MdiWindow::MdiWindow( QWidget *parent ) : QMainWindow( parent ) {
  setWindowTitle( tr( "MDI" ) );
  QWorkspace* workspace = new QWorkspace;
  setCentralWidget( workspace );
  connect( workspace, SIGNAL(windowActivated(QWidget *)), this, SLOT(enableActions()));
  QSignalMapper* mapper = new QSignalMapper( this );

  //my problem is in this line
  connect( mapper, SIGNAL(mapped(QWidget*)), workspace, SLOT(setActiveWindow(QWidget*)) );

  createActions();
  createMenus();
  createToolbars();
  statusBar()->showMessage( tr("Done") );
  enableActions();
}

class ButtonWidget : public QWidget {
  Q_OBJECT

public:
  ButtonWidget(QStringList texts, QWidget *parent = 0);

signals:
  void clicked(const QString &text);

private:
  QSignalMapper *signalMapper;
};

ButtonWidget::ButtonWidget(QStringList texts, QWidget *parent)
  : QWidget(parent)
{
  signalMapper = new QSignalMapper(this);

  QGridLayout *gridLayout = new QGridLayout;
  for (int i = 0; i < texts.size(); ++i) {
    QPushButton *button = new QPushButton(texts[i]);
    connect(button, SIGNAL(clicked()), signalMapper, SLOT(map()));
    signalMapper->setMapping(button, texts[i]);
    gridLayout->addWidget(button, i / 3, i % 3);
  }

  connect(signalMapper, SIGNAL(mapped(const QString &)),
          this, SIGNAL(clicked(const QString &)));

  setLayout(gridLayout);
}

connect(signalMapper, SIGNAL(mapped(const QString &)),
        this, SIGNAL(clicked(const QString &)));

TestClass::TestClass() {
  widget = new ButtonWidget(QStringList() << "Foo" << "Bar");
  connect(widget, SIGNAL(clicked(const QString &)),
          this, SLOT(onButtonClicked(const QString &)));
}

void TestClass::onButtonClicked(const QString &btnText) {
  if (btnText == "Foo") {
    // Do stuff.
  }
  else {
    // Or something else.
  }
}

mySignalMapper:
    [ obj1 -> 42       ]
    [ obj2 -> "obiwan" ]
    [ obj3 -> myWidget ]

connect(obj1,mySignal,mySignalMapper,SLOT(map())); // idem for obj2 and obj3

(obj2 emits "mySignal")
-> (mySignalMapper::map slot is called)
-> (sender() == obj2, associated data = "obiwan")
-> (mySignalMapper emits mapped("obiwan"))