToItem1 = new MyItem;
    supposedPointerToItem1 = qgraphicsitem_cast<MyItem*> (m_scene->items().first());
    supposedPointerToItem1->setTestProperty("Another test that may fail");//Failed

}

MainWindow::~MainWindow()
{
}

And finally here is the screenshot of the results i get

The program ended suddenly

Solution

What can fail will fail.

So it is better to check. QGraphicsScene::items() returns all the items so it will also return the QGraphicsTextItem and that is what happens in your case: the first item is not MyItem but the QGraphicsTextItem.

On the other hand you are creating a second MyItem unnecessarily, also I do not see the need to use Q_OBJECT or inherit from QObject.

#ifndef MYITEM_H
#define MYITEM_H

#include <QGraphicsRectItem>

class QGraphicsTextItem;

class MyItem :public QGraphicsRectItem
{
public:
    enum {Type = QGraphicsItem::UserType + 200};
    MyItem(QGraphicsItem *parent = nullptr);
    QString getTestProperty() const;
    void setTestProperty(const QString &p_newValue);
    int type() const override;
private:
    QGraphicsTextItem* test_txt;
};

#endif // MYITEM_H

#include "myitem.h"

#include <QDebug>

MyItem::MyItem(QGraphicsItem *parent): QGraphicsRectItem(0,0,100,100, parent)
{
    setFlag(QGraphicsTextItem::ItemIsMovable);
    test_txt = new QGraphicsTextItem(this);
    setData(0,QVariant("hi there"));
}

void MyItem::setTestProperty(const QString & p_newValue) {
    setData(0,p_newValue);
    qDebug()<<data(0).toString();;
    test_txt->setPlainText(data(0).toString());
}

QString MyItem::getTestProperty() const {
    return  data(0).toString();
}

int MyItem::type() const
{
    return Type;
}

#include "mainwindow.h"
#include "myitem.h"

#include <QGraphicsScene>
#include <QGraphicsView>

MainWindow::MainWindow(QWidget *parent) : QMainWindow(parent)
{
    setMinimumSize(800,600);
    m_scene = new QGraphicsScene;
    m_view = new QGraphicsView;
    m_view->setScene(m_scene);

    MyItem* item1 = new MyItem();
    m_scene->addItem(item1);
    setCentralWidget(m_view);
    item1->setTestProperty("hello there :)");

    QList<QGraphicsItem*> items = m_scene->items();
    for(QGraphicsItem *item : qAsConst(items)){
        if(MyItem* myitem = qgraphicsitem_cast<MyItem *>(item)){
            myitem->setTestProperty("Another test that may fail");
        }
    }
}