File: examplewindow.h (For use with gtkmm 3, not gtkmm 2)
#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H
#include <gtkmm/window.h>
#include <gtkmm/comboboxtext.h>
class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();
protected:
  //Signal handlers:
  void on_combo_changed();
  //Child widgets:
  Gtk::ComboBoxText m_Combo;
};
#endif //GTKMM_EXAMPLEWINDOW_H
File: main.cc (For use with gtkmm 3, not gtkmm 2)
#include "examplewindow.h"
#include <gtkmm/application.h>
int main(int argc, char *argv[])
{
  auto app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
  ExampleWindow window;
  //Shows the window and returns when it is closed.
  return app->run(window);
}
File: examplewindow.cc (For use with gtkmm 3, not gtkmm 2)
#include "examplewindow.h"
#include <iostream>
ExampleWindow::ExampleWindow()
{
  set_title("ComboBoxText example");
  //Fill the combo:
  m_Combo.append("something");
  m_Combo.append("something else");
  m_Combo.append("something or other");
  m_Combo.set_active(1);
  add(m_Combo);
  //Connect signal handler:
  m_Combo.signal_changed().connect(sigc::mem_fun(*this,
              &ExampleWindow::on_combo_changed) );
  show_all_children();
}
ExampleWindow::~ExampleWindow()
{
}
void ExampleWindow::on_combo_changed()
{
  Glib::ustring text = m_Combo.get_active_text();
  if(!(text.empty()))
    std::cout << "Combo changed: " << text << std::endl;
}