Trying to use boundingRect in Qt - c++

Question

What is wrong with class Kwadrat? I have an error:
Invalid new-expression of abstract class type 'Kwadrat'
Kwadrat* kwadrat = new Kwadrat(20);
I want a moving square on the screen (when it hit 370 on X or 370 on Y coordinate it comes back to middle).
when Kwadrat is classic QGraphicsRectItem I have a bug with (0,0) coordinate.
#include <QApplication>
#include <QGraphicsView>
#include <QGraphicsScene>
#include <QPixmap>
#include "poruszanie.h"
#include <QRectF>
#include <QGraphicsRectItem>
class Kwadrat : public QGraphicsItem
{
Q_OBJECT
public:
Kwadrat(int size)
: QGraphicsItem(NULL) // we could parent, but this may confuse at first
{
m_boundingRect = QRectF(0, 0, size, size);
}
QRectF boundingRect() const
{
return m_boundingRect;
}
private:
QRectF m_boundingRect;
};
int main(int argc, char *argv[])
{
QApplication a(argc, argv);
QGraphicsScene*scena=new QGraphicsScene();
// Poruszanie*kwadrat=new Poruszanie();
// kwadrat->setRect(0,0,20,20);
// kwadrat->setBrush(QBrush(Qt::white));
// scena->addItem(kwadrat);
Kwadrat*kwadrat=new Kwadrat(20);
kwadrat->setBrush(QBrush(Qt::white));
scena->addItem(kwadrat);
kwadrat->setFlag(QGraphicsItem::ItemIsFocusable);
kwadrat->setFocus();
QGraphicsView *widok=new QGraphicsView(scena);
widok->setBackgroundBrush(QBrush(Qt::yellow));
widok->setMinimumSize(400,400);
widok->show();
return a.exec();
}

Answer 1

To write your own graphics item, you first create a subclass of
QGraphicsItem, and then start by implementing its two pure virtual
public functions: boundingRect(), which returns an estimate of the
area painted by the item, and paint(), which implements the actual
painting.
From http://doc.qt.io/qt-5/qgraphicsitem.html#details
So you need to implement the pure virtual function paint to do the painting of your QGraphicsItem to get rid of the error, your compiler should have shown that, besides telling you that your class is abstract (because of the missing function implementation).