The Art of the Metaobject Protocol Chapters 5 and 6

The Art of the Metaobject Protocol 

Chapters 5and 6 

Gregor Kiczales, Jim des Rivieres, and DanielG.Bobrow 

Xerox Palo Alto Research Center 

c 1991 MIT Press. The material i ncl uded here compri ses chapters 5 and 6 of \The Artof 

theMetaobject Protocol." MITPress is expected to publishthis entire workinthesummer 

of 1991. Pl ease see page 1 f or f urther i nformati on about thecopyri ght ofthis material.

ontent s 

5 Concepts 3 

Introducti on :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 3 

Metaobj ects : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 3 

Cl asses :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 4 

Sl ot Denitions : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 4 

Generi c Functi ons :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 5 

Methods : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 

Speci al i zers :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 

Method Combinati ons : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 6 

Inheri tance Structure of Metaobj ect Classes : : : : : : : : : : : : : : : : : : : : : : 6 

ImplementationandUser Specialization : : : : : : : : : : : : : : : : : : : : 8 

ProcessingoftheUser Interface Macros : : : : : : : : : : : : : : : : : : : : : : : 11 

Compile-le ProcessingoftheUser Interface Macros : : : : : : : : : : : : : 12 

The defcl ass Macro : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 12 

The defmethod Macro : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 16 

ProcessingMethod Bodies :: : : : : : : : : : : : : : : : : : : : : : : : : : : : 17 

The def generi c Macro : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 19 

Subprotocols :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 20 

Metaobj ect I nitializationProtocols : : : : : : : : : : : : : : : : : : : : : : : 20 

Class FinalizationProtocol : : : : : : : : : : : : : : : : : : : : : : : : : : : 22 

Instance Structure Protocol : :: : : : : : : : : : : : : : : : : : : : : : : : : : 23 

Funcallable Instances :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 25 

Generi c Functi on Invocati on Protocol :: 

: : : : : : : : : : : : : : : : : : : : 26 

Dependent Mai ntenance Protocol : : : : : : : : : : : : : : : : : : : : : : : : : 26 

6 Generic Functi ons and Methods 29 

add-dependent : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 30 

add-di rect-method :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 31 

add-di rect-subcl ass : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 32 

add-method : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 33 

i

ii 

CONTENTS 

allocate-instance : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 34 

class-:::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 35 

compute-applicable-methods :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 35 

compute-applicable-methods-using- cl asses : : : : : : : : : : : : : : : : : : : : : : : 36 

compute-class-precedence-list : :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : 38 

compute-default-ini targs : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 39 

compute-discriminati ng- f uncti on : : : : : : : : : : : : : : : : : : : : : : : : : : : 40 

compute-eecti ve-method :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 41 

compute-eecti ve-sl ot-denition : : : : : : : : : : : : : : : : : : : : : : : : : : : : 42 

compute-slots :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 43 

direct-slot-denition-class : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 45 

eecti ve-sl ot-denition-class : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 45 

ensure-class : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 46 

ensure-class-using- cl ass : : :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 47 

ensure-generi c-functi on : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 49 

ensure-generi c-functi on-using- cl ass : : :: 

: : : : : : : : : : : : : : : : : : : : : : : 50 

eql -speci al i zer-object : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 52 

extract-lambda- l i st : :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 52 

extract-speci al i zer-names : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 53 

nalize-inheri tance : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 54 

nd-method-combination : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 54 

funcallable-standard-instance-access : : : : : : : : : : : : : : : : : : : : : : : : : 55 

generi c-functi on- :::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 56 

Initializationof Class Metaobj ects : : : : : : : : : : : : : : : : : : : : : : : : : : 57 

Initializationof Generi c Functi on Metaobj ects : : : : : : : : : : : : : : : : : : : : 61 

Initializationof Method Metaobj ects : : : : : : : : : : : : : : : : : : : : : : : : : 64 

Initializationof Slot DenitionMetaobj ects : : : : : : : : : : : : : : : : : : : : : 67 

intern-eql-speci al i zer : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 70 

make-i nstance : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 70 

make-method-lambda : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 71 

map-dependents : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 73 

method- ::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 74 

Readers f or Class Metaobj ects : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 75 

Readers f or Generi c Functi on Metaobj ects :: 

: : : : : : : : : : : : : : : : : : : : : 79 

Readers f or Method Metaobj ects :: : : : : : : : : : : : : : : : : : : : : : : : : : : 81 

Readers f or Slot DenitionMetaobj ects : :: : : : : : : : : : : : : : : : : : : : : : 83 

reader-method-class : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 86 

remove-dependent : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 87 

remove-di rect-method :: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 89 

remove-di rect-subcl ass : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 90 

remove-method : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 91 

set-funcallable-instance-functi on : : :: 

: : : : : : : : : : : : : : : : : : : : : : : : 91

CONTENTS 

iii 

(setf class-name) : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 92 

(setf generi c-functi on-name) : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 92 

(setf slot-val ue-using- cl ass) : : :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : 93 

slot-boundp-using- cl ass : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 94 

slot-denition- :::: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 95 

slot-makunbound-using- cl ass :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : 96 

slot-val ue-using- cl ass : :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : 97 

speci al i zer-di rect-generic-functi: ons : : : : : : : : : : : : : : : : : : : : : : : : : 98 

speci al i zer-di rect-methods : :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : 99 

standard-instance-access : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :100 

update- dependent : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :101 

validate-supercl ass : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :102 

writer-method-class :: 

: : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :103

iv 

CONTENTS

In this part of the book, weprovidethedetailedspeci cati on of a metaobj ect protocol 

for CLOS. Our work withthis protocol has al ways been rooted i n our ownimplementation 

of CLOS, PCL. This has made itpossible for ustohave auser community, whichinturn 

has provided us wi th feedback on this protocol as it has evol ved. As a result, much of the 

desi gn presented here i s well-tested and stable. As this is bei ng written, those parts have 

been i mplemented not onlyinPCL, but inatleastthree other CLOS implementationswe 

knowof . Other parts of theprotocol, even though they have been i mplementedinone f orm 

or another i n PCL andother i mplementations, are less wel l worked out. Work remai ns 

to improve not only theeaseofuse of these protocols, but also thebal ance they provide 

between user extensibilityandimplementor freedom. 

Inprepari ng this speci cati on, itisourhope that i t will provideabasi s f or the users and 

implementors whowishtowork withametaobj ect protocol for CLOS. This documentshould 

not be construed as any sort of nal word or standard, but rather onl y as documentation 

of what has been done sofar. We l ook f orward to seei ngtheimprovements, both smal l and 

large, whichwehope this publicationwill catalyze. 

Tothis end, f or Part I I only(chapters 5 and6), wegrantpermissionto prepare revisions 

or other deri vati ve works incl udinganyamount of the ori gi nal text. We askonly that 

you properl y acknowledge the source of the ori gi nal text andexplicitly allowsubsequent 

revisionsandderi vati ve works under the sameterms. Tofurther facilitate improvements in 

this work, wehave made the electronic source for these chapters publiclyavai l able; it can 

be accessed byanonymous FTPfromthe /pcl/mop di rectory on arisia.xerox.com. 

In additionto theval uable feedback f romusers of PCL, theprotocol desi gn presented 

here has beneted fromdetailedcomments andsuggesti ons bythe followingpeople: Kim 

Barrett, Scott Cyphers, Harley Davis, Patrick Dussud, John Foderaro, Richard P. Gabriel, 

DavidGray,DavidA. Moon, Andreas Paepcke, Chris Richardson,Wal ter van Roggen, and 

Jon L. White. Wearevery grateful to each of them. Any remai ning errors, inconsistenci es 

or desi gn deci enci es are the responsibilityof theauthors al one.

2 CONTENTS

a 

ter 

on e ts 

Introduction 

The CLOS Speci cati on [X3J13 , CLtLII]descri bes the standard Programmer Interface for 

the Common Lisp Object System(CLOS). This document extends that speci cati on by 

deningametaobj ect protocol for CLOS|that i s, a descri pti on of CLOS i tsel f as an extensible 

CLOS program. In this descri ption, thefundamental elements of CLOS programs 

(classes, slot denitions, generi c functi ons, methods, speci al i zers andmethod combinati ons) 

are represented byrst-class objects. The behavior of CLOS is provided bythese objects, 

or, more preci sel y, bymethods speci al i zed to the classes of these objects. 

Because these objects represent pi eces of CLOS programs, andbecause thei r behavior 

provides the behavior of theCLOS language i tsel f , they are considered meta- l evel objects 

or metaobj ects. The protocol followed bythemetaobj ects to providethebehavior of CLOS 

is calledtheCLOS Metaobj ect Protocol (MOP). 

etaobjects 

For each kindof programel ement there is a corresponding basic metaobject class. These 

are the classes: class, slot-denition, generi c- functi on, method and method-combi - 

nation. A met aobj ect cl ass is a subcl ass of exactl y one ofthese classes. The results are 

undened i f an attempt ismade todene a cl ass that i s a subclassofmore than one basi c 

metaobj ect cl ass. Amet aobj ect is aninstance of a metaobj ect cl ass. 

Each metaobj ect represents oneprogramel ement. Associ ated witheachmetaobj ect i s 

the information required to serve its role. This incl udes i nformati on that mightbe provided 

directly ina user interface macro such as def cl ass or def method. It also incl udes 

informati on computed indirectlyfromother metaobj ects such as that computed fromcl ass 

inheri tance or the full set of methods associ ated wi th a generi c functi on. 

3

4 CHAPTER 5. CONCEPTS 

Much of the informati on associ ated with a metaobj ect i s i n the f ormof connecti ons 

to other metaobj ects. This interconnecti on means that the roleofametaobj ect i s al ways 

based on that of other metaobj ects. As an introducti on to this interconnected structure, 

this sectionpresents a partial enumerationof thekinds ofinformati on associ ated wi th each 

kindof metaobj ect. More detailedinformati on i s presented l ater. 

lasses 

Acl ass met aobj ect determines the structure andthedefault behavior of its instances. The 

followinginformati on i s associ ated withclass metaobj ects: 

The name, i f there i s one, i s avai l abl e as an obj ect. 

The di rect subclasses, di rect supercl asses andclass precedence list are avai l able as lists 

of cl ass metaobj ects. 

The slots dened directly in the cl ass are avai l able as a list of di rect sl ot denition 

metaobj ects. The slotswhich are accessible ininstances of the cl ass are avai l abl e as a 

list of eecti ve slotdenitionmetaobj ects. 

The documentationis avai l able as a stringornil. 

The methods whichuse the cl ass as a speci al i zer, andthegeneri c functi ons associ ated 

with those methods are availabl e as l i sts of method and generi c functi on metaobj ects 

respecti vel y. 

Sl ot e niti ons 

Asl ot de ni t i on met aobj ect containsinformati onabout thedenitionof aslot. There are two 

kinds ofslotdenitionmetaobj ects. Adi rect sl ot denitionmetaobj ect i s used to represent 

thedi rect denitionof aslot inaclass. This corresponds roughlyto theslotspeci ers found 

indef cl ass forms. Aneecti ve slotdenitionmetaobj ect i s used to representinformati on, 

incl udinginheri ted i nformati on, about a sl ot whi ch i s accessi ble ininstances of a particular 

class. 

Associ ated witheachclass metaobj ect i s a l i st of di rect sl ot denitionmetaobj ects representingtheslotsdened 

directlyinthe class. Al so associ ated witheachclass metaobj ect 

is a list of eecti ve slot denition metaobj ects representingthe set of sl ots accessi ble in 

instances of that cl ass. 

The followinginformati on i s associ ated with both di rect and eecti ve slotdenitions 

metaobj ects: 

The name, al l ocati on, andtype areavai l able as formsthat couldappear i n a def cl ass 

form.

ETAO JECTS 5 

The initialization f orm, if there is one, i s avai l abl e as a f ormthat could appear i n a 

def cl ass form. The initializationformtogether withits lexical environment isavai l able 

as a f uncti on of no arguments which, when cal l ed, returns theresult of eval uati ng the 

initializationforminits lexical environment. This is calledthe init unct i on of the slot. 

The slotllinginitializationarguments are avai l able as alist of symbol s. 


Certainother i nformati on i s onl y associ ated withdi rect sl ot denitionmetaobj ects. This 

informati on applies only to thedi rect denitionof theslotinthe class (it is not i nheri ted). 

The functi on names of those generi c functi ons f or whichthere are automati cal l y generated 

reader andwriter methods. This informati on i s avai l abl e as l i sts of f uncti on names. 

Anyaccessors speci ed i n the def cl ass formare broken downinto thei r equival ent readers 

andwriters in thedi rect sl ot denition. 

Informati on, incl udinginheri ted i nformation,whichapplies tothedenitionof aslot in 

aparticular class in whi ch i t i s accessi bl e i s associ ated only with eecti ve slotdenition 

metaobj ects. 

