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## 2 Introduction

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:

1. A structured framework for the creation of basic classes with attributes and methods using singular inheritance similar to CLOS.
2. Type checking, and slot unbinding.
3. Method definitions similar to CLOS.
4. Simple and complex class browsers.
5. Edebug support for methods.
6. Imenu updates.
7. Byte compilation support of methods.
8. Help system extensions for classes and methods.
9. Several base classes for interesting tasks.
10. Simple test suite.
11. Public and private classifications for slots (extensions to CLOS)
12. Customization support in a class (extension to CLOS)

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:

Method dispatch

  EIEO does not support method dispatch for built-in types and multiple arguments types. In other words, method dispatch only looks at the first argument, and this one must be an EIEIO type.

Support for metaclasses

  There is just one default metaclass, `eieio-default-superclass`, and you cannot define your own. The `:metaclass` tag in `defclass` is 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.

EQL specialization

  EIEIO does not support it.

`:around` method tag

  This CLOS method tag is non-functional.

:default-initargs in `defclass`

  Each slot can have an `:initform` tag, so this is not really necessary.

Mock object initializers

  Each class contains a mock object used for fast initialization of instantiated objects. Using functions with side effects on object slot values can potentially cause modifications in the mock object. EIEIO should use a deep copy but currently does not.

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