CLOS

The Common Lisp Object System, a powerful system for object-oriented programming which forms part of Common Lisp.

CLOS differs from most other object-oriented programming environments in the following ways:

It offers multiple dispatch, or multimethods .

Therefore, methods are not considered to live within *Classes*; they are conceptually grouped into generic function instead, a form of mixin.

CLOS doesn t provide encapsulation; that is considered to be the job of a different part of Common Lisp, the package system.

*Inheritance* can cause methods to be combined together in arbitrarily complicated ways at the discretion of the programmer, and not merely overridden by one another.

CLOS is dynamic, meaning that not only the contents, but also the structure of its objects can be modified at runtime. CLOS supports changing class definitions on-the-fly (even when instances of the class in question already exist) as well as changing the class membership of a given instance through the change-class operator.

CLOS has multiple inheritance and, unofficially, a meta-object protocol.