Features
The basic building blocks of CLOS are methods, classes, instances of those classes, and generic functions. CLOS provides macros to define those: defclass, defmethod, and defgeneric. Instances are created with the method make-instance.
Classes can have multiple superclasses, a list of slots (member variables in C++/Java parlance) and a special metaclass. Slots can be allocated by class (all instances of a class share the slot) or by instance. Each slot has a name and the value of a slot can be accessed by that name using the function slot-value. Additionally special generic functions can be defined to write or read values of slots. Each slot in a CLOS class must have a unique name.
CLOS is a multiple dispatch system. This means that methods can be specialized upon any or all of their required arguments. Most OO languages are single-dispatch, meaning that methods are only specialized on the first argument. Another unusual feature is that methods do not "belong" to classes; classes do not provide a namespace for generic functions or methods. Methods are defined separately from classes, and they have no special access (e.g. "this", "self", or "protected") to class slots.
Methods in CLOS are grouped into generic functions. A generic function is an object which is callable like a function and which associates a collection of methods with a shared name and argument structure, each specialized for different arguments. Since Common Lisp provides non-CLOS classes for structures and built-in data types (numbers, strings, characters, symbols, ...), CLOS dispatch works also with these non-CLOS classes. CLOS also supports dispatch over individual objects (eql specializers). CLOS does not by default support dispatch over all Common Lisp data types (for example dispatch does not work for fully specialized array types or for types introduced by deftype). However, most Common Lisp implementations provide a metaobject protocol which allows generic functions to provide application specific specialization and dispatch rules.
Dispatch in CLOS is also different from most OO languages:
- Given a list of arguments, a list of applicable methods is determined.
- This list is sorted according to the specificity of their parameter specializers.
- Selected methods from this list are then combined into an effective method using the method combination used by the generic function.
- The effective method is then called with the original arguments.
This dispatch mechanism works at runtime. Adding or removing methods thus may lead to changed effective methods (even when the generic function is called with the same arguments) at runtime. Changing the method combination also may lead to different effective methods.
For example,
; Declare the common argument structure prototype.(defgenericf(xy)); Define an implementation for (f integer y), where y matches all types.(defmethodf((xinteger)y)1)(f12.0)=>1; Define an implementation for (f integer real).(defmethodf((xinteger)(yreal))2)(f12.0)=>2;Dispatch changed at runtime.
Like the OO systems in most dynamic languages, CLOS does not enforce encapsulation. Any slot can be accessed using the slot-value function or via (optionally auto-generated) accessor methods. To access it via slot-value you have to know the name of the slot. CL programmers use the language's package facility to declare which functions or data structures are intended for export.
Apart from normal ("primary") methods, there also are :before, :after, and :around "auxiliary" methods. The former two are invoked prior to, or after the primary method, in a particular order based on the class hierarchy. An :around method can control whether the primary method is executed at all. Additionally, the programmer can specify whether all possible primary methods along the class hierarchy should be called or just the one providing the closest match.
The Standard Method-Combination provides the primary, before, after and around methods explained above. There are other Method-Combinations with other method types. New (both simple and complex) Method-Combinations and method types can be defined.
CLOS allows multiple inheritance. When the default order in which methods are executed in multiple inheritance is not correct, the programmer may resolve the diamond inheritance problems by specifying the order of method combinations.
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 also allows one to add, redefine and remove methods at runtime. The Circle-Ellipse Problem is readily solved in CLOS, and most OOP design patterns either disappear or are qualitatively simpler. [2]
CLOS is not a prototype language: classes must be defined before objects can be instantiated as members of that class.
