QT: Using QPainter on child widgets

Question

I'm having a QT/C++ problem with a simple QWidget program that draws an ellipse inside a child QWidget.

The program is composed of:
(1) A parent QWidget
(2) A child QWidget (used as the drawing surface for an ellipse)
(3) A draw QPushButton

Here is part of the code (QPushButton Slot and Signal code omitted for simplicity)

void Draw::paintEvent(QPaintEvent *event) {
    QPainter painter;
    painter.begin(child_Widget);    //The line with the problem
    painter.setRenderHint(QPainter::Antialiasing, true);
    painter.setPen(QPen(Qt::black, 12, Qt::DashDotLine, Qt::RoundCap));
    painter.setBrush(QBrush(Qt::green, Qt::SolidPattern));
    painter.drawEllipse(50, 50, 100, 100);
    painter.end();}

Line 2 painter.begin(child_Widget); doesn't do anything. The program draws the ellipse only if I replace line 2 with painter.begin(this); but that draws on the parent QWidget and not on the child QWidget as desired.

p.s. I have the child_Widget housed inside a GroupBox which in turn is housed inside a QVBoxLayout Any Suggestion?

Thanks.

Answer 1

As stated in the QPainter documentation

Warning: When the paintdevice is a widget, QPainter can only be used inside a paintEvent() function or in a function called by paintEvent(); that is unless the Qt::WA_PaintOutsidePaintEvent widget attribute is set. On Mac OS X and Windows, you can only paint in a paintEvent() function regardless of this attribute's setting.

If you want to draw on that widget you will need to do so from it's own paintEvent().