Custom QGraphicsRectItem

main.cpp

#include <QtWidgets>

class RectItem : public QGraphicsRectItem
{
public:
    RectItem()
    {
        setRect(0, 0, 40, 40);
    }
};

int main(int argc, char *argv[])
{
    QApplication app(argc, argv);

    QGraphicsView view;

    QGraphicsScene *scene = new QGraphicsScene(&view);
    RectItem *rectItem = new RectItem;
    scene->addItem(rectItem);

    view.setScene(scene);
    view.resize(400, 400);
    view.show();

    return app.exec();
}