If you want your code to show properly formatted doubles no matter what locale, then you should do this:

Qt Code: 
  1. class DoubleDelegate: public QStyledItemDelegate
  2. {
  3.     Q_OBJECT
  4.     public:
  5.  
  6.     DoubleDelegate(QObject* parent=0) : QStyledItemDelegate(parent) { }
  7.     virtual ~DoubleDelegate() { }
  8.  
  9.     virtual QString displayText(const QVariant &value, const QLocale &locale) const
  10.     {
  11.         if (value.type() == QVariant::Double) {
  12.             return locale.toString( value.toDouble(), 'f', 2 );
  13.         }
  14.         return QStyledItemDelegate::displayText(value, locale);
  15.     }
  16. };
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Your original code will substitute a comma for a period in all cases, regardless of locale. Maybe that's what you want to do, but an American will certainly be confused seeing a table where the entries are all of the form "123,45".