#ifndef GTKMM_EXAMPLE_DNDWINDOW_H
#define GTKMM_EXAMPLE_DNDWINDOW_H

#include <gtkmm/label.h>
#include <gtkmm/window.h>
#include <gtkmm/box.h>
#include <gtkmm/button.h>

class DnDWindow : public Gtk::Window
{

public:
  DnDWindow();
  virtual ~DnDWindow();

protected:
  //Signal handlers:
  virtual void on_button_drag_data_get(const Glib::RefPtr<Gdk::DragContext>& context, GtkSelectionData* selection_data, guint info, guint time);
  virtual void on_label_drop_drag_data_received(const Glib::RefPtr<Gdk::DragContext>& context, int x, int y, GtkSelectionData* selection_data, guint info, guint time);

  //Member widgets:
  Gtk::HBox m_HBox;
  Gtk::Button m_Button_Drag;
  Gtk::Label m_Label_Drop;
};

#endif // GTKMM_EXAMPLE_DNDWINDOW_H

File: dndwindow.cc

#include "dndwindow.h"
#include <iostream>

DnDWindow::DnDWindow()
: m_Button_Drag("Drag Here\n"),
  m_Label_Drop("Drop here\n")
{
  set_title("DnD example");

  add(m_HBox);

  //Targets:
  std::list<Gtk::TargetEntry> listTargets;
  listTargets.push_back( Gtk::TargetEntry("STRING") );
  listTargets.push_back( Gtk::TargetEntry("text/plain") );

  //Drag site:

  //Make m_Button_Drag a DnD drag source:
  m_Button_Drag.drag_source_set(listTargets);
		
  //Connect signals:
  m_Button_Drag.signal_drag_data_get().connect( SigC::slot(*this, &DnDWindow::on_button_drag_data_get));

  m_HBox.pack_start(m_Button_Drag);

  //Drop site:

  //Make m_Label_Drop a DnD drop destination:
  m_Label_Drop.drag_dest_set(listTargets);

  //Connect signals:
  m_Label_Drop.signal_drag_data_received().connect( SigC::slot(*this, &DnDWindow::on_label_drop_drag_data_received) );

  m_HBox.pack_start(m_Label_Drop);

  show_all();
}

DnDWindow::~DnDWindow()
{
}

void DnDWindow::on_button_drag_data_get(const Glib::RefPtr<Gdk::DragContext>&, GtkSelectionData* selection_data, guint, guint)
{
  //TODO: The gtkmm API needs to change to use a Gtk::SelectionData instead of a GtkSelectionData.
  //That should happen for gtkmm 2.4.
  
  gtk_selection_data_set (selection_data, selection_data->target, 8, (const guchar*)"I'm Data!", 9);
}

void DnDWindow::on_label_drop_drag_data_received(const Glib::RefPtr<Gdk::DragContext>& context, int, int, GtkSelectionData* selection_data, guint, guint time)
{
  //TODO: The gtkmm API needs to change to use a Gtk::SelectionData instead of a GtkSelectionData.
  //That should happen for gtkmm 2.4.
  
  if ((selection_data->length >= 0) && (selection_data->format == 8))
  {
    std::cout << "Received \"" << (gchar *)(selection_data->data) << "\" in label " << std::endl;
  }

  context->drag_finish(false, false, time);
}

File: main.cc

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

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

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

  return 0;
}

There is a more complex example in examples/dnd.