Promote custom widget with custom constructor in QT Creator

Question

I know, this is basically the same question, but my problem goes further.

The following tree explains my structure:

         QWidget
            |
       CustomWidget
        |        |
    MyTable  MyWidgetAroundIt

I have promoted MyTable in Qt Designer. So, I can add it to MyWidgetAroundIt. That worked quite well. The only problem is, CustomWidget requires it's parent to be a CustomWidget too, its constructor looks like:

CustomWidget(CustomWidget* parent) : QWidget(parent), _specialValue(parent->getSpecialValue)

This causes compile errors, as the designer generated code tries to initialize MyTable with a QWidget*, instead of the CustomWidget*. What could/should I do to prevent this and/or give the designer a hint about this requirement?

Answer 1

A widget whose parent can't be a QWidget is not a widget anymore. Your design breaks the Liskov Substitution Principle and has to be fixed.

You're free to enable special functionality if the widget happens to be of a certain type, but a widget must be usable with any widget for a parent.

Thus:

CustomWidget(QWidget* parent = nullptr) :
  QWidget(parent)
{
  auto customParent = qobject_cast<CustomWidget*>(parent);
  if (customParent)
    _specialValue = customParent->specialValue();
}

or:

class CustomWidget : public QWidget {
  Q_OBJECT
  CustomWidget *_customParent = qobject_cast<CustomWidget*>(parent());
  SpecialType _specialValue = _customParent ? _customParent->specialValue() : SpecialType();

  SpecialType specialValue() const { return _specialValue; }
public:
  CustomWidget(QWidget * parent = nullptr) : QWidget(parent) {}
};