Head file

protected:
    void paintEvent(QPaintEvent *);    //drawing operation will be done here

CPP file

void Widget::paintEvent(QPaintEvent *QPaintEvent)
{
    QPainter painter(this);    //create a QPainter object, this means painter devices is component of widget

    // using drawLine to draw a line start from (0,0) to end of (100,100)
    painter.drawLine(QPoint(0,0), QPoint(100,100));

    // drawing a rectangle, outline is red, fulfilled with green
    // setting up art of pen and brush.
    QPen pen;
    pen.setColor(QColor(255,0,0));    // red pen

    QBrush brush(QColor(0,255,0,125));    // green of brush and 50% of transparent
    painter.setPen(pen);    // add the pen
    painter.setBrush(brush);    // add the brush
    painter.drawRect(50,50,100,100);    // draw a rectangle
}

References

Qt5.9中QPainter类用法 https://blog.csdn.net/naibozhuan3744/article/details/79038084