You can't use virtual functions in GObjects. At all. Introducing a virtual function to a class changes the resulting object layout in a way that is wholly incompatible with GObject.

If you need something virtual-like before a class hierarchy has been transitioned away from GObject, you _must_ use the normal GObject mechanisms for that (i.e. adding an entry to the GObjectClass-ish vtable and then writing a (non-virtual) wrapper method that calls explicitly through that function pointer).

-mental