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

If you think of methods called on objects as static methods that pass an implicit 'this' into the function it can make thinking OO in C easier.

For example:

String s = "hi";
System.out.println(s.length());

becomes:

string s = "hi";
printf(length(s)); // pass in s, as an implicit this

Or something like that.