I have probably (surely?) made a mistake somewhere but...impossible to find it out! Could you help please?

Qt Code: 
  1. class CircleParameter : public QWidget
  2. {
  3.   Q_OBJECT
  4.  
  5. public:
  6.   CircleParameter(QWidget *parent=0);
  7.   virtual ~CircleParameter() {};
  8.  
  9. public slots:
  10.   void setRadius(int radius);
  11.  
  12. protected:
  13.   void paintEvent(QPaintEvent *e);
  14.  
  15. private:
  16.   int radius;
  17. };
  18.  
  19. CircleParameter::CircleParameter(QWidget *parent)
  20.   : QWidget(parent)
  21. {
  22.   printf("creating widget\n");
  23.   this->radius = DEFAULT_RADIUS;
  24.   this->resize(MAX_RADIUS*2, MAX_RADIUS*2);
  25.   if (updatesEnabled()) printf("yes\n"); else printf("no\n");
  26.   if (isVisible()) printf("yes\n"); else printf("no\n");
  27. }
  28.  
  29. void
  30. CircleParameter::setRadius(int radius)
  31. {
  32.   printf("setRadius()\n");
  33.   if (updatesEnabled()) printf("yes\n"); else printf("no\n");
  34.   if (isVisible()) printf("yes\n"); else printf("no\n");
  35.   this->radius = radius>0 ? radius<=MAX_RADIUS ? radius : MAX_RADIUS : 1;
  36.   this->update();
  37. }
  38.  
  39. void
  40. CircleParameter::paintEvent(QPaintEvent *e)
  41. {
  42.   printf("paintEvent()\n");
  43.   QPainter painter(this);
  44.   QPen pen;
  45.   pen.setColor(Qt::red);
  46.   painter.setPen(pen);
  47.   painter.drawEllipse(this->size().width()/2-this->radius, this->size().height()/2-this->radius, radius*2, radius*2);
  48. }
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Outputs:
- at creation time:
Qt Code: 
  1. creating widget
  2.     yes
  3.     no
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- when moving a slider connected to CircleParameter::setRadius():
Qt Code: 
  1. setRadius()
  2.     yes
  3.     yes
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Issue: why CircleParameter:: paintEvent() never executed?