For certain slots, the l ocati on of the slotininstances of the class is avai l able. 

eneri c 

uncti ons 

Ageneri c unct i on met aobj ect containsinformati on about a generi c functionover andabove 

theinformati on associ ated wi th each of the generi c functi on's methods. 

The name isavai l abl e as a f uncti on name. 

The methods associ ated with the generi c functi on are avai l able as a list of method 

metaobj ects. 

The defaul t cl ass f or thi s generi c functi on's method metaobj ects i s available as a class 

metaobj ect. 

The lambdalist is available as alist. 

The method combinati on i s avai l able as a method combinati onmetaobj ect. 


The argument precedence order i s avai l able as apermutationof those symbol s f romthe 

lambdalist whichname therequi red arguments of thegeneri c functi on.

6 CHAPTER 5 . CONCEPTS 

The decl arati ons areavai l able as alist of decl arati ons. 

erminology ote: 

There i s someambiguityinCommon Lispabout thetermsusedto identifythevari ous 

parts of decl are speci al forms. Inthis document, thetermdecl arat i on is usedtorefer 

to an obj ect that couldbeanargument toadecl are speci al form. For example, in 

thespeci al form(decl are (speci al *g1*)), the list (speci al *g1*) is adecl arati on. 

et o s 

Amet hod met aobj ect containsinformati on about aspeci c method. 

The qualiers are avai l able as alist of of non-nul l atoms. 

The lambdalist is available as alist. 

The speci al i zers are avai l able as alist of speci al i zer metaobj ects. 

The functi on i s availabl e as a f uncti on. This functi on can be appliedtoarguments and 

alist of next methods using applyor funcall. 

When themethodi s associ ated wi th a generi c functi on, that generi c functi onmetaobj ect 

is avai l able. Amethod can be associ ated withat most one generi c functi on at a ti me. 


S 

ecializers 

Aspeci al i zer metaobj ect represents the speci al i zers of a method. Class metaobj ects are 

themselves speci al i zer metaobj ects. Aspeci al kindof speci al i zer metaobj ect i s used for eql 

speci al i zers. 

et o o inati ons 

Amet hod combi nat i on met aobj ect represents theinformati onabout themethod combination 

bei ngused by a generi c functi on. 

ote: 

This document does not speci fy the structure of method combinati on metaobj ects. 

In eritance tructure o etaobject lasses 

The inheri tance structure of thespeci ed metaobj ect cl asses i s shown inTabl e 5. 1. 

Each cl ass marked witha\3" isanabst ract cl ass and is not i ntended to be instantiated. 

The results are undened i f an attempt ismade tomake aninstance of one ofthese classes 

withma e- i nstance.

NHER TANCE S TR CT RE O ETAO JECT C ASSES 7 

Metaobject Class i rect upercl asses 

standard-object 

(t) 

funcallable-standard-object (standard-object functi on) 

3 metaobject (standard-object) 

3 generi c- f uncti on (metaobject 

funcallable-standard-object) 

standard-generi c- f uncti on (generi c- f uncti on) 

3 method (metaobject) 

standard-method 

(method) 

3 standard-accessor-method (standard-method) 

standard-reader- method (standard-accessor-method) 

standard-wri ter-method (standard-accessor-method) 

3 method-combinati on (metaobject) 

3 slot-denition (metaobject) 

3 direct-slot-denition (slot-denition) 

3 e ecti ve- sl ot-denition (slot-denition) 

3 standard-slot-denition (slot-denition) 

standard-direct-slot- (standard-slot-denition 

denition 

di rect- sl ot-denition) 

standard-e ecti ve- sl ot- (standard-slot-denition 

denition 

e ecti ve- sl ot-denition) 

3 speci al i zer (metaobject) 

eql-speci alizer 

(speci alizer) 

3 cl ass (speci alizer) 

built-in-class 

(class) 

forward- ref erenced-class (class) 

standard-class 

(class) 

funcallable-standard-class (class) 

a e .1 r r r . 

r a a - a r 

- - a e e - 

a a - e e - r a a e- 

a a - a .


The classes standard-class, standard-di rect- sl ot-denition, standard-e ecti veslot-denition, 

standard-method, standard-reader- method, standard-writermethod 

and standard-generic-functi on are cal l ed standard met aobj ect cl asses. For 

each kindof metaobj ect, this is the class theuser interface macros presented in theCLOS 

Speci cati on use bydefault. These are also the classes on whichuser specializations are 

normal l y based. 

The classes built-in-class, funcallable-standard-class and forward- ref erencedcl 

ass are speci al -purpose cl ass metaobj ect cl asses. Built-in classes are instances of the 

cl ass built-in-class. The class funcallable-standard-class provides a speci al kindof 

instances descri bed i n the section called \Funcal lable Instances. " When the denition of 

a class references another cl ass which has not yet been dened, an instance of forwardreferenced-class 

is used as a stand-inuntil the class is actuallydened. 

The classstandard-object is the de aul t di rect su ercl ass of the classstandard-class. 

When an i nstance of the class standard-class i s created, andnodi rect superclasses are 

explicitly speci ed, it defaults to theclass standard-object. In this way, anybehavior 

associ ated withtheclassstandard-object will beinheri ted, directly or indirectly, byall 

instances of the classstandard-class. Asubcl ass of standard-class may have adierent 

cl ass as i ts default di rect supercl ass, but that cl ass must beasubcl ass of the classstandardobject. 

The same is trueforfuncallable-standard-class and funcallable-standard-object. 

The classspeci alizercaptures onlythemost basi c behavior of method speci al i zers, and 

is not itself intended to be instantiated. The classcl ass is adi rect subcl ass of speci alizer 

re ecti ng theproperty that cl asses by themselves can be used as method speci al i zers. The 

cl ass eql-speci alizer is used for eql speci al i zers. 

le entati on an ser S ecializati on 

The purpose of the Metaobj ect Protocol is to provideusers withapowerful mechanismfor 

extendingandcustomizingthebasi c behavior of theCommon LispObject System. Asan 

object-ori ented descri pti on of the basi c CLOS behavior, theMetaobj ect Protocol makes i t 

possible to create these extensionsbydeningspeci al i zed subclasses of existingmetaobj ect 

classes. 

The Metaobj ect Protocol provides this capability without interferingwith the implementor's 

abilityto devel op high-performance i mplementations. This bal ance between user 

extensibilityandimplementor freedomis mediatedbyplacingexplicit restrictions on each. 

Some ofthese restrictions are general |they apply to theentire class graphandtheapplicabilityof 

all methods. These are presented in this section. 

The followingadditional terminol ogy i s used to presentthese restrictions:

NHER TANCE S TR CT RE O ETAO JECT C ASSES 9 

Metaobj ects are divided i nto three categori es. Those dened i n this document are 

called s eci ed; those dened by animplementati on but not mentioned i n this document 

are called imlementation-s eci c; andthose dened byaportable programare 

called ort abl e. 

Aclass is inter osed between twoother cl assesand if andonlyif there i s some 

path, followingdi rect superclasses, fromtheclass to theclass whichincl udes . 

Amethod i s s eci al i ed t o a class if andonly if that cl ass i s i n the list of speci al i zers 

associ ated withthemethod; andthemethodis inthe list of methods associ ated with 

somegeneri c functi on. 

Ina givenimplementation,aspeci ed methodis saidtohave been romot ed if andonly 

if thespeci al i zers of the method,: :: ,aredened i n this speci cati on as the classes 

::: , but intheimplementation,oneormore of the speci al i zers i, is a supercl ass 

of the cl ass gi ven i n the speci cati i. on 

For a gi ven generi c functi on and set of arguments, a method e tends amethod 

if andonlyif: 

(i) and are both associ ated withthegiven generi c functi on, 

(ii) and are both applicable to thegiven arguments, 

(iii) the speci al i zers and qualiers of the methods are such that when the generi c 

functionis called, is executed bef ore , 

(iv) will beexecuted if andonlyif call-ne t-methodis invoked fromwithinthe 

body of and 

(v) call-ne t-method is invoked fromwithinthebody of , therebycausing 

to beexecuted. 

For a gi ven generi c functi on and set of arguments, a method o erri des amethod 

if andonlyif conditionsithrough i v above hol d and 

,therebyprevent- 

(v) call-ne t-methodis not i nvoked fromwithinthebody of 

ing frombei ngexecuted. 

estrictions on mplementations 

Implementations are allowedlatitude tomodifythe structure of speci ed cl asses andmethods. 

This incl udes: theinterpositionof implementation-speci c classes; thepromoti on of 

speci ed methods; andthe consolidati on of two or more speci ed methods into a singl e 

method speci al i zed to i nterposed classes. 

Any such modicati ons arepermitted onlyso long as f or any portabl e cl ass p that i s 

asubcl ass of one ormore speci ed cl asses::: i, the followingconditionsaremet:


In theactual cl ass precedence list of p, the classes ::: i must appear i n the same 

order as they wouldhave if noimplementation-speci c modicati ons had been made. 

The method applicabi l i ty of any speci ed generi c functi on must bethesameinterms 

of behaviorasitwouldhave been had no implementation-speci c changes been made. 

This incl udes speci ed generi c functi ons that have had portable methods added. In this 

context, theexpression \thesameintermsofbehavior" means that methods withthe 

samebehavior as those speci ed are applicable, andinthesameorder. 

 

oportabl e cl ass p may inheri t, byvirtueofbei ng adirect or indi rect subcl ass of a 

speci ed cl ass, any sl ot f or whichthename isasymbol accessi ble inthe common-lispuser 

package or exported byanypackage dened i n the ASI Common Lisp standard. 

Implementations are free to dene implementation-speci c before- and after-methods 

on speci ed generi c functi ons. Implementations are also free to dene implementationspeci 

c around-methods withextendingbehavior. 

estrictions on ortable rograms 

Portableprograms are allowed to dene subclassesofspeci ed classes, andarepermittedto 

dene methods onspeci ed generi c functi ons, withthe following restrictions. The results 

are undened i f anyof these restrictionsisviolated. 

Portableprograms must not redene anyspeci ed classes, generi c functi ons, methods or 

method combinati ons. Anymethod dened byaportable programon a speci ed generi c 

functi on must have atleastonespeci al i zer that i s nei ther a speci ed cl ass nor an eql 

speci al i zer whose associ ated val ue isaninstance of a speci ed cl ass. 

Portableprograms may dene methods that extendspeci ed methods unless thedescri p- 

ti on of the speci ed method explicitlyprohibits this. Unless there i s a speci c statement 

to thecontrary, these extendingmethods must return whatever val ue was returned by 

the cal l to call-ne t-method. 

Portable programs may dene methods that overridespeci ed methods onlywhen the 

descri pti on of the speci ed method explicitlyallows this. Typically, when a method i s 

allowed to be overridden, asmal l number of related methods will need to be overridden 

as well. 

An example of this is thespeci ed methods onthegeneri c functi ons add-dependent, 

remove- dependent and map-dependents. Overridingaspeci ed method on one of 

these generi c functi ons requires that the correspondingmethod on the other twogeneri c 

functi ons beoverridden as well. 

Portablemethods onspeci ed generi c functi onsspeci al i zedto portablemetaobj ect cl asses 

must bedened bef ore any instances of those cl asses (or anysubclasses) are created, either 

directly or indirectly byacall toma e- i nstance. Methods can be dened af ter

PROCES S N O THE SER NTER ACE ACROS 11 

instances are created by allocate-instance however. Portable metaobj ect cl asses cannot 

be redened. 

mplementation ote: 

The purpose of this last restriction is to permit implementations toprovideperformance 

optimi zati ons byanal yzi ng, at the timetherst instance of a metaobj ect cl ass 

is initialized,what portablemethods will beapplicabletoit. This canmake itpossible 

to optimize calls tothose speci ed generi c functi ons whichwouldhave noapplicable 

portable methods. 

ote: 

The speci cati on technol ogy used in this document needs further devel opment. The 

concepts of obj ect- ori ented protocols andsubcl ass specialization are intuitivel y familiar 

to programmers of object-ori ented systems; the protocols presented here t 

quite naturally into this framework. onethel ess, i n prepari ng this document, we 

have found it di cul t to gi ve speci cati on-qualitydescri ptions of theprotocols in a 

way that makes i t cl ear what extensionsusers can andcannot write. Object-ori ented 

protocol speci cation is inherently about speci fying l eeway, andthi s seems di cult 

usingcurrent technol ogy. 

rocessi n o t e ser Inter ace acros 

Alist inwhichtherst el ement isoneofthesymbol s def cl ass, def method, def generi c, 

dene- method-combination, generic-functi on, generi c- et or generi c- l abel s, and 

whichhas proper syntaxfor that macrois calledauser inter ace macro orm. Thisdocument 

provides an extended speci cati on of the def cl ass, def method and def generi c macros. 

The user interface macros def cl ass, def generi c and def methodcanbeusednot onlyto 

dene metaobj ects that are i nstances of the corresponding standard metaobj ect cl ass, but 

also to dene metaobj ects that are i nstances of appropriate portable metaobj ect cl asses. 

