gtkmm, like most GUI toolkits, is event-driven. When an event occurs, such as the press of a mouse button, the appropriate signal will be emitted by the Widget that was pressed. Each Widget has a different set of signals that it can emit. To make a button click result in an action, we set up a signal handler to catch the button's "clicked" signal.
gtkmm uses the libsigc++ library to implement signals. Here is an example line of code that connects a Gtk::Button's "clicked" signal with a signal handler called "on_button_clicked":
m_button1.signal_clicked().connect( sigc::mem_fun(*this, &HelloWorld::on_button_clicked) );
For more detailed information about signals, see the appendix.
For information about implementing your own signals rather than just connecting to the existing gtkmm signals, see the appendix.