First off, please note that this manual cannot serve as a complete introduction to object oriented programming and generic functions in LISP. Although EIEIO is not a complete implementation of the Common Lisp Object System (CLOS) and also differs from it in several aspects, it follows the same basic concepts. Therefore, it is highly recommended to learn those from a textbook or tutorial first, especially if you only know OOP from languages like C++ or Java. If on the other hand you are already familiar with CLOS, you should be aware that EIEIO does not implement the full CLOS specification and also differs in some other aspects which are mentioned below (also see CLOS compatibility).
EIEIO supports the following features:
Due to restrictions in the Emacs Lisp language, CLOS cannot be completely supported, and a few functions have been added in place of setf. Here are some important CLOS features that EIEIO presently lacks:
eieio-default-superclass, and you cannot define your own. The
defclassis ignored. Also, functions like
find-class, which should return instances of the metaclass, behave differently in EIEIO in that they return symbols or plain structures instead.
:initformtag, so this is not really necessary.