Motif/libxm is very similar to that. With two important exceptions: the user code does not use type-checking macros and “stringly-typed” API for set a property to something is more or less a part of the overall ecosystem (well, Xm is just an layer built on top of Xt).

Gtk+ was originally an implementation of whatever parts of Motif were used by Gimp, built on an underlying object model that has nothing to do with Xt (which has both good and bad implications).