Cannot `dynamic_cast` custom wrapped widget from Gtk::Builder

[Josh Bialkowski <josh bialkowski gmail com>]

Hello, 

I have a custom widget that I've implemented in C and then used gmmproc[1] to generate C++ bindings for that widget. The process was mostly smooth, but I cannot seem to use my generated C++ bindings in Gtk::Builder::get_widget. I can post the full source code if needed, but probably there is something simple that I a missing so I'll start with what I think is most relevant: 

My widget is a relatively simple extension of GtkDrawingArea which adds the ability to pan and zoom using mouse motion and mouse wheel. It's called GtkPanZoomArea. I have created a Glade catalog XML file which allows me to use my custom widget in glade. I have successfully used my widget in an application using the plain-C API, retrieving a pointer to an instance of this widget like so:

~~~
GtkPanZoomArea* panzoom_ =
      GTK_PANZOOM_AREA(builder->get_object("overhead-view")->gobj());
~~~
I have gone through the process of using `h2def.py` and `enum.py`, and written a custom `gen_extra_defs.cc` (using `glibmm_generate_extra_defs`) to generate def files, and I have used `gmmproc` to generate the `mm-` wrapper `Gtk::PanZoomArea`. 

Everything compiles, but when I change the above code to:

~~~
Gtk::PanZoomArea panzoom_;
builder->get_widget("overhead-view", panzoom_)
~~~

the member is assigned nullptr and I see the following errors printed to stderr:

~~~
: CRITICAL **: 13:53:12.873: Gtk::Builder::get_widget(): dynamic_cast<> failed.
~~~

The following works:

~~~
Gtk::DrawingArea* drawingarea;
builder->get_widget("overhead-view", drawingarea);
~~~

But then this subsequently does not work:

~~~
auto panzoom_ = dynamic_cast<Gtk::PanZoomArea*>(drawingarea);
~~~

My guess is that the most likely place for there to be an error is in the `.hg` file so here is the class declaration I have in `mm/panzoomarea.hg`:

~~~
class PanZoomArea : public Gtk::DrawingArea {
  _CLASS_GOBJECT(PanZoomArea, GtkPanZoomArea, GTK_PANZOOM_AREA,
                 Gtk::DrawingArea, GtkDrawingArea);
~~~

And here is the type declaration in `panzoomarea.h`:

~~~
G_DECLARE_DERIVABLE_TYPE(GtkPanZoomArea, gtk_panzoom_area, GTK, PANZOOM_AREA,
                         GtkDrawingArea);
~~~

Does anyone know what I'm doing wrong here? The code for `get_widget_checked` in `builder.ccg`[2] looks like it would print errors if there were any problem retrieving the widget and checking the type and since it does neither of these, it seems to be a problem in `Glib::wrap`. For some reason `Glib::wrap` is constructing a C++ object other than my object. It appears to be constructing a `Gtk::DrawingArea`. Assuming the `Glib::wrap` uses the gobject type registry, I added the following in my `main()`:

~~~
auto _ignoreme = Gtk::PanZoomArea::get_type();
(void)_ignoreme;
~~~

but this did not fix the problem. 

[1]: https://developer.gnome.org/gtkmm-tutorial/stable/chapter-wrapping-c-libraries.html.en
[2]: https://fossies.org/linux/gtkmm/gtk/src/builder.ccg#290