signal_enter_notify_event() does nothing

[HeavyJoost <heavyjoost gmail com>]

I want a function to be called when the mouse goes over a button, I use 
the following line:

toolUndo->signal_enter_notify_event().connect(sigc::mem_fun(*this,&MainWindow::nothingExec));

where nothing exec is:

bool MainWindow::nothingExec(GdkEventCrossing* event){
  std::cout << "nothing to do" << std::endl;
  return false;
}

I also tried return true; but that doesn't make a difference. Can 
someone tell me how to make this code to work?

Thanks, HeavyJoost