Inheriting from Gtk::CellRendererCombo

On ggredit the user can change properties of the selected object using a properties browser, implemented using the Gtk::TreeView object. On my previous post "Different CellRenderers on the same Column" I explain the way I use for renderer each property with a different cell-renderer. Later I see that the deceased crow designer use the same method.
For select enumeration values (i.e. alignment can be left, right or center) I would like to use a Gtk::CellRendererCombo, but my derived class never start editing. In effect a simple Gtk::CellRendererCombo inherited class like this:

#include <gtkmm.h>

class CellRendererComboDerived : public Gtk::CellRendererCombo

{
public:
 CellRendererComboDerived() :
  Glib::ObjectBase (typeid(CellRendererComboDerived)),
  Gtk::CellRendererCombo()

 {
 }

 virtual ~CellRendererComboDerived()
 {

 }
};

never start edit. On the gtkmm mailing list someone ask for this "subclassing Gtk::CellRendererCombo" issue but no one answer.
I start to write my own combo as a custom cell renderer, following the gtkmm example. At the end the custom cell renderer result a poor combo.
So I come back to the Gtk::CellRendererCombo, first I look at the gtkmm source. This is the constructor of the CellRendererCombo:

CellRendererCombo::CellRendererCombo()
:

  // Mark this class as non-derived to allow C++ vfuncs to be skipped.
  Glib::ObjectBase(0),
  Gtk::CellRendererText(Glib::ConstructParams(cellrenderercombo_class_.init()))

{
  

}

The CellRendererCombo call the Glib::ObjectBase constructor passing zero instead of the typeid of the class.
I try to revrite the CellRendererComboDerived class in this way:

#include <gtkmm.h>

class CellRendererComboDerived : public Gtk::CellRendererCombo

{
public:
 CellRendererComboDerived() :
  Glib::ObjectBase (0),
  Gtk::CellRendererCombo()

 {
 }

 virtual ~CellRendererComboDerived()
 {

 }
};

and the cell renderer start work as expected, when the user click on the cell the edit start.

The editing_started signal

For modify the combo used as editable from the CellRendererCombo we must catch the signal_editing_started signal.
The gtkmm documentation says that this signal does not work yet (Bug 301597).

Note that this signal does not work yet in gtkmm. See http://bugzilla.gnome.org/show_bug.cgi?id=301597

I try to use it:

#include <gtkmm.h>

#include <iostream>

class CellRendererComboDerived : public Gtk::CellRendererCombo

{
public:
 CellRendererComboDerived() :
  Glib::ObjectBase (0),
  Gtk::CellRendererCombo()

 {
  this->signal_editing_started().connect(
    sigc::mem_fun(*this, &CellRendererComboDerived::on_editing_started));

 }

 virtual ~CellRendererComboDerived()
 {
 }

 virtual void on_editing_started(Gtk::CellEditable* editable, const Glib::ustring& path)

 {
  std::cout << "Editing started editable: " << editable << std::endl;

 }
};

the signal is emitted but the editable pointer is always null.
Murray Cumming on Bugzilla tell us to use the C API (g_signal_connect()). I never use the C API, so I first have a look at how to use g_signal_connect(), we have to give a callback to this function, and the callback must be a static function. So we can not pass a class member function, but we can pass a raw gpointer parameter and use this for invoke a class member function. The code may be more clear, this is the header:

#ifndef CELLRENDERERCOMBODERIVED_H_
#define CELLRENDERERCOMBODERIVED_H_

#include <gtkmm.h>

class CellRendererComboDerived : public Gtk::CellRendererCombo
{
public:
    CellRendererComboDerived();
    virtual ~CellRendererComboDerived();

    virtual void on_editing_started(Gtk::ComboBox* combo_ptr, const Glib::ustring& path);
};

#endif /* CELLRENDERERCOMBODERIVED_H_ */

and this is the implementation:

#include "CellRendererComboDerived.h"

#include <iostream>

void cb_editing_started(GtkCellRenderer* cell,
                        GtkCellEditable* editable,
                        const gchar* path,
                        gpointer data)
{
    reinterpret_cast< CellRendererComboDerived* >(data)->on_editing_started (
            Glib::wrap(reinterpret_cast< GtkComboBox* >(editable), true), path);
};

CellRendererComboDerived::CellRendererComboDerived() :
    Glib::ObjectBase (0),
    Gtk::CellRendererCombo()
{
    g_signal_connect (this->gobj(), "editing-started", G_CALLBACK(cb_editing_started), (gpointer)this);
}

CellRendererComboDerived::~CellRendererComboDerived()
{
}

void CellRendererComboDerived::on_editing_started(Gtk::ComboBox* combo_ptr, const Glib::ustring& path)
{
    std::cout << "Editing started columns: " << combo_ptr->get_model()->get_n_columns() << std::endl;
}

On the constructor we use g_signal_connect() for connect the function cb_editing_started to the signal editing-started, and we give also the this pointer.
Whe the signal was emitted the function cb_editing_started was called, the function receive on the data parameter the pointer we give on the contructor. So the function cast the pointer to the CellRendererComboDerived class. For create a gtkmm Gtk::ComboBox class from the GtkCellEditable we can use the Glib::wrap() function.
Now I need to implement this on ggredit...