Subclassing in Gtk+ 3.0

[David Nečas <yeti physics muni cz>]

Hello, how is subclassing intended to work in Gtk+ 3.0?

Now I subclass GtkButton with

struct _MyButton {
    GtkButton parent;
    /* My fields go here. */
    ...
};

According to

    http://mail.gnome.org/archives/gtk-devel-list/2008-June/msg00014.html

object struct definitions will be moved to private files.  So I will not
be able to do the above because GtkButton is not available (note the I'm
not talking about access to GtkButton fields, the above code simply
won't compile with GtkButton being an incomplete type).

So, what will replace this mechanism?

Yeti