Install an event filter to detect QEvent::Close, e.g.

Qt Code: 
  1. #include <QtGui>
  2.  
  3. class EventSignaller : public QObject
  4. {
  5.  Q_OBJECT
  6.  
  7.  signals:
  8.   void closing();
  9.  
  10.  protected:
  11.   bool eventFilter(QObject *object, QEvent *event)
  12.   {
  13.     if(event->type() == QEvent::Close)
  14.       emit(closing());
  15.  
  16.     return QObject::eventFilter(object, event);
  17.   }
  18. };
  19.  
  20. int main(int argc, char *argv[])
  21. {
  22.   QApplication app(argc, argv);
  23.  
  24.   QWidget w1;
  25.   w1.setWindowTitle("w1");
  26.   w1.show();
  27.  
  28.   QWidget w2;
  29.   w2.setWindowTitle("w2");
  30.   w2.show();
  31.  
  32.   EventSignaller s;
  33.   w1.installEventFilter(&s);
  34.   w2.connect(&s, SIGNAL(closing()), SLOT(close()));
  35.  
  36.   return app.exec();
  37. }
  38.  
  39. #include "main.moc"
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