qImage not showing inside custom promoted widget

[A Former User]

Hi, I am trying to recreate the Qt Scribble example. I got it working by creating everything manually, now I am trying to use Qt Designer.

I need a custom class that subclasses QWidget so that I can reimplement the paint and mouse click events for scribbling.
I wrote the class and headers (paintarea.h, and paintarea.cpp). And then promoted a QWidget in QTDesigner into this class.

Code is:
paintarea.h

#ifndef PAINTAREA_H
#define PAINTAREA_H

#include <QWidget>
#include <QtWidgets>

class PaintArea : public QWidget
{
Q_OBJECT
public:
explicit PaintArea(QWidget *parent = 0);

private:
QImage image;

signals:

public slots:
};
#endif // PAINTAREA_H

paintarea.cpp

PaintArea::PaintArea(QWidget *parent) : QWidget(parent)
{
image = QImage (500, 500, QImage::Format_ARGB32_Premultiplied);
image.fill(qRgb(200,0,0));
}

Nothing displays inside the widget. I have tried for example a qSlider, and it shows up. But qImage won't work for some reason. I have tried isNull() to determine if the image is null, but it is not. It's just invisible.

[SGaist]

Hi and welcome to devnet,

QImage is not a QWidget, if you want to see what it contains, you need to either use a QLabel to show it or paint it in a widget.

[A Former User]

@SGaist But why does the QImage show if I code everything manually without using a promoted class in QTDesigner? Then the code above works.

I'd like to use QTDesigner because it makes the UI work a lot faster but it seems to add a lot of headache as well.

[SGaist]

The exact same code shows a red image ?

[A Former User]

@SGaist Yes, it's actually supposed to be a white background that you can paint on (like in the Qt Scribble example). And it works if I add all the elements manually, but in another project that uses QtDesigner with a QWidget promoted to PaintArea class, the QImage no longer shows up.