Object-orientation in C

Question

What would be a set of nifty preprocessor hacks (ANSI C89/ISO C90 compatible) which enable some kind of ugly (but usable) object-orientation in C?

I am familiar with

Answer 1

The GNOME desktop for Linux is written in object-oriented C, and it has an object model called "GObject" which supports properties, inheritance, polymorphism, as well as some other goodies like references, event handling (called "signals"), runtime typing, private data, etc.

It includes preprocessor hacks to do things like typecasting around in the class hierarchy, etc. Here's an example class I wrote for GNOME (things like gchar are typedefs):

Class Source

Class Header

Inside the GObject structure there's a GType integer which is used as a magic number for GLib's dynamic typing system (you can cast the entire struct to a "GType" to find it's type).