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>
#include <gtkmm/liststore.h>
class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();
protected:  
  //Signal handlers:
  void on_combo_changed();
  //Tree model columns:
  class ModelColumns : public Gtk::TreeModel::ColumnRecord
  {
  public:
    ModelColumns()
    { add(m_col_id); add(m_col_name); }
    Gtk::TreeModelColumn<int> m_col_id;
    Gtk::TreeModelColumn<Glib::ustring> m_col_name;
  };
  ModelColumns m_Columns;
  //Child widgets:
  Gtk::ComboBox m_Combo;
  Glib::RefPtr<Gtk::ListStore> m_refTreeModel;
};
#endif //GTKMM_EXAMPLEWINDOW_H
File: examplewindow.cc (For use with gtkmm 3, not gtkmm 2)
#include "examplewindow.h"
#include <gtkmm/stock.h>
#include <iostream>
ExampleWindow::ExampleWindow()
{
  set_title("ComboBox example");
  //Create the Tree model:
  //m_refTreeModel = Gtk::TreeStore::create(m_Columns);
  m_refTreeModel = Gtk::ListStore::create(m_Columns);
  m_Combo.set_model(m_refTreeModel);
  //Fill the ComboBox's Tree Model:
  Gtk::TreeModel::Row row = *(m_refTreeModel->append());
  row[m_Columns.m_col_id] = 1;
  row[m_Columns.m_col_name] = "Billy Bob";
  m_Combo.set_active(row);
  /*
  Gtk::TreeModel::Row childrow = *(m_refTreeModel->append(row.children()));
  childrow[m_Columns.m_col_id] = 11;
  childrow[m_Columns.m_col_name] = "Billy Bob Junior";
  childrow = *(m_refTreeModel->append(row.children()));
  childrow[m_Columns.m_col_id] = 12;
  childrow[m_Columns.m_col_name] = "Sue Bob";
  */
  row = *(m_refTreeModel->append());
  row[m_Columns.m_col_id] = 2;
  row[m_Columns.m_col_name] = "Joey Jojo";
  row = *(m_refTreeModel->append());
  row[m_Columns.m_col_id] = 3;
  row[m_Columns.m_col_name] = "Rob McRoberts";
  /*
  childrow = *(m_refTreeModel->append(row.children()));
  childrow[m_Columns.m_col_id] = 31;
  childrow[m_Columns.m_col_name] = "Xavier McRoberts";
  */
  //Add the model columns to the Combo (which is a kind of view),
  //rendering them in the default way:
  m_Combo.pack_start(m_Columns.m_col_id);
  m_Combo.pack_start(m_Columns.m_col_name);
  //Add the ComboBox to the window.
  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()
{
  Gtk::TreeModel::iterator iter = m_Combo.get_active();
  if(iter)
  {
    Gtk::TreeModel::Row row = *iter;
    if(row)
    {
      //Get the data for the selected row, using our knowledge of the tree
      //model:
      int id = row[m_Columns.m_col_id];
      Glib::ustring name = row[m_Columns.m_col_name];
      std::cout << " ID=" << id << ", name=" << name << std::endl;
    }
  }
  else
    std::cout << "invalid iter" << std::endl;
}
File: main.cc (For use with gtkmm 3, not gtkmm 2)
#include "examplewindow.h"
#include <gtkmm/application.h>
int main(int argc, char *argv[])
{
  Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
  ExampleWindow window;
  //Shows the window and returns when it is closed.
  return app->run(window);
}