To make itpossible for portable metaobj ect cl asses to properl y process the information 

appeari nginthemacroform,this documentprovidesalimitedspeci cati on of the processing 

of these macro forms. 

User interface macro forms can be e al uat ed or com iled and later e ecuted. The eect 

of eval uati ng or executing a user interface macro formis speci ed in terms of calls to 

speci ed functi ons andgeneri c functi ons whichprovidetheactual behavior of themacro. 

The arguments recei ved bythese functi ons andgeneri c functi ons arederi ved i n a speci ed 

way fromthemacro form. 

Convertingauser interface macro forminto thearguments to theappropri ate f uncti ons 

and generi c functi ons has two maj or aspects: theconversionof themacro argument syntax 

into a f ormmore suitabl e f or l ater processing, andtheprocessingof macro arguments which 

are f orms tobeeval uated (i ncl udingmethod bodies).


In thesyntax of the def cl ass macro, the init ormand de aul t-initarg-initial- al ue- orm 

arguments are f orms which will be eval uated one or more ti mes af ter the macro formis 

eval uated or executed. Speci al processingmust bedone onthese arguments to ensure that 

thelexi cal scope ofthe f orms iscaptured properl y. This is done bybuildingafuncti on of 

zero arguments which, when cal l ed, returnstheresult of eval uati ng the f ormintheproper 

lexical environment. 

In the syntax of the def method macro the orm argument is a list of forms that 

comprise thebody of themethod denition. This list of formsmust beprocessed speci al l y 

to capture thelexi cal scope ofthemacro form. In addition, thelexical functi ons avai l able 

only in the body of methods must be introduced. To allowthis andanyother speci al 

processing (such as sl ot access optimization), a speci al i zable protocol is used f or processing 

thebody of methods. This is discussed in the secti on \ProcessingMethod Bodies." 

o ile le rocessi n o t e ser nter ace acros 

It is common practi ce f or Common Lispcompilers, while processingale or set of les, to 

mai ntain informati on about thedenitionsthat have been compiledso far. Among other 

things, this makes i t possible to ensure that a gl obal macro denition (def macro form) 

whichappears inale will aect uses of themacrolater inthat le. This informati onabout 

the state of thecompilationis calledthe com ile- le en ironment . 

When compilingles containingCLOS denitions, it is useful to mai ntain certain additional 

i nformati on i n the compile-le environment. This can make itpossible to issue 

vari ous kinds of warnings (e.g., lambda list congruence) and todovari ous performance 

optimi zati ons that wouldnot otherwise bepossible. 

At this time, there i s such si gnicant vari ance i n the way existingCommon Lisp implementations 

handle compile-le environments that i t wouldbepremature to speci fy this 

mechanism. Consequently, this document speci es onlythebehavior of eval uati ng or executinguser 

interface macro forms. What f uncti ons andgeneri c functi ons are calledduring 

compile-le processingof auser interface macro formis not speci ed. Implementations are 

free to dene anddocument thei r ownbehavior. Users may need to check i mplementationspeci 

c behavior before attemptingtocompile certainportable programs. 

e e class acro 

The eval uati on or executi on of a def cl ass formresults inacall tothe ensure-cl ass functi on. 

The arguments recei ved by ensure-cl ass are deri ved fromthe def cl ass formina dened 

way. The exact macroexpansi on of the def cl ass formis not dened, onlythe rel ati onship 

between the arguments to the def cl ass macro and thearguments recei ved bythe ensurecl 

ass functi on. Examples of typical def cl ass formsandsample expansions are shown in 

Figures 5.1 and5.2.


e class pla e mo i o ec rap ics o ec 

al i e i i orm accessor pla e al i e 

spee 

e a l i i ar s e i e e 

e s re class pla e 

irec s perclasses mo i o ec rap ics o ec 

irec slo s lis lis ame al i e 

i i orm 

i i c io lam a 

rea ers pla e al i e 

ri ers se pla e al i e 

lis ame spee 

irec e a l i i ar s lis lis e i e 

e 

lam a e 

e .1 e a r r 

r r r e e- a . 

The name argument todef cl ass becomes the val ue of therst argument to ensurecl 

ass. This is theonlypositional argument accepted by ensure-cl ass; al l other arguments 

are keyword arguments. 

The di rect -su ercl asses argument todef cl ass becomes the val ue of the :direct-supercl 

asses keyword argument toensure-cl ass. 

The di rect sl ot s argument todef cl ass becomes the val ue ofthe :direct-slots keyword 

argumenttoensure-cl ass. Speci al processingof this val ue isdone toregularize the f orm 

of each sl ot speci cati on and toproperl y capture thelexi cal scope oftheinitialization 

forms. This is done byconvertingeachslot speci cation to a property list called a 

canoni cal i ed sl ot s eci cat i on. The resulting list of canonicalizedslot speci cati ons is 

theval ue of the :direct-slots keyword argument. 

Canonicalizedslot speci cati ons are later used as thekeyword arguments to a generi c 

functi onwhichwill, inturn,pass themto ma e- i nstance for use as a set of i nitialization 

arguments. Each canonicalizedslot speci cation is formed fromthe correspondingslot 

speci cati on as f ol l ows: 

The name oftheslotistheval ue of the :name property. This property appears i n 

every canonicalizedslot speci cati on.


e class ss pla e 

mac ma s ep 

loca or ss mac 

loca or mac loca io 

rea er mac spee 

rea er mac 

me aclass as er class 

a o er op io oo ar 

e s re class ss 

irec s perclasses pla e 

irec slo s lis lis ame mac 

rea ers mac spee mac 

ma s ep 

loca or ss mac mac loca io 

me aclass as er class 

a o er op io oo ar 

e . e a r r 

r r r e e- a . r r 

r r r r r r 

r r r .


When the :initformsl ot opti on i s present in theslotspeci cati on, then both the 

:initformand :initfuncti on properties are presentinthecanonicalizedslot speci - 

cati on. The val ue ofthe :initformproperty is theinitializationform. The val ue 

of the :initfuncti on property is a functi on of zero arguments which, when cal l ed, 

returns theresult of eval uati ng theinitialization f orminits proper l exical environment. 

If the :initformsl ot opti on i s not present in theslotspeci cati on, then ei ther 

the :initfuncti on property will not appear, or i ts val ue will be f al se. I n such cases, 

theval ue of the :initformproperty,orwhether i t appears, is unspeci ed. 

The val ue of the :initargs property is a list of the val ues of each :initarg slot 

option. If there are no :initarg sl ot options, then ei ther the :initargs property will 

not appear or i ts val ue will betheempty list. 

The val ue of the :readers propertyis a list of theval ues of each :reader and :accessor 

sl ot option. If there are no :reader or :accessor sl ot options, then ei ther the 

:readers property will not appear or i ts val ue will betheempty list. 

The val ue of the :wri ters propertyis a list of theval ues speci ed by each :wri ter 

and :accessor sl ot option. The val ue speci ed bya:wri ter sl ot option is just the 

val ue of theslotoption. The val ue speci ed byan:accessor sl ot opti on i s a two 

el ement list: therst el ement isthesymbol setf, the secondelement istheval ue of 

theslotoption. If there are no :wri ter or :accessor sl ot options, then ei ther the 

:wri ters property will not appearoritsval ue will betheemptylist. 

The val ue of the :documentati onpropertyis theval ue ofthe :documentationslot 

option. If there i s no :documentationsl ot option,then ei ther the :documentati on 

property will not appear or i ts val ue will be f al se. 

Al l other sl ot optionsappear as the val ues of properties withthesamename asthe 

sl ot option. ote that this incl udes not only theremai ning standard sl ot options 

(:allocationand :type), but alsoanyother optionsandval ues appeari ngintheslot 

speci cati on. If oneofthese sl ot optionsappears more than once, the val ue of the 

property will be alist of thespeci ed val ues. 

An implementation is free to addadditional properties to the canonicalized slot 

speci cati on provided these are not symbols accessi ble in the common-lisp-user 

package, or exported byanypackage dened i n the ASI Common Lisp standard. 

Returningtothe correspondence between arguments to the def cl ass macro and the 

arguments recei ved bythe ensure-class functi on: 

The de aul t ini t args cl ass option,ifitispresentinthe def cl ass form,becomes the val ue 

of the :di rect- def ault-initargs keyword argumenttoensure-class. Speci al processing 

of this val ue isdone toproperl y capture thelexi cal scope ofthedefault val ue f orms. This 

is done byconvertingeachdefault initargintheclassoptioninto a canoni cal i ed de aul t 

ini t arg. The resulting list of canonicalizeddefault ini targs i s the val ue of the :directdef 

ault-initargs keyword argument toensure-cl ass.


Acanonicalizeddefault initargis alist of three elements. The rst el ement isthename; 

the secondis theactual f ormitself; andthethirdis afuncti on of zero arguments which, 

when cal l ed, returnstheresult of eval uati ng thedefault val ue f orminits proper l exical 

environment. 

The met acl ass cl ass option, if it is presentinthe def cl ass form, becomes the val ue of 

the :metacl ass keyword argument toensure-cl ass. 

The documentation cl ass option,ifitispresentinthe def cl ass form,becomes the val ue 

of the :documentation keyword argument toensure-class. 

Any other cl ass optionsbecome theval ue of keyword arguments withthesamename. 

The val ue of thekeyword argument isthe tai l of the classoption. An error is signal ed 

if anyclass opti on appears more than once i n the def cl ass form. 

In the cal l to ensure-cl ass, every el ement of i ts arguments appears in thesameleftto-rightorder 

as the correspondingelement ofthe def cl ass form,except that the order of 

theproperti es of canonicalized slot speci cati ons is unspeci ed. The val ues of properties 

in canonicalized slot speci cati ons do followthi s orderi ng requirement. Other orderi ng 

relationshipsinthekeyword arguments to ensure-cl ass are unspeci ed. 

The result of the cal l to ensure-cl ass is returned as theresult of eval uati ngor executing 

the def cl ass form. 

e e et o acro 

The eval uati on or executi on of a def methodformrequires rst that the bodyof themethod 

be convertedto a method f uncti on. This process i s descri bedinthenext secti on. The result 

of this process i s a method f uncti on andaset of additional i nitializationarguments to be 

used when creati ngthenewmethod. Given these twoval ues, the eval uati on or executi on of 

a def method formproceeds inthree steps. 

The rst step ensures theexistence of a generi c functi on withthespeci ed name. This 

is done bycallingthefuncti on ensure-generic-functi on. The rst argument inthis call 

is thegeneri c functi on name speci ed i n the def method form. 

The secondstepis the creati on of the newmethodmetaobj ect bycalling ma e- i nstance. 

The class of the new method metaobj ect i s determined by calling generi c- f uncti onmethod-class 

on the result of the cal l to ensure-generi c- f uncti on fromtherst step. 

The initializationarguments recei ved bythe cal l to ma e- i nstance are as f ol l ows: 

The val ue of the :qualiers initialization argument is a list of the qualiers which 

appeared i n the def method form. ospeci al processingisdone onthese val ues. The 

order of the elements of this list is thesameasinthe def method form. 

The val ue of the :lambda-list initializationargument istheunspeci al i zed l ambdalist 

fromthe def method form.


The val ue ofthe :speci alizers initializationargument is a list of the speci al i zers for 

themethod. For speci al i zers which are classes, thespeci al i zer i s the classmetaobj ect 

itself. In the case of eql speci al i zers, i t will beaneql-speci alizer metaobj ect obtained 

bycalling intern-eql-speci alizer on the result of eval uati ng the eql speci al i zer formin 

thelexical environment ofthe def method form. 

The val ue ofthe :functi on initializationargument isthemethod f uncti on. 

The val ue ofthe :decl arations initializationargument isalistofthedecl arati ons from 

the def method form. If there are nodecl arati ons inthemacro form,this initialization 

argument either doesn't appear, or appears withaval ue of theempty list. 

The val ue ofthe :documentation initializationargument isthedocumentation string 

fromthe def method form. If there i s nodocumentationstringinthemacro formthis 

initializationargument either doesn't appear, or appears withaval ue of f al se. 

Anyother i nitializationargumentproduced i n conjuncti on withthemethodfunctionare 

al so i ncl uded. 

The implementationis free to incl ude additional i nitializationarguments provided these 

are not symbol s accessi ble inthe common-lisp-user package, or exported byanypackage 

dened i n the ASI Common Lisp standard. 

In thethird step, add-method is calledtoaddthenewl y created method to thesetof 

methods associ ated withthegeneri c functi on metaobj ect. 

The result of the cal l to add-methodis returned as theresult of eval uati ngor executing 

the def method form. 

An example showingatypical def method formandasample expansion is shown in 

Figure 5. 3. The processingof themethod body for this method i s shown inFigure 5. 4. 

rocessi n et o o ies 

Bef ore a method can be created, the list of formscomprisingthe method body must be 

converted to a method f uncti on. This conversionis a two step process. 

ote: 

The body of methods can al so appear i n the :initial-methods opti on of def generi c 

forms. Initial methods arenot considered byanyof theprotocols speci ed i n this document. 

