Qt stylesheet and moc subtleties

written on Monday, October 6, 2008

You may know I am a big fan of Qt sylesheets. While playing around with them I stumbled across a subtle behavior which had me puzzled for a while.

Often when I need to slightly override the behavior of a widget, I do not create separate .h/.cpp files for them. Instead, I write the code directly in the .cpp file where this widget will be used.

Here is an example, assuming SomeWidget is a widget used elsewhere in the application, and it contains an instance of a custom widget of class InternalWidget.

This is SomeWidget.h:

class SomeWidgetPrivate;
class SomeWidget : public QWidget {
public:
  SomeWidget(QWidget* parent = 0);
  /* Rest of SomeWidget declaration */
private:
  SomeWidgetPrivate* const d;
};

And this is SomeWidget.cpp:

class InternalWidget : public QWidget {
protected:
  virtual void mousePressEvent(QMouseEvent*) {
    /* Custom code here */
  }
};

struct SomeWidgetPrivate {
  InternalWidget* mInternalWidget;
};

SomeWidget::SomeWidget(QWidget* parent)
: QWidget(parent)
, d(new SomeWidgetPrivate) {
  d->mInternalWidget = new InternalWidget(this);
}

So far so good, I create InternalWidget inside SomeWidget and it works well. Now let's say I want to apply a stylesheet to SomeWidget by calling setStylesheet() on it. Qt allows me to define styles for classes, so if I want to paint the background of InnerWidget instances in blue, I can write a stylesheet like this:

InternalWidget {
  background-color: blue;
}

Alas, it doesn't work. After scratching my head for a while, I realized that Qt stylesheet system probably uses the metaobject information to know the exact class of a widget instance. By defining my internal widget the way I did, moc didn't have a chance to run on it, so mInternalWidget appeared like an instance of QWidget, not InternalWidget.

To get the stylesheet to apply, one must get moc to run on the widget classes, so it's important to:

1. Declare the widget in a header file
2. Add the Q_OBJECT macro (otherwise moc while happily ignore it)