Here is a very simple example, demonstrating a drag and drop Copy operation:
File: dndwindow.h (For use with gtkmm 3, not gtkmm 2)
#ifndef GTKMM_EXAMPLE_DNDWINDOW_H
#define GTKMM_EXAMPLE_DNDWINDOW_H
#include <gtkmm/box.h>
#include <gtkmm/label.h>
#include <gtkmm/window.h>
#include <gtkmm/button.h>
class DnDWindow : public Gtk::Window
{
public:
  DnDWindow();
  virtual ~DnDWindow();
protected:
  //Signal handlers:
  void on_button_drag_data_get(
          const Glib::RefPtr<Gdk::DragContext>& context,
          Gtk::SelectionData& selection_data, guint info, guint time);
  void on_label_drop_drag_data_received(
          const Glib::RefPtr<Gdk::DragContext>& context, int x, int y,
          const Gtk::SelectionData& selection_data, guint info, guint time);
  //Member widgets:
  Gtk::Box m_HBox;
  Gtk::Button m_Button_Drag;
  Gtk::Label m_Label_Drop;
};
#endif // GTKMM_EXAMPLE_DNDWINDOW_H
File: dndwindow.cc (For use with gtkmm 3, not gtkmm 2)
#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::vector<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::mem_fun(*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::mem_fun(*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>&,
        Gtk::SelectionData& selection_data, guint, guint)
{
  selection_data.set(selection_data.get_target(), 8 /* 8 bits format */,
          (const guchar*)"I'm Data!",
          9 /* the length of I'm Data! in bytes */);
}
void DnDWindow::on_label_drop_drag_data_received(
        const Glib::RefPtr<Gdk::DragContext>& context, int, int,
        const Gtk::SelectionData& selection_data, guint, guint time)
{
  const int length = selection_data.get_length();
  if((length >= 0) && (selection_data.get_format() == 8))
  {
    std::cout << "Received \"" << selection_data.get_data_as_string()
        << "\" in label " << std::endl;
  }
  context->drag_finish(false, false, time);
}
File: main.cc (For use with gtkmm 3, not gtkmm 2)
#include "dndwindow.h"
#include <gtkmm/application.h>
int main (int argc, char *argv[])
{
  Glib::RefPtr<Gtk::Application> app = Gtk::Application::create(argc, argv, "org.gtkmm.example");
  DnDWindow dndWindow;
  //Shows the window and returns when it is closed.
  return app->run(dndWindow);
}
There is a more complex example in examples/others/dnd.