What is the purpose of this updateImage variable?

In general it should all be more or less like this:

Qt Code: 
  1. void X::mousePressEvent(QMouseEvent *me) {
  2.     m_pt = me->pos();
  3. }
  4.  
  5. void X::mouseReleaseEvent(QMouseEvent *me) {
  6.   addLineToPixmap(m_pt, me->pos());
  7.   m_pt = QPoint();
  8. }
  9.  
  10. void X::mouseMoveEvent(QMouseEvent *me) {
  11.   addLineToPixmap(m_pt, me->pos());
  12.   m_pt = me->pos();
  13. }
  14.  
  15. void X::addLineToPixmap(QPoint from, QPoint to) {
  16.   QPainter p(&m_pixmap);
  17.   p.drawLine(from, to);
  18.   update();
  19. }
  20.  
  21. void X::paintEvent(...) {
  22.   QPainter p(this);
  23.   p.drawPixmap(m_pixmap);
  24. }
  25.  
  26. private:
  27.   QPoint m_pt;
  28.   QPixmap m_pixmap;
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