Qt painter drawPolyline in paintEvent

void MainWindow::paintEvent(QPaintEvent *w)
{
    QPainter p(this);
    p.setPen(QPen(Qt::red,10));
    QPointF pt(19,19);
    QPointF pts[10];
    int t=QTime::currentTime().second();
    qsrand(t);
    for(int i=0;i<10;i++)
    {

        pt.setX(qrand()%200);
        pt.setY(qrand()%300);
        pts[i]=pt;
    }
    p.drawPolyline(pts,10);
}

环境 Qt4.8.4 in windows vs2010