The rst step occurs duringmacro- expansi on of the macro form. In this step, the 

method l ambda list, decl arati ons andbody areconverted to a lambda expression called a 

met hod lambda. This conversionis based on i nformati on associ ated withthegeneri c functi on 

denitionineect at the timethemacro formis expanded.


e me o mo e e ore p posi io l e l 

op io al isi l p 

e color 

se o ori i p 

e isi l p s o mo e p color 

le e s re e eric c io mo e 

a me o 

ma e i s a ce e eric c io me o class 

ali iers e ore 

speciali ers lis i class posi io 

i er e l speciali er 

lam a lis p l op io al isi l p 

e color 

c io c io met hod- l ambda 

a i io al i i ar 

a i io al i i ar 

e . e e r rr . 

r a e- e - a a r r 

. r r r r r 

r r r a e- e - a a. 

le e s re e eric c io mo e 

ma e me o lam a 

class pro o pe e eric c io me o class 

lam a p l op io al isi l p e color 

se o ori i p 


en ironment 

e . r r e e r r . r 

r r r . 

r r e e r r .


The generi c functi on denitionis obtained bycalling ensure-generi c- f uncti on witha 

rst argument ofthegeneri c functi on name speci ed i n the macro form. The :lambda-list 

keyword argument isnot passed in this call. 

Giventhegeneri c functi on, productionof themethod l ambdaproceeds bycalling ma e- 

method-lambda. The rst argument inthis call is thegeneri c functi on obtained as described 

above. The secondargument istheresult of calling cl ass-prototype on the result 

of calling generic-functi on-method-class on the generi c functi on. The thirdargument 

is a lambda expression formed fromthemethod l ambda list, decl arati ons andbody. The 

fourth argument isthemacro- expansionenvironment ofthemacro form;this is theval ue of 

the environment argument tothe def method macro. 

The generi c functi on ma e- method-lambda returns twoval ues. The rst is the 

method l ambda itself. The secondis alist of initializationarguments andval ues. These are 

incl uded i n the initializationarguments when the method i s created. 

Inthe secondstep,themethod l ambdais convertedtoafuncti onwhichproperlycaptures 

thelexi cal scope ofthemacro form. This is done byhavingthemethod l ambda appear i n 

themacro- expansi on as the argument ofthe functi onspeci al form. Duringthesubsequent 

eval uati on of the macro- expansion, theresult of the functi on speci al formis themethod 

functi on. 

e e eneric acro 

The eval uationor executi on of a def generi c formresults inacall tothe ensure-generi c- 

functi onfuncti on. The arguments recei ved by ensure-generi c- f uncti onare deri ved from 

the def generi c forminadened way. Aswithdef cl ass and def method, theexact macroexpansi 

on of the def generi c formis not dened, only the rel ati onshipbetween the arguments 

to themacro and thearguments recei ved by ensure-generic-functi on. 

The unct i on-name argument to def generi c becomes the rst argument to ensuregeneri 

c- f uncti on. This is theonlypositional argument accepted by ensure-generi c- 

functi on; al l other arguments are keyword arguments. 

The lambda- l i st argumenttodef generi c becomes theval ue ofthe :lambda-listkeyword 

argument toensure-generi c- f uncti on. 

For each of the options :argument-precedence-order, :documentati on, :generi c- 

functi on-class and :method-class, theval ue oftheopti on becomes the val ue ofthe 

keyword argument with the samename. If theopti on does not appear i n the macro 

form, thekeyword argument does not appear i n the resulting cal l to ensure-generi c- 

functi on. 

For the option decl are, the list of decl arati ons becomes the val ue of the :decl arations 

keyword argument. If the decl are opti on does not appear i n the macro form,the :declarations 

keyword argument does not appear i n the cal l to ensure-generi c- f uncti on.


The handlingof the :method-combination opti on i s not speci ed. 

The result of the cal l to ensure-generic-functi onis returned as theresult of eval uati ng 

or executingthe def generi c form. 

ub rotocols 

This sectionprovides an overviewof the Metaobj ect Protocols. The detailedbehavi or of each 

functi on, generi c functi on andmacro in theMetaobj ect Protocol is presented in Chapter 6. 

The remai nder of this chapter is intended to emphasi ze connecti ons among the parts of 

theMetaobj ect Protocol, andtoprovidesomeexamples of thekinds ofspecializationsand 

extensionstheprotocols are desi gned to support. 

etao ect nitializati on rotocols 

Likeother objects, metaobj ects can be created bycalling ma e- i nstance. The initialization 

arguments passed to ma e- i nstance are used to initializethemetaobj ect i n the usual way. 

The set of l egal i nitializationarguments, andthei r i nterpretation, depends on thekindof 

metaobj ect bei ngcreated. Implementationsandportableprograms are free toextendtheset 

of l egal i nitializationarguments. Detailedinformati onabout theinitializationof eachkind 

of metaobj ect are provided i n Chapter 6; this sectionprovides an overviewandexamples of 

this behavior. 

nitializationof Class Metaobjects 

Class metaobj ects created withma e- i nstance are usuallyanon mous; that i s, they have 

no proper name. Ananonymous classmetaobj ect can be given a proper name using (setf 

nd-class) and (setf class-name). 

When a cl ass metaobj ect i s created withma e- i nstance, itisinitializedintheusual 

way. The initializationarguments passed to ma e- i nstance are use to establishthedenitionof 

the class. Each i nitializationargument ischecked for errors and associ ated with 

theclassmetaobj ect. The initializationarguments correspond roughlyto thearguments accepted 

bythe def cl ass macro, and more closely to thearguments accepted bythe ensurecl 

ass functi on. 

Some classmetaobj ect cl asses al l owthei r i nstances to be redened. When permissible, 

this is done bycalling rei nitialize-instance. This is discussed in thenext secti on. 

An exampl e of creati ngananonymous classdirectlyusing ma e- i nstance follows: 

le ero 

propellor propellor 

ma e i s a ce s a ar class 

ame m class oo

S PROTOCO S 21 

irec s perclasses lis i class pla e 

anot her- anon mous- cl ass 

irec slo s ame x 

i i orm 

i i c io ero 

i i ar s x 

rea ers posi io x 

ri ers se posi io x 

ame 

i i orm 

i i c io ero 

i i ar s 

rea ers posi io 

ri ers se posi io 

irec e a l i i ar s e i e propellor propellor 

ei nitializationof Class Metaobjects 

Some classmetaobj ect cl asses al l ow thei r i nstances to be reinitialized. This is done by 

calling rei nitialize-instance. The initializationarguments have thesameinterpretation 

as i n cl ass i nitialization. 

If theclassmetaobj ect was nal i zed bef ore the cal l to rei nitialize-instance, nalizeinheri 

tance will be cal l ed agai n once al l the initializationarguments have been processed 

and associ ated with theclassmetaobj ect. In addition, once nalizationis complete, any 

dependents of theclassmetaobj ect will beupdated bycalling update-dependent. 

nitializationof Generi c Functi onandMethodMetaobjects 

An exampl e of creati ng a generi c functi on andamethod metaobj ect, and then addingthe 

methodto thegeneri c functi on i s shownbel ow. This example is comparable to themethod 

denitionshown inFigure 5. 3. 

le ma e i s a ce s a ar e eric c io 

lam a lis p l op io al isi l p e 

me o class e eric c io me o class 

m l iple al e i lam a i i ar s 

ma e me o lam a 

class pro o pe me o class 

lam a p l op io al isi l p e color 

se o ori i p 


il


a me o 

appl ma e i s a ce me o class 

c io compile il lam a 

speciali ers lis i class posi io 

i er e l speciali er 

ali iers 

lam a lis p l op io al isi l p 

e color 

i i ar s 

lass inalizati on rotocol 

Class nal i at i on is theprocess of computingtheinformation a class inheri ts fromits supercl 

asses andprepari ng toactuallyallocate instances of the class. The classnalization 

process i ncl udes computingthe class's class precedence list, thefull set of slots accessible 

in instances of the classandthefull set of default initializationarguments for the class. 

These val ues are associ ated withtheclassmetaobj ect and can be accessed bycallingthe 

appropri ate reader. In addition, theclassnalizationprocess makes deci si ons about how 

instances of the classwill beimplemented. 

To support f orward-referenced superclasses, and to account for the f act that not al l 

classes are actuallyinstantiated,classnalizationis not done aspart of the initializationof 

theclassmetaobj ect. I nstead, nalizationis done asaseparate protocol, invoked bycalling 

thegeneri c functi on nalize-inheri tance. The exact poi nt atwhichnalize-inheri tance 

is calleddepends onthe class of theclassmetaobj ect; f or standard-class it is calledsometime 

af ter al l the classes superclasses are dened, but nolaterthan when the rst instance 

of the class is allocated(by allocate-instance). 

The rst step of class nalizationis computingtheclassprecedence list. Doi ng this rst 

allows subsequent steps to access theclassprecedence list. This step is performed bycalling 

thegeneri c functi on compute-class-precedence-l i st. The val ue returned fromthis call is 

associ ated withtheclassmetaobj ect and can be accessed bycallingthe cl ass-precedencelist 

generi c functi on. 

The second step is computingthefull set of slots that will be accessi ble ininstances of 

the class. This step is performed bycallingthegeneri c functi on compute-slots. The result 

of this call is alist of eecti ve slotdenitionmetaobj ects. This val ue i s associ ated withthe 

cl ass metaobj ect and can be accessed bycallingthe cl ass-slots generi c functi on. 

The behavior of compute-slots is itself layered, consisting of calls to e ecti ve- sl otdenition-class 

and compute-e ecti ve- sl ot-denition. 

The nal step of class nalizationis computingthefull set of initializationarguments 

for the class. This is done bycallingthegeneri c functi on compute-def ault-initargs. The 

val ue returned bythi s generi c functi on i s associ ated withtheclassmetaobj ect and can be 

accessed bycalling cl ass-def ault-initargs.


If theclasswas previously nal i zed, nalize-inheri tance may cal l ma e- i nstancesobsolete. 

The circumstances under whichthis happens are descri be inthe secti on of the 

CLOS speci cati on cal l ed \RedeningClasses." 

Forward-referenced cl asses, whichprovideatemporary denitionfor a class whichhas 

been referenced but not yet dened, can never be nal i zed. An error is signalledif nalizeinheri 

tance is calledonaforward-referenced cl ass. 

nstance Structure 

rotocol 

The instance structure protocol is responsible for implementingthe behavior of the slot 

access functi ons like slot-value and (setf slot-value). 

For each CLOS sl ot access functi on other than slot-e ists-p, there is a corresponding 

generi c functi on whichactuallyprovides the behavior of thefuncti on. When cal l ed, theslot 

access functi on nds thepertinent eecti ve slotdenitionmetaobj ect, cal l s the correspondinggeneri 

c functi on andreturns its result. The arguments passed onto thegeneri c functi on 

incl ude oneadditional val ue, the class of the obj ect argument, whichalways immediately 

precedes the obj ect argument 

The correspondences between sl ot access functi on and underl ying sl ot access generi c 

functionare as follows: 

lot ccess Functi on Corresponding lot ccess 

Generi c Functi on 

slot-boundp 

slot-ma unbound 

slot-value 

(setf slot-value) 

slot-boundp-using-class 

slot-ma unbound-using-class 

slot-value- usi ng-class 

(setf slot-value-using-class) 

At thelowest l evel , the instance structure protocol provides only limitedmechanisms 

for portable programs to control the implementationof instances and todi rectl y access 

the storage associated withinstances without goi ng through the indirectionof slot access. 

This is done toallowportable programs toperformcertai n commonlyrequested sl ot access 

optimi zati ons. 

In particular, portable programs can control theimplementati on of , and obtain di rect 

access to, sl ots with allocation:instance and type t. These are cal l ed di rect l accessi bl e 

sl ot s. 

The rel evant speci ed around-method on compute-slots determines the implementati 

on of i nstances bydeci dinghoweach sl ot i n the instance will be stored. For each directly 

accessi bl e sl ot, this methodallocates alocat i on and associ ates i t withtheeecti ve slotdefinitionmetaobj 

ect. The l ocati on can be accessed bycallingthe slot-denition-location


generi c functi on. Locati ons arenon-negati ve integers. For a gi ven class, the l ocati ons increase 

consecutivel y, in the order that the di rectl y accessi bl e sl ots appear i n the list of 

eecti ve slots. ( ote that here, the next paragraph, andthespeci cati on of thi s aroundmethod 

are the onlypl aces where the val ue returned by compute-slots is descri bed as a 

list rather than a set. ) 

Given the l ocati on of a di rectl y accessi bl e sl ot, the val ue of that sl ot i n an i nstance 

can be accessed with theappropriate accessor. For standard-class, thi s accessor i s the 

functi on standard-instance-access. For funcallable-standard-class, thi s accessor i s 

thefuncti on funcallable-standard-instance-access. I n each case, the arguments to the 

accessor are theinstance andthe slot location, in that order. See the denition of each 

accessor i n Chapter 6 for additional restrictionsontheuse of these functi on. 

