QT pomoc z klasą abstrakcyjną

 C:\Test_oryginal\test\test.cpp:30: błąd: cannot allocate an object of abstract type 'WidgetWithBackground'
         bar = new WidgetWithBackground(widget);
                                              ^

 class WidgetWithBackground : public QWidget
{
public:
     WidgetWithBackground(QWidget * parent = 0);
         virtual ~WidgetWithBackground ();


protected:

   virtual void  paintBackground(QPainter & painter) = 0;

 class WidgetWithBackground;

    class TestWidget : public QMainWindow,  private Ui::TestForm
    {
       Q_OBJECT
    public:
        TestWidget(QMainWindow *parent = 0);

        void initialize();
        void connections();
    public slots:
       void ComboBoxChoiceChanged(int index );
        void SpinBoxValueChanged(double val);

    private slots:
    void on_tab_destroyed();

    void on_stackedWidget_destroyed();

    private:
        void closeEvent ( QCloseEvent * e );
    QTabWidget *tabWidget;

    WidgetWithBackground *bar;

    };
 

 QWidget * widget = stackedWidget->widget(0);
        widget = stackedWidget->widget(0);
        bar = new WidgetWithBackground(widget);//dokladnie w tym miejscu
        bar->setValueFont(font());
        bar->resize(120,120);
        QLayout * layout = new QVBoxLayout();
        layout->addWidget(bar);
        widget->setLayout(layout);

   virtual void  paintBackground(QPainter & painter) = 0;

 class WidgetWithBackground : public QWidget
{
public:
    WidgetWithBackground(QWidget * parent = 0);
    void  paintBackground(QPainter & painter);
    ~WidgetWithBackground ();
}