I have read Object oriented programming with ANSI-C and implemented my own class system in C with inheritance and polymorphism.

Taking a look at Writing Bug-free C Code the chapter about class is really just an example how to use structs with pointers that don't expose the underlying implementation.

If you want to learn oop read the Oop with Ansi-C, where classes similar to c++ with certain limitations are possible. The book explains everything in detail with including source code[http://www.cs.rit.edu/~ats/books/ooc-14.01.03.tar.gz].