Portable programs arepermitted to aect andrelyonthe al l ocati on of l ocati ons only 

in the following limited way: Byrst deningaportable primary method on computeslots 

whichorders the returned val ue inapredictable way, andthen rel yingonthedened 

behavior of thespeci ed around-method to assign locationstoalldi rectl y accessi ble slots. 

Portable programs may compile-incalls to low-level accessors whichtakeadvantage of the 

resultingpredictabl e al l ocati on of sl ot l ocati ons. 

ample: 

Thefollowingexampleshows theuse of this mechanismtoimplementanewcl ass metaobject 

cl ass, ordered-class andclass option:slot-order. This opti on provides control over 

the al l ocati on of sl ot l ocati ons. In this simple example implementation, the :slot-order 

opti on i s not i nheri ted bysubclasses; it controls onlyinstances of the class itself. 

e class or ere class s a ar class 

slo or er i i orm 

i i ar slo or er 

rea er class slo or er 

e me o comp e slo s class or ere class 

le or er class slo or er class 

sor cop lis call ex me o 

lam a a 

posi io slo e i i io ame a or er 

posi io slo e i i io ame a or er 

Fol l owingis thesource codetheuser of this extensi on wouldwrite. ote that because 

thecode above doesn't implement inheri tance of the :slot-order option, thefuncti on 

distance must not be calledoninstances of subclasses of point; i t can onlybe calledon 

instances of point itself. 

e class poi 

x i i orm


i i orm 

me aclass or ere 

slo or er x 

class 

e is a ce poi 

s r / exp s a ar i s a ce access poi 

exp s a ar i s a ce access poi 

. 

In more realistic uses of this mechanism, the calls to the low-level i nstance structure 

accessors wouldnot actuallyappear textuallyinthesource program, but rather would 

be generated byameta- l evel anal ysis programrunduringtheprocess of compilingthe 

source program. 

uncalla le nstances 

Instances of cl asses which are themselves i nstances of funcallable-standard-class or one 

of i ts subclasses are called uncal l abl e i nstances. Funcallable instances can onlybecreated 

by allocate-instance (f uncallable-standard-class). 

Like standardinstances, funcallableinstances have slotswiththenormal behavior. They 

dier fromstandard i nstances i n that they can be used as functi ons aswell; that i s, they 

can be passed to funcall and apply,andthey can be stored as thedenitionof afuncti on 

name. Associ ated witheachfuncallable instance i s the functi on whichit runs when it is 

called. This functi on can be changed withset-funcallable-instance-f uncti on. 

ample: 

The followingsimple example shows theuse of funcallable instances to create a si mple, 

defstruct-like facility. (Funcallable instances are useful when a programneeds to construct 

andmai ntainaset of functi ons andinformati on about those f uncti ons. They make 

it possible tomai ntainboth as the same object rather than two separate obj ects l i nked, 

for example, byhash tables.) 

e class co s r c or 

ame i i ar ame accessor co s r c or ame 

iel s i i ar iel s accessor co s r c or iel s 

me aclass calla le s a ar class 

e me o i i iali e i s a ce a er c co s r c or e 

i slo s ame iel s c 

se calla le i s a ce c io 

c 

lam a 

le e ma e arra le iel s


se are e ame 

e 

se c ma e i s a ce co s r c or 

ame posi io iel s x 

se p call c 

eneric uncti on n ocati on rotocol 

Associ ated witheachgeneri c functi on i s i ts discriminati ng functi on. Each ti me thegeneri c 

functionis called,thediscriminati ngfunctionis calledtoprovidethebehavior of thegeneri c 

functi on. The discriminati ng functi on recei ves the full set of arguments recei ved by the 

generi c functi on. It must lookup andexecute the appropriate methods, andreturn the 

appropriate val ues. 

The discriminati ngfuncti oni s computedbythehighest l ayer of thegeneri c functi oni nvocati 

on protocol, compute-discriminati ng-functi on. Whenever a generi c functi on metaobject 

is initialized,reinitialized,oramethodis added or removed, thediscriminati ngfunction 

is recomputed. The newdiscriminati ng functi on i s then stored withset-funcallableinstance-functi 

on. 

Discriminati ngfuncti ons cal l compute-applicable-methods and compute-applicablemethods-using-classes 

to compute themethods applicable to thegeneri c functi ons arguments. 

Applicable methods arecombined by compute-e ecti ve-method to produce an 

e ect i e met hod. Provisionsaremade to allowmemoizationof themethod applicability 

and eecti ve methods computations. (See the descri pti on of compute-discriminati ngfuncti 

on for details.) 

The body of method denitionsareprocessed by ma e- method-lambda. The result 

of thi s generi c functionis alambda expression whichis processed byeither compileor the 

le compiler toproduce a met hod unct i on. The arguments recei ved bythemethod f uncti on 

are controlled bythe call-method formsappeari ng intheeecti ve methods. By default, 

method f uncti ons accept twoarguments: a list of arguments to thegeneri c functi on, anda 

list of next methods. The list of next methods corresponds tothenext methods argument 

to call-method. If call-method appears withadditional arguments, these will bepassed 

to themethod f uncti ons aswell; inthese cases, ma e- method-lambda must have created 

themethod l ambdas to expect additional arguments. 

e en ent aintenance rotocol 

It is convenient f or portable metaobj ects to be able to memoize informati on about other 

metaobj ects, portabl e or otherwise. Because cl ass and generi c functi on metaobj ects can


be reinitialized, andgeneri c functi on metaobj ects can be modied byaddingandremoving 

methods, a means must beprovided to update this memoi zed i nformation. 

The dependent mai ntenance protocol supports this byprovidingaway to register an 

obj ect whichshouldbenoti ed whenever a cl ass or generi c functi on i s modied. An obj ect 

which has been registered this wayis called a de endent of the cl ass or generi c functi on 

metaobj ect. The dependents of cl ass and generi c functi on metaobj ects are mai ntained with 

add-dependent and remove- dependent. The dependents of a cl ass or generi c functi on 

metaobj ect can be accessed withmap-dependents. Dependents are noti ed about amodicati 

on bycalling update-dependent. (See the speci cati on of update-dependent for 

detaileddescri pti on of the circumstances under whichit is called.) 

To prevent conicts between twoportable programs, or between portable programs and 

theimplementation,portable codemust not register metaobj ects themselves as dependents. 

Instead, portable programs which need to record a metaobj ect as a dependent, should 

encapsulate that metaobj ect i n some other kind of obj ect, and record that obj ect as the 

dependent. The results are undened i f this restriction is violated. 

ample: 

This example shows a general facility for encapsulatingmetaobj ects before recording 

themas dependents. The facilitydenes a basi c kindof encapsulating obj ect: an updater. 

Specializationsofthebasi c cl ass can be dened withappropri ate speci al updati ng 

behavior. In this way, informati on about theupdati ng requi red i s associ ated wi th each 

updater rather than withthemetaobj ect bei ngupdated. 

Updaters are used to encapsulate anymetaobj ect which requires updati ng when a 

gi ven cl ass or generi c functi on i s modied. The functi on record-updater is called to 

both create an updater and addit tothedependents of the cl ass or generi c functi on. 

Methods onthegeneri c functi on update-dependent, speci al i zed to the speci c classof 

updater do theappropriate update work. 

e class p a er 

epe e i i ar epe e rea er epe e 

e recor p a er class epe ee epe e res i i ar s 

le p a er appl ma e i s a ce class epe e epe e 

i i ar s 

a epe e epe ee p a er 

p a er 

A ush-cache- updater simply ushes the cache ofthedependentwhenitis updated. 

e class l s cac e p a er p a er 

e me o p a e epe e epe ee p a er l s cac e p a er 

res ar s


eclare i ore ar s 

l s cac e epe e p a er

a 

ter 

ene n t ons n 

et o s 

This chapter descri bes each of the functi ons andgeneri c functi ons that make uptheCLOS 

Metaobj ect Protocol. The descri ptionsappear inalphabeti cal order withtheexception that 

al l the reader generi c functi ons for each kindof metaobj ect are grouped together. So, f or 

example, method-functi on wouldbefound withmethod-qualiers and other method 

metaobj ect readers under \Readers f or Method Metaobj ects. " 

The descri pti on of f uncti ons follows thesame f ormas used in theCLOS speci cati on. 

The descri pti on of generi c functi ons issimilar to that i n the CLOS speci cati on, but some 

minor changes have been made intheway methods arepresented. 

Thefollowingis anexampleof the f ormat f or the syntaxdescri pti on of a generi c functi on: 

gf1 

op io al 

e 

This descri ption indicates that gf1 is a generi c functi on with tworequired parameters, 

and , an optional parameter and akeyword parameter . 

The descri pti on of a generi c functionincl udes a descri pti on of i ts behavior. This provides 

thegeneral behavi or, or protocol of thegeneri c functi on. All methods dened on thegeneri c 

functi on, both portable andspeci ed, must have behavior consistentwiththis descri ption. 

Every generi c functi on descri bed i n this section is an instance of the classstandardgeneri 

c- f uncti on and uses standard method combinati on. 

The descri pti on of a generi c functionalsoincl udes descri ptionsofthespeci ed methods 

for that generi c functi on. In thedescri pti on of these methods, a met hod signat ure is used 

to descri be theparameters andparameter speci al i zers of each method. The followingis an 

example of the f ormat f or a method si gnature: 

29

30 add-dependent Chapter 6 

gf1 ri mar et hod 

cl ass op io al e 

This signature indicates that this primary method on the generi c functi on gf1has two 

required parameters, named and . In addition, there i s an optional parameter 

and akeyword parameter . This signature also indicates that the method's parameter 

speci al i zers are the classes named cl ass and t. 

The descri pti on of each method i ncl udes a descri pti on of the behavior particular to 

that method. 

An abbreviatedsyntax is used when referringtoamethod dened el sewhere i n the 

document. This abbreviatedsyntax incl udes the name of thegeneri c functi on, thequal - 

iers, andthe parameter speci al i zers. A reference to the method with the signature 

shown above iswritten as: gf1 (class t). 

a e en ent eneri c unct i on 

S 

add-dependent 

met aobj ect de endent 

A 

The met aobj ect argument isaclassorgeneri c functi on metaobj ect. 

The de endent argument i s an obj ect. 

The val ue returned bythi s generi c functi on i s unspeci ed. 

P 

Thi s generi c functi on adds de endent to the dependents of met aobj ect . If de endent is 

al ready inthe set of dependents it is not added agai n (no error is signal ed). 

The generi c functi on map-dependents can be cal l ed to access the set of dependents 

of a cl ass or generi c functi on. The generi c functi on remove- dependent can be calledto 

remove an obj ect f romthe set of dependents of a cl ass or generi c functi on. The eect of 

calling add-dependent or remove- dependent while acall to map-dependents on the 

same cl ass or generi c functionis inprogress i s unspeci ed. 

The situati ons inwhichadd-dependent is calledare not speci ed. 

M 

add-dependent ri mar et hod 

cl ass s a ar class de endent 

obehavior is speci ed for this method beyond that whichis speci ed for the generi c 

functi on.

Generi c Functi ons andMethods add-di rect-method 31 

This method cannot be overridden unless the followingmethods are overridden as 

well: 

remove-dependent (standard-class t) 

map-dependents (standard-class t) 


cl ass calla le s a ar class de endent 


functi on. 


well: 

remove-dependent (funcallable-standard-class t) 

map-dependents (funcallable-standard-class t) 


generi c- unct i on s a ar e eric c io de endent 


functi on. 


well: 

remove-dependent (standard-generic-functi ont) 

map-dependents (standard-generic-functi ont) 

R 

See the\Dependent Mai ntenance Protocol" sectionfor remarks about theuse of this facility. 

a irect et o eneri c unct i on 

S 

add-direct-method 

s eci al i er met hod 

A 

The s eci al i er argument isaspeci al i zer metaobj ect. 

The met hod argument isamethod metaobj ect. 

The val ue returned bythi s generi c functi on i s unspeci ed.

32 add-di rect-subcl ass Chapter 6 

P 

Thi s generi c functionis calledtomai ntaina set of backpoi nters fromaspeci al i zer to the set 

of methods speci al i zed to i t. If met hod i s al ready intheset, itisnot added agai n (no error 

is signal ed). 

Thi s set can be accessed as a list by callingthegeneri c functi on speci alizer-directmethods. 

Methods are removed fromthesetby remove- di rect- method. 

The generi c functi onadd-di rect- methodis calledby add-methodwhenever a method 

is added to a generi c functi on. It is calledonce f or each of the speci al i zers of the method. 

ote that i n cases where a speci al i zer appears more than once i nthespeci al i zers of a method, 

thi s generi c functi on will be calledmore than once withthesamespeci al i zer as argument. 

The results are undened i f the s eci al i er argument isnot one ofthespeci al i zers of the 

met hod argument. 

M 

add-direct-method ri mar et hod 

s eci al i er class 

met hod me o 

This method i mplements thebehavior of thegeneri c functi on f or cl ass speci al i zers. o 

behavior is speci ed for this method beyond that which is speci ed for the generi c 

functi on. 


well: 

remove-di rect- method(class method) 

speci alizer-di rect- generi c- f uncti ons (class) 

speci alizer-di rect- methods (class) 

add-direct-method ri mar et hod 

s eci al i er e l speciali er 

met hod me o 

This method i mplements thebehavior of thegeneri c functi on for eql speci al i zers. o 


functi on. 

a irect su class eneri c unct i on 

S 

add-direct-subcl ass 

su ercl ass subcl ass 

A 

The su ercl ass argument isaclassmetaobj ect.

Generi c Functi ons andMethods add-method 33 

The subcl ass argument isaclassmetaobj ect. 


P 

Thi s generi c functionis calledto mai ntai n a set of backpoi nters froma class to its di rect 

subclasses. Thi s generi c functi on adds subcl ass to the set of di rect subclasses of su ercl ass. 

Whenaclass is initialized,thi s generi c functionis calledonce for each di rect supercl ass 

of the class. 

When a class is reinitialized, thi s generi c functionis calledonce f or each added di rect 

supercl ass of the class. The generi c functi on remove- direct-subcl ass is called once for 

each del eted di rect supercl ass of the class. 

M 

add-direct-subcl ass ri mar et hod 

su ercl ass class 

subcl ass class 


functi on. 


well: 

remove-direct-subcl ass (class class) 

cl ass-direct-subcl asses (class) 

a et o eneri c unct i on 

S 

add-method 

generi c- unct i on met hod 

A 

The generi c- unct i on argument i s a generi c functi on metaobj ect. 


The generi c- unct i on argument isreturned. 

P 

Thi s generi c functi on associ ates an unattached method wi th a generi c functi on. 

An error is signal ed i f the lambdalist of themethod i s not congruent withthelambda 

list of thegeneri c functi on. An error is alsosignal ed i f the method i s al ready associ ated 

withsomeother generi c functi on.

34 allocate-instance Chapter 6 

If thegivenmethod agrees withanexistingmethod of the generi c functiononparameter 

speci al i zers andqualiers, theexistingmethod i s removed bycalling remove-method before 

thenewmethodis added. See the secti on of the CLOSSpeci cati on cal l ed \Agreement 

on Parameter Speci al i zers and ualiers" for a denitionof agreement inthis context. 

Associatingthemethod withthegeneri c functi on then proceeds in four steps: (i) add 

met hod to the set returned by generi c- f uncti on-methods and arrange f or methodgeneri 

c- f uncti on to return generi c- unct i on; (ii) call add-di rect- method f or each of the 

method's specializers; (iii) call compute-discriminating-functi on and install its result 

withset-funcallable-instance-functi on; and(iv)update the dependents of thegeneri c 

functi on. 

The generi c functi on add-method can be calledbytheuser or theimplementation. 

M 

add-method ri mar et hod 

generi c- unct i on s a ar e eric c io 

met hod s a ar me o 


functi on. 

allocate instance eneri c unct i on 

S 

allocate-instance 

cl ass res ini t args 

A 

The cl ass argument isaclassmetaobj ect. 

The ini t args argument consists of alternati ng initializationargument names andval ues. 

The val ue returned i s a newlyallocatedinstance of cl ass. 

P 

Thi s generi c function is called to create a new, uninitializedinstance of a class. The interpretation 

of theconcept ofan\uninitialized"instance depends ontheclassmetaobj ect 

class. 

Before al l ocati ngthenewi nstance, cl ass-nalized-pis calledto see if cl ass has been 

nal i zed. If it has not been nal i zed, nalize-inheritance is calledbef ore the newi nstance 

is allocated.

Generi c Functi ons andMethods class-::: 35 

M 

allocate-instance ri mar et hod 

cl ass s a ar class res ini t args 

This method allocates storage in theinstance for each sl ot withallocation:instance. 

These sl ots are unbound. Slots withanyother al l ocati on are i gnored bythis method (no 

error is signal ed). 


cl ass calla le s a ar class res ini t args 

This method allocates storage in theinstance for each sl ot withallocation:instance. 

These sl ots are unbound. Slots withanyother al l ocati on are i gnored bythis method (no 

error is signal ed). 

The funcallable instance functi on of the instance i s undened|theresults are unde- 

ned i f theinstance i s appliedtoarguments bef ore set-funcallable-instance-functi on 

has been used to set thefuncallable instance functi on. 


cl ass il i class res ini t args 

This method si gnals an error. 

class eneri c unct i on 

The following generi c functi ons are descri bed together under \Readers for Class 

Metaobj ects" (page 75): cl ass-def ault-initargs, cl ass-di rect- def ault-initargs, cl assdirect-slots, 

cl ass-direct-subcl asses, cl ass-direct-supercl asses, cl ass-nalized-p, 

cl ass-name, cl ass-precedence-l i st, cl ass-prototype and cl ass-slots. 

co ute a lica le et o s eneri c unct i on 

S 

compute-applicable-methods 

generi c- unct i on arguments 

A 


The arguments argument isalistofobjects. 

Thi s generi c functi on returnsapossiblyemptylist of method metaobj ects.

36 compute-applicable-methods-using- cl asses Chapter 6 

P 

Thi s generi c functi on determines the method applicabilityof ageneri c functiongivenalist 

of requi red arguments. The returned list of method metaobj ects i s sorted by precedence 

order with themost speci c method appeari ng rst. If nomethods areapplicable to the 

suppliedarguments theemptylistisreturned. 

When a generi c functi on i s i nvoked, the discriminati ng functi on must determine the 

ordered l i st of methods applicable to thearguments. Dependingonthegeneri c functionand 

thearguments, this is done inoneofthree ways: usingamemoi zedval ue; calling computeapplicable-methods-using-classes; 

or calling compute-applicable-methods. (Refer 

to thedescri pti on of compute-discriminati ng-functi on for thedetails of this process.) 

The arguments argumentispermittedtocontainmore el ements than the generi c functi on 

accepts requi red arguments; in these cases the extra arguments will beignored. An error 

is signal ed i f arguments containsfewer el ements than the generi c functi on accepts required 

arguments. 

The list returned bythi s generi c functi on will not be mutated bytheimplementation. 

The results are undened i f a portable programmutates the list returned bythi s generi c 

functi on. 

M 

compute-applicable-methods ri mar et hod 


arguments 

This method si gnals an error if anymethod of the generi c functi on has a speci al i zer 

whichis nei ther a cl ass metaobj ect nor an eql speci al i zer metaobj ect. 

Otherwise, this method computes thesortedlistofapplicable methods accordingto 

therules descri bed i n the secti on of the CLOS Speci cati on cal l ed \Method Sel ecti on 

and Combinati on." 

This method can be overridden. Because of theconsistency requirements between 

thi s generi c functi on and compute-applicable-methods-usi ng-classes, doi ngsomay 

require al so overriding compute-applicable-methods-usi ng-classes (standardgeneri 

c- f uncti ont). 

co ute a lica le et o s usi n classes eneri c unct i on 

S 

compute-applicable-methods-using-classes 

generi c- unction cl asses 

A 


The cl asses argument is alist of class metaobj ects.

Generi c Functi ons andMethods compute-applicable-methods-using- cl asses 37 

Thi s generi c functi on returns twoval ues. The rst is a possibly empty list of method 

metaobj ects. The secondis either true or f al se. 

P 

Thi s generi c functionis calledtoattempt todeterminethemethod applicabilityof ageneri c 

functiongiven onlythe classes of therequi red arguments. 

If it is possible to completely determinethe ordered l i st of applicable methods based 

onlyonthesupplied classes, thi s generi c functi on returnsthat list as its rst val ue andtrue 

as i ts secondval ue. The returnedlist of method metaobj ects i s sorted byprecedence order, 

themost speci c method comingrst. If nomethods areapplicabl e to arguments withthe 

speci ed classes, theemptylist and true are returned. 

If it is not possible tocompletelydeterminetheordered l i st of applicable methods based 

onlyonthesuppliedclasses, thi s generi c functionreturnsanunspeci ed rst val ue andfalse 

as i ts secondval ue. 

When a generi c functi on i s i nvoked, the discriminati ng functi on must determine the 

ordered l i st of methods applicable to thearguments. Dependingonthegeneri c functionand 

thearguments, this is done inoneofthree ways: usingamemoi zedval ue; calling computeapplicable-methods-using-classes; 

or calling compute-applicable-methods. (Refer 

to thedescri pti on of compute-discriminati ng-functi on for thedetails of this process.) 

The followingconsistency relationshipbetweencompute-applicable-methods-usi ngcl 

asses and compute-applicable-methods must be mai ntained: for anygiven generi c 

functi on and set of arguments, if compute-applicable-methods-using-classes returns 

a second val ue of true, the rst val ue must beequal to theval ue that wouldbereturned 

by a corresponding cal l to compute-applicable-methods. The results are undened i f a 

portable method on ei ther of these generi c functi ons causes this consistency to be violated. 



functi on. 

M 

compute-applicable-methods-using-classes ri mar et hod 


cl asses 

If anymethod of the generi c functionhas a speci al i zer whichis nei ther a cl ass metaobj ect 

nor an eql speci al i zer metaobj ect, this method si gnals an error. 

In cases where the generi c functi on has no methods witheql speci al i zers, or has no 

methods with eql speci al i zers that could be applicabl e to arguments of the supplied 

classes, this method returnstheordered l i st of applicable methods asitsrst val ue and 

true as its secondval ue. 

Otherwise this methodreturnsanunspeci ed rst val ue and f al se as i ts secondval ue.

38 compute-class-precedence-l i st Chapter 6 

This method can be overridden. Because of theconsistency requirements between 

thi s generi c functi on and compute-applicable-methods, doi ng somay require also 

overriding compute-applicable-methods (standard-generi c- f uncti ont). 

R 

Thi s generi c functi on exists to allowuser extensionswhichalter method l ookup rules, but 

whichbase the newrules onlyonthe classes of therequi red arguments, to takeadvantage 

of the class-based method l ookup memoi zati on found inmany implementations. (There i s 

of course norequirement f or an i mplementationto providethis optimi zati on.) 

Such an extensioncanbeimplementedbytwomethods, oneonthi s generi c functionand 

one oncompute-applicable-methods. Whenever the user extensionis ineect, the rst 

method will return a secondval ue of true. This shouldallowthe implementationto absorb 

these cases i nto its ownmemoizationscheme. 

To get appropriate performance, other kinds of extensions may require methods on 

compute-discriminati ng-functi on whichimplement thei r ownmemoi zati on scheme. 

co ute class rece ence list eneri c unct i on 

S 

compute-class-precedence-list 

cl ass 

A 


The val ue returned bythi s generi c functionis alist of class metaobj ects. 

P 

Thi s generi c-functionis calledtodeterminetheclassprecedence list of aclass. 

The result is alist whichcontains each of cl ass and its supercl asses once andonlyonce. 

The rst el ement ofthe list is cl ass and thelastelement istheclassnamed t. 

All methods onthi s generi c functi onmust compute theclassprecedence list as afuncti on 

of the ordered di rect supercl asses of the supercl asses of cl ass . The results are undened i f 

therules used to compute theclassprecedence list dependonanyother factors. 

Whenaclass is nal i zed, nalize-inheri tance calls thi s generi c functionand associ ates 

thereturned val ue with theclassmetaobj ect. The val ue canthen be accessed bycalling 

cl ass-precedence-l i st. 



functi on.

Generi c Functi ons andMethods compute-default-ini targs 39 

M 

compute-class-precedence-list ri mar et hod 

cl ass class 

This method computes theclassprecedence l i st accordingtotherules descri bed i n the 

secti on of the CLOS Speci cati on cal l ed \DeterminingtheClass Precedence List." 

This method si gnals anerror if cl ass or anyof its superclasses is a forwardreferenced 

class. 

This method can be overridden. 

co ute e ault initar s eneri c unct i on 

S 

compute-def ault-initargs 

cl ass 

A 


The val ue returned bythi s generi c function is a list of canonicalized default initialization 

arguments. 

P 

Thi s generi c-functionis calledtodeterminethedefault initializationarguments for a class. 

The result is alist of canonicalizeddefault initializationarguments, withnoduplication 

among initializationargument names. 

All methods onthi s generi c functi on must compute thedefault initializationarguments 

as a f uncti on of only: (i) the class precedence list of cl ass , and (ii) the di rect default 

initializationarguments of each cl ass i n that list. The results are undened i f the rules used 

to compute thedefault initializationarguments dependonanyother factors. 

Whenaclass is nal i zed, nalize-inheri tance calls thi s generi c functionand associ ates 

thereturned val ue with theclassmetaobj ect. The val ue canthen be accessed bycalling 

cl ass-def ault-initargs. 



functi on.

40 compute-discriminati ng- f uncti on Chapter 6 

M 

compute-def ault-initargs ri mar et hod 

cl ass s a ar class 

compute-def ault-initargs ri mar et hod 

cl ass calla le s a ar class 

These methods compute thedefault initializationarguments accordingtotherules described 

i n the secti on of the CLOS Speci cati on cal l ed \Defaultingof InitializationArguments." 

These methods signal an error i f cl ass or anyof its superclasses is aforwardreferenced 

class. 

These methods can be overridden. 

co ute iscri inati n uncti on eneri c unct i on 

S 

compute-discriminati ng-functi on 

generi c- unct i on 

A 


The val ue returned bythi s generi c functi on i s a functi on. 

P 

Thi s generi c functioniscalledtodeterminethediscriminati ngfunctionforageneri c functi on. 

When a generi c functionis called, the inst al l ed discriminati ng function is calledwiththe 

full set of arguments recei ved bythegeneri c functi on, andmust implement thebehavior of 

callingthegeneri c functi on: determiningtheordered set of applicablemethods, determining 

theeecti ve method, andrunningtheeecti ve method. 

To determinethe ordered set of applicable methods, the discriminati ng functi on rst 

calls compute-applicable-methods-using-classes. If compute-applicable-methodsusing-classes 

returns a second val ue of f al se, the discriminati ng functi on then cal l s 

compute-applicable-methods. 

When compute-applicable-methods-using-classes returns a second val ue of true, 

thediscriminati ngfuncti on i s permittedto memoi ze the rst returned val ueas follows. The 

discriminati ngfuncti on mayreuse the list of applicable methods without calling computeapplicable-methods-using-classes 

agai n provided that: 

(i) thegeneri c functi on i s bei ng cal l ed agai n withrequi red arguments whichare 

instances of the same classes, 

(ii) thegeneri c functi on has not been rei nitialized,

Generi c Functi ons andMethods compute-eecti ve-method 41 

(iii) nomethod has been added to or removed fromthegeneri c functi on, 

(iv) f or al l the speci al i zers of al l the generi c functi on's methods whichare classes, 

thei r cl ass precedence lists have not changed and 

(v) f or any such memoi zed val ue, the classprecedence list of the cl ass of each of 

therequi red arguments has not changed. 

Determinati on of the eecti ve method i s done bycalling compute-e ecti ve- method. 

When the eecti ve method i s run, each method's functionis called, and recei ves as arguments: 

(i) a list of thearguments to thegeneri c functi on, and(ii) whatever other arguments 

are speci ed i n the call-method formindicatingthat the method shouldbe called. (See 

ma e- method-lambda for more i nformati on about howmethod f uncti ons are called.) 

The generi c functi on compute-discriminating-functi on is called, and its result 

installed, by add-method, remove- method, initialize-instance and rei nitializeinstance. 

M 

compute-discriminati ng-functi on ri mar et hod 


obehaviorisspeci ed for this method beyond that speci ed for the generi c functi on. 


co ute e ecti e et o eneri c unct i on 

S 

compute-e ecti ve- method 

generi c- unct i on met hod- combi nat i on met hods 

A 


The met hod- combi nat i on argument isamethod combinati on metaobj ect. 

The met hods argument isalistofmethod metaobj ects. 

Thi s generi c functi on returns twoval ues. The rst is an eecti ve method, the secondisa 

list of eecti ve method options. 

P 

Thi s generi c functionis calledtodeterminetheeecti vemethodfromasortedlist of method 

metaobj ects. 

An eecti ve method i s a f ormthat descri bes how the applicable methods are to be 

combined. Insideofeecti ve method f orms arecall-method formswhichindi cate that a 

particular methodis tobe called. The arguments to the call-methodformindi cate exactl y

42 compute-eecti ve-sl ot-denition Chapter 6 

howthe method f uncti on of the method shouldbe called. (See ma e- method-lambda for 

more details about method f uncti ons.) 

An eecti ve method opti on has the sameinterpretation andsyntax as either the :arguments 

or the :generi c- f uncti onoptioninthelong f ormof dene- method-combination. 

More i nformati on about the f ormand interpretation of eecti ve methods andeecti ve 

method options can be found under the descri pti on of the dene- method-combinati on 

macro in theCLOS speci cati on. 

Thi s generi c functi on can be calledbytheuser or theimplementation. Itiscalledby 

discriminati ng functi ons whenever a sorted list of applicable methods must beconvertedto 

an eecti ve method. 

M 

compute-e ecti ve- method ri mar et hod 


met hod- combi nat i on 

met hods 

This method computes theeecti ve method accordingtotherules of themethod combinati 

on type implementedby met hod- combi nat i on. 


co ute e ecti e sl ot e nition eneri c unct i on 

S 

compute-e ecti ve- sl ot-denition 

cl ass name di rect - sl ot - de nitions 

A 


The name argument i s a sl ot name. 

The di rect -sl ot -de nitions argument isanordered l i st of di rect sl ot denitionmetaobjects. 

The most speci c di rect sl ot denitionmetaobj ect appears rst in the list. 

The val ue returned bythi s generi c functionis aneecti ve slotdenitionmetaobj ect. 

P 

Thi s generi c functi on determines the eecti ve slotdenitionfor aslot inaclass. It is called 

by compute-slots once for each sl ot accessi ble ininstances of cl ass. 

Thi s generi c functi on uses thesuppliedlist of di rect sl ot denitionmetaobj ects to compute 

theinheri tance of sl ot properties for a singl e sl ot. The returned eecti veslotdenition 

represents the resul t of computingthe inheri tance. The name of the new eecti ve slot 

denitionis thesameasthename ofthedi rect sl ot denitionssupplied.

Generi c Functi ons andMethods compute-slots 43 

The class of theeecti ve slotdenitionmetaobj ect i s determined bycalling e ecti veslot-denition-class. 

The eecti ve slot denition is then created by calling ma e- 

instance. The initializationarguments passed in this call to ma e- i nstance are used to 

initialize theneweecti ve slotdenitionmetaobj ect. See \I nitializationof Slot Denition 

Metaobj ects" f or details. 

M 

compute-e ecti ve- sl ot-denition ri mar et hod 


name 

di rect -sl ot -de nitions 

This method i mplements theinheri tance anddefaultingof slot options followingtherules 

descri bed i n the \Inheri tance of Slots andOptions" section of theCLOS Speci cati on. 

This method can be extended, but theval ue returned bytheextendingmethod must 

be theval ue returned bythis method. 

compute-e ecti ve- sl ot-denition ri mar et hod 


name 

di rect -sl ot -de nitions 

This method i mplements theinheri tance anddefaultingof slot options followingtherules 

descri bed i n the \Inheri tance of Slots andOptions" section of theCLOS Speci cati on. 

This method can be extended, but theval ue returned bytheextendingmethod must 

be theval ue returned bythis method. 

co ute sl ots eneri c unct i on 

S 

compute-slots 

cl ass 

A 


The val ue returned i s a set of eecti ve slotdenitionmetaobj ects. 

P 

Thi s generi c functi on computes a set of eecti ve slotdenition metaobj ects f or the class 

cl ass . The result is alist of eecti ve slotdenitionmetaobj ects: one for each sl ot that will 

be accessi ble ininstances of cl ass. 

Thi s generi c functi on proceeds in 3 steps:

44 compute-slots Chapter 6 

The rst step collects thefull set of di rect sl ot denitionsfromthesupercl asses of cl ass. 

The di rect sl ot denitions are then col l ected i nto individual lists, one l i st for each sl ot 

name associ ated withanyof thedi rect sl ot denitions. The slotnames are compared with 

eql . Each such list is then sorted into class precedence list order. Di rect sl ot denitions 

comingfromcl asses earl i er i n the classprecedence list of cl ass appear bef ore those coming 

f romcl asses l ater i n the class precedence list. For each sl ot name, the generi c functi on 

compute-e ecti ve- sl ot-denition is calledto compute an eecti ve slotdenition. The 

result of compute-slots is alist of these eecti ve slotdenitions, in unspeci ed order. 

In thenal step, the l ocati on for each eecti ve slotdenitionis set. This is done by 

speci ed around-methods; portable methods cannot take over this behavior. For more 

informati on on the slotdenitionlocations, see the secti on \Instance Structure Protocol." 



functi on. 

M 

compute-slots ri mar et hod 


This method i mplements thespeci ed behavior of thegeneri c functi on. 


compute-slots ri mar et hod 


This method i mplements thespeci ed behavior of thegeneri c functi on. 


compute-slots round- et hod 


This method i mplements thespeci ed behavior of computingandstoringslot locations. 

This method cannot be overridden. 

compute-slots round- et hod 


This method i mplements thespeci ed behavior of computingandstoringslot locations. 

This method cannot be overridden.

Generi c Functi ons andMethods direct-slot-denition-class 45 

irect sl ot e nition class eneri c unct i on 

S 

direct-slot-denition-class 


A 


The ini t args argument isasetofinitializationarguments andval ues. 

The val ue returned i s a subcl ass of the classdi rect- sl ot-denition. 

P 

Whena class is initialized, each of thecanonicalizedslot speci cati ons must beconverted 

to a di rect sl ot denitionmetaobj ect. Thi s generi c functionis calledtodeterminetheclass 

of that di rect sl ot denitionmetaobj ect. 

The ini t args argument issimplythecanonicalizedslot speci cati on f or the slot. 

M 

direct-slot-denition-class ri mar et hod 


res ini t args 

This method returnstheclassstandard-di rect- sl ot-denition. 


direct-slot-denition-class ri mar et hod 



This method returnstheclassstandard-di rect- sl ot-denition. 


e ecti e sl ot e nition class eneri c unct i on 

S 

e ecti ve-slot-denition-class 


A 


The ini t args argument isasetofinitializationarguments andval ues.

46 ensure-class Chapter 6 

The val ue returned i s a subcl ass of the classe ecti ve- sl ot-denition-class. 

P 

Thi s generi c function is called by compute-e ecti ve- sl ot-denition to determine the 

cl ass of the resultingeecti veslotdenitionmetaobj ect. The ini t args argumentisthesetof 

initializationarguments andval ues that will bepassedtoma e- i nstance when theeecti ve 

sl ot denitionmetaobj ect i s created. 

M 

e ecti ve-slot-denition-class ri mar et hod 


res i i ar s 

This method returnstheclassstandard-e ecti ve- sl ot-denition. 


e ecti ve-slot-denition-class ri mar et hod 


res i i ar s 

This method returnstheclassstandard-e ecti ve- sl ot-denition. 


ensure class 

unct i on 

S 

ensure-cl ass 

name e allo o er e s 

A 

The name argument isasymbol . 

Some of the keyword arguments accepted bythis function are actually processed by 

ensure-cl ass-using-class, others are processedduringinitializationof theclassmetaobj ect 

(as descri bed i n the sectioncalled\Initializationof Class Metaobj ects"). 

The result is aclass metaobj ect. 

P 

This functionis calledtodene orredene aclasswiththespeci ed name, and can becalled 

by theuser or theimplementation. It is thefuncti onal equival ent ofdef cl ass, andis called 

by theexpansi on of the def cl ass macro.

Generi c Functi ons andMethods ensure-class-using- cl ass 47 

The behavior of this functi on i s actuallyimplemented bythegeneri c functi on ensurecl 

ass-using-class. When ensure-class is called,itimmediatelycalls ensure-class-usi ngcl 

ass and returnsthat result as its own. 

The rst argument toensure-class-using-class is computedas follows: 

If name names a cl ass (nd-class returnsaclasswhen cal l ed withname) use that cl ass. 

Otherwise use nil. 

The secondargument isname. The remai ningarguments are the complete set of keyword 

arguments recei ved bythe ensure-class functi on. 

ensure class usi n class eneri c unct i on 

S 

ensure-cl ass-using-class 

cl ass name e irec e a l i i ar s irec slo s 

irec s perclasses ame 

me aclass 

allo o er e s 

A 

The cl ass argument isaclassmetaobj ect or nil. 

The name argument isaclassname. 

The :metacl ass argument isaclassmetaobj ect cl ass or a cl ass metaobj ect cl ass name. 

If thi s argument isnot supplied, it defaults to theclassnamed standard-class. I f a cl ass 

name issupplied,itisinterpreted as theclasswiththat name. If a cl ass name issupplied, 

but there i s nosuch class, an error is signal ed. 

The :direct-supercl asses argument isalistofwhicheachelementisaclassmetaobj ect 

or a cl ass name. An errorissignal ed i f thi s argument isnot a proper list. 

For the interpretationof additional keyword arguments, see \Initializationof Class Metaobjects" 

(page 57). 

The result is aclass metaobj ect. 

P 

Thi s generi c functionis calledtodene ormodifythedenitionof anamedcl ass. It i s cal l ed 

by the ensure-cl ass functi on. I t can al so be calleddirectly. 

The rst step performed bythi s generi c functi on i s to compute thesetofinitialization 

arguments whichwill beusedtocreateor reinitialize thenamed cl ass. The initialization 

arguments are computed f romthe full set of keyword arguments recei ved bythi s generi c 

functionas follows:

48 ensure-class-using- cl ass Chapter 6 

The :metacl ass argument isnot i ncl uded i n the initializationarguments. 

If the :direct-supercl asses argument was recei ved bythi s generi c functi on, it is converted 

into a list of class metaobj ects. This conversion does not aect the structure 

of the supplied :direct-supercl asses argument. For each el ement in the :directsupercl 

asses argument: 

If theelement isaclassmetaobj ect, that cl ass metaobj ect i s used. 

If theelement names a class, that cl ass metaobj ect i s used. 

Otherwise aninstance of the classforward-ref erenced-class i s created andused. 

The proper name of thenewl y created forward referenced cl ass metaobj ect i s set to 

name. 

Al l other keyword arguments are i ncl uded directlyintheinitializationarguments. 

If the cl ass argument is nil, a new cl ass metaobj ect i s created by callingthe ma e- 

instance generi c functi on withtheval ue of the :metacl ass argumentasitsrst argument, 

andthepreviouslycomputedinitializationarguments. The proper name of thenewl ycreated 

cl ass metaobj ect i s set to name. The newl y created cl ass metaobj ect i s returned. 

If the cl ass argument isaforward referenced class, changecl ass is called to change 

its class to theval ue speci ed bythe :metacl ass argument. The classmetaobj ect i s then 

reinitializedwiththepreviously initializationarguments. (This is a documented violation 

of the general constraintthat changecl ass not be used withclass metaobj ects. ) 

If the class of the cl ass argumentisnot the sameasthe class speci ed bythe :metacl ass 

argument, an error is signal ed. 

Otherwise, theclassmetaobj ect cl ass is redened bycallingthe rei nitialize-instance 

generi c functi on with cl ass and theinitializationarguments. The cl ass argument isthen 

returned. 

M 

ensure-cl ass-using-class ri mar et hod 

cl ass class 

name 

e me aclass 

irec s perclasses 

allo o er e s 

This methodimplements thebehavior of thegeneri c functioninthecasewhere the cl ass 

argument is a class. 

This method can be overridden.

Generi c Functi ons andMethods ensure-generic-functi on 49 


cl ass or ar re ere ce class 

name 

e 

me aclass 


allo o er e s 


argument isaforward referenced cl ass. 


cl ass ll 

name 

e me aclass 


allo o er e s 


argument isnil. 

ensure eneric uncti on unct i on 

S 

ensure-generi c- f uncti on 

unct i on-name e allo o er e s 

A 

The unct i on-name argument isasymbol or a l i st of the f orm se s mbol . 

Some of the keyword arguments accepted bythis function are actually processed by 

ensure-generi c- f uncti on-using-class, others are processed duringinitializationof the 

generi c functi on metaobj ect (as descri bed i n the section called \Initializationof Generi c 

Functi on Metaobj ects"). 

The result is ageneri c functi on metaobj ect. 

P 

This functionis calledto dene a gl oballynamed generi c functi on or to speci fy or modify 

optionsanddecl arati ons that pertain to a globallynamed generi c functi on as a whol e. It 

can be calledbytheuser or theimplementation.

50 ensure-generi c-functi on-using- cl ass Chapter 6 

It is the functi onal equival ent of def generi c, andis calledbythe expansi on of the 

def generi c and def method macros. 

The behavior of this functi on i s actuallyimplemented bythegeneri c functi on ensuregeneri 

c- f uncti on-using-class. When ensure-generic-functi onis called,itimmediately 

calls ensure-generic-functi on-using-class and returnsthat result as its own. 

The rst argument toensure-generi c- f uncti on-usi ng-class is computedas follows: 

If unct i on-name names a non-generi c functi on, a macro, or a speci al form, an error is 

signal ed. 

If 

unct i on-name names a generi c functi on, that generi c functi on metaobj ect i s used. 

Otherwise, nil is used. 

The secondargument is unct i on-name. The remai ningarguments are the complete set 

of keyword arguments recei ved by ensure-generic-functi on. 

ensure eneric uncti on usi n class eneri c unct i on 

S 

ensure-generi c- f uncti on-using-class 


unct i on-name 

e ar me prece e ce or er eclara io s 

oc me a io e eric c io class 

lam a lis me o class 

me o com i a io ame 

allo o er e s 

A 

The generi c- unct i on argument i s a generi c functi on metaobj ect or nil. 

The unct i on-name argument isasymbol or a l i st of the f orm se s mbol . 

The :generi c- f uncti on-class argument isaclassmetaobj ect or a cl ass name. If it is 

not supplied, it defaults to theclassnamed standard-generi c- f uncti on. If a class name 

is supplied, it is interpreted as theclasswiththat name. If a cl ass name issupplied, but 

there i s nosuch class, an error is signal ed. 

For the interpretation of additional keyword arguments, see \Initializationof Generi c 

Functi on Metaobj ects" (page 61). 

The result is ageneri c functi on metaobj ect.

Generi c Functi ons andMethods ensure-generic-functi on-using- cl ass 51 

P 

The generi c functi on ensure-generi c- f uncti on-usi ng-class is calledto dene ormodify 

thedenition of a globally named generi c functi on. It is called bythe ensure-generi c- 

functi on functi on. I t can al so be calleddirectly. 

The rst step performed bythi s generi c functi on i s to compute thesetofinitialization 

arguments whichwill beusedtocreateor reinitialize thegloballynamed generi c functi on. 

These i nitializationarguments are computedfromthefullsetofkeyword arguments recei ved 

by thi s generi c functionas follows: 

The :generi c- f uncti on-class argument isnot i ncl uded i n the initializationarguments. 

If the :method-class argument was recei ved by thi s generi c functi on, it is converted 

into a class metaobj ect. This is done bylookinguptheclassname withnd-class. If 

there i s nosuch class, an error is signalled. 

Al l other keyword arguments are i ncl uded directlyintheinitializationarguments. 

If the generi c- unct i on argumentisnil,aninstance of the class speci ed bythe :generi c- 

functi on-class argument i s created by calling ma e- i nstance with thepreviously computedinitializationarguments. 

The functionname unct i on-name is set toname thegeneri c 

functi on. The newl y created generi c functi on metaobj ect i s returned. 

If the class of the generi c- unct i on argument isnot the sameasthe class speci ed by 

the :generi c- f uncti on-class argument, an error is signal ed. 

Otherwise thegeneri c functi on generi c- unct i on is redened bycallingthe rei nitializeinstance 

generi c functi on with generi c- unct i on and the initializationarguments. The 

generi c- unct i on argument isthen returned. 

M 

ensure-generi c- f uncti on-using-class ri mar et hod 

generi c- unct i on e eric c io 


e e eric c io class 

allo o er e s 

This method i mplements thebehavior of thegeneri c functioninthecasewhere unct i onname 

names an existinggeneri c functi on. 

This method can be overridden.

52 extract-lambda- l i st Chapter 6 

ensure-generi c- f uncti on-using-class ri mar et hod 

generi c- unct i on ll 


e e eric c io class 

allo o er e s 

This method i mplements thebehavior of thegeneri c functioninthecasewhere unct i onname 

names nofuncti on, generi c functi on, macro or speci al form. 

e l s ecializer o ect unct i on 

S 

eql-speci alizer-object 

e l-s eci al i er 

A 

The e l-s eci al i er argument isaneql speci al i zer metaobj ect. 

The val ue returned bythis functi on i s an object. 

P 

This functi on returns the object associ ated with e l-s eci al i er duringinitialization. The 

val ue isguaranteed to be eql to theval ue ori gi nallypassed to intern-eql-speci alizer,but 

it is not necessarily eq to that val ue. 

This functionsignals an error if e l-s eci al i er is not an eql speci al i zer. 

e tract la a list unct i on 

S 

e tract-l ambda-list 

s eci al i ed- l ambda- l i st 

A 

The s eci al i ed- l ambda- l i st argumentisaspeci al i zed l ambdal i st as acceptedby def method. 

The result is anunspeci al i zed l ambda list.

Generi c Functi ons andMethods extract-speci al i zer-names 53 

P 

This functi ontakes a speci al i zed l ambdalistandreturnsthelambdalist withthespeci al i zers 

removed. This is anon-destructi ve operati on. Whether the result shares anystructure with 

theargument isunspeci ed. 

If the s eci al i ed- l ambda- l i st argument does not have l egal syntax, an error is signal ed. 

This syntax checkingdoes not check the syntax of theactual speci al i zer names, only the 

syntax of thelambdalist andwhere the speci al i zers appear. 

E 

ex rac lam a lis p posi io 

ex rac lam a lis p posi io x 

ex rac lam a lis a e l x c res i 

e tract s ecializer na es unct i on 

S 

e tract-speci alizer-names 

s eci al i ed- l ambda- l i st 

A 

The s eci al i ed- l ambda- l i st argumentisaspeci al i zed l ambdal i st as acceptedby def method. 

The result is alist of speci al i zer names. 

P 

This functiontakes a speci al i zed l ambdalist andreturns its speci al i zer names. This is anondestructi 

veoperati on. Whether theresult shares structure withtheargumentisunspeci ed. 

The results are undened i f the result of this functi on i s modied. 

The result of this functi on will be alist withanumber of el ements equal to thenumber 

of requi red arguments in s eci al i ed- l ambda- l i st . Speci al i zers are defaulted to thesymbol t. 

If the s eci al i ed- l ambda- l i st argument does not have l egal syntax, an error is signal ed. 

This syntax checkingdoes not check the syntax of theactual speci al i zer names, only the 

syntax of thelambdalist andwhere the speci al i zers appear. 

E 

ex rac speciali er ames p posi io 

ex rac speciali er ames p posi io x 

ex rac speciali er ames a e l x c res i

54 nd-method-combinati on Chapter 6 

nalize in eritance eneri c unct i on 

S 

nalize-inheri tance 

cl ass 

A 



P 

Thi s generi c functionis calledtonalizeaclass metaobj ect. This is descri bedintheSecti on 

named \Class FinalizationProtocol." 

After nalize-inheri tance returns, theclassmetaobj ect i s nal i zed andtheresult of 

calling cl ass-nalized-pon the classmetaobj ect will be true. 

M 

nalize-inheri tance ri mar et hod 




obehavior is speci ed for these methods beyondthat whichis speci ed for the generi c 

functi on. 


cl ass or ar re ere ce class 


n et o co inati on eneri c unct i on 

S 

nd-method-combinati on 


met hod- combi nat i on-t e-name 

met hod- combi nat i on-o tions 

A 

The generi c- unct i on argument i s a generi c functi on metaobj ect.

Generi c Functi ons andMethods funcallable-standard-instance-access 55 

The met hod- combi nat i on-t e-name argumentisasymbol whichnames a type ofmethod 

combinati on. 

The met hod- combi nat i on-o tions argument is alist of arguments to themethod combinati 

on type. 

The val ue returned bythi s generi c functi on i s a method combinati on metaobj ect. 

P 

Thi s generi c functionis calledtodeterminethemethod combinati on obj ect usedby a generi c 

functi on. 

R 

Further detai l s of method combinati on metaobj ects are not speci ed. 

uncalla le stan ar instance access unct i on 

S 

funcallable-standard-instance-access 

instance l ocation 

A 

The instance argument i s an obj ect. 

The locat i on argument is a slot location. 

The result of this functi on i s an object. 

P 

This functionis calledto providedi rect access to a sl ot i n an i nstance. By usurpingthe 

normal sl ot l ookupprotocol, this functi on i s i ntended to providehighlyoptimi zed access to 

the slots associated withaninstance. 

The following restrictionsapply to theuse of this functi on: 

The instance argument must beafuncallable instance (i t must have been returned by 

allocate-instance (f uncallable-standard-class)). 

The instance argument cannot be annon-updated obsolete instance. 

The locat i on argument must be a l ocati on of one ofthedi rectl y accessi bl e sl ots of the 

instance' s cl ass. 

The slotmust bebound. 

The results are undened i f anyof these restrictionsarenot met.

56 generi c-functi on- ::: Chapter 6 

eneric uncti on eneri c unct i on 

The followinggeneri c functi ons are descri bed together under \Readers f or Generi c Functi 

on Metaobj ects" (page 79): generi c- f uncti on-argument-precedence-order, generi c- 

functi on-decl arati ons, generi c- f uncti on-lambda-list, generi c- f uncti on-methodcl 

ass, generi c- f uncti on-method-combination, generi c- f uncti on-methods and generi c- 

functi on-name.

Generi c Functi ons andMethods Initializationof Class Metaobj ects 57 

nitializati on o lass etao ects 

Aclass metaobj ect can be created by calling ma e- i nstance. The initializationarguments 

establishthedenitionof the class. Aclass metaobj ect can be redened bycalling 

rei nitialize-instance. Some classes of class metaobj ect do not support redenition; in 

these cases, rei nitialize-