Comment 123 for bug 219385

small intermission from a lurker:

(In reply to Karl Tomlinson (:karlt) from comment #101)
> > GDK_DRAWABLE() adds code for an extra function call per caller.
>
> Actually two function calls: gdk_drawable_get_type and g_type_check_instance.

Only if you don't compile with -DG_DISABLE_CAST_CHECKS which people definitely should do for releases. In that case those casts are just casts.

FWIW, using GNOME coding style, I'd make every cast use the macro both because of style ad because I know there's no performance difference, but huge debuggability improvements (once you turn it on).