Unlike a regular ComboBox, a ComboBoxEntry contains a Entry widget for entering of arbitrary text. So that this Entry can interact with the drop-down list of choices, you must specify which of your model columns are the text column, with set_text_column(). For instance:

m_combo.set_text_column(m_columns.m_col_name);

When you select a choice from the drop-down menu, the value from this column will be placed in the Entry.

Because the user may enter arbitrary text, an active model row isn't enough to tell us what text the user has inputted. Therefore, you should retrieve the Entry widget with the ComboBoxEntry::get_entry() method and call get_text() on that.

Source Code

File: examplewindow.h

#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H

#include <gtkmm/window.h>
#include <gtkmm/comboboxentrytext.h>
#include <gtkmm/liststore.h>

class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();

protected:  
  //Signal handlers:
  virtual 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<Glib::ustring> m_col_id; //The data to choose - this must be text.
    Gtk::TreeModelColumn<Glib::ustring> m_col_name;
  };

  ModelColumns m_Columns;

  //Child widgets:
  Gtk::ComboBoxEntry m_Combo;
  Glib::RefPtr<Gtk::ListStore> m_refTreeModel;
};

#endif //GTKMM_EXAMPLEWINDOW_H

File: main.cc

#include <gtkmm/main.h>
#include "examplewindow.h"

int main(int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);

  ExampleWindow window;
  //Shows the window and returns when it is closed.
  Gtk::Main::run(window);

  return 0;
}

File: examplewindow.cc

#include "examplewindow.h"
#include <gtkmm/stock.h>
#include <iostream>

ExampleWindow::ExampleWindow()
{
  set_title("ComboBoxEntry 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";
  /*
  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:
  //This is automatically rendered when we use set_text_column().
  //m_Combo.pack_start(m_Columns.m_col_id);
  m_Combo.pack_start(m_Columns.m_col_name);

  m_Combo.set_text_column(m_Columns.m_col_id);

  //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::Entry* entry = m_Combo.get_entry();
  //Note: to get changes only when the entry has been completed, 
  //instead of on every key press, connect to Entry::signal_changed() 
  //instead of ComboBoxEntry::signal_changed.

  if(entry)
  {
    std::cout << " ID=" << entry->get_text() << std::endl;
  }
}

Source Code

File: examplewindow.h

#ifndef GTKMM_EXAMPLEWINDOW_H
#define GTKMM_EXAMPLEWINDOW_H

#include <gtkmm/window.h>
#include <gtkmm/comboboxentrytext.h>

class ExampleWindow : public Gtk::Window
{
public:
  ExampleWindow();
  virtual ~ExampleWindow();

protected:
  //Signal handlers:
  virtual void on_combo_changed();

  //Child widgets:
  Gtk::ComboBoxEntryText m_Combo;
};

#endif //GTKMM_EXAMPLEWINDOW_H

File: main.cc

#include <gtkmm/main.h>
#include "examplewindow.h"

int main(int argc, char *argv[])
{
  Gtk::Main kit(argc, argv);

  ExampleWindow window;
  //Shows the window and returns when it is closed.
  Gtk::Main::run(window);

  return 0;
}

File: examplewindow.cc

#include "examplewindow.h"
#include <gtkmm/stock.h>
#include <iostream>

ExampleWindow::ExampleWindow()
{
  set_title("ComboBoxEntryText example");

  //Fill the combo:
  m_Combo.append_text("something");
  m_Combo.append_text("something else");
  m_Combo.append_text("something or other");

  add(m_Combo);

  //Connect signal handler:
  m_Combo.signal_changed().connect(sigc::mem_fun(*this,
              &ExampleWindow::on_combo_changed) );

  m_Combo.property_has_frame() = false;
  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;
}