GTK is built entirely on C and it uses many OOP concepts. I have read through the source code of GTK and it is pretty impressive, and definitely easier to read. The basic concept is that each "class" is simply a struct, and associated static functions. The static functions all accept the "instance" struct as a parameter, do whatever then need, and return results if necessary. For Example, you may have a function "GetPosition(CircleStruct obj)". The function would simply dig through the struct, extract the position numbers, probably build a new PositionStruct object, stick the x and y in the new PositionStruct, and return it. GTK even implements inheritance this way by embedding structs inside structs. pretty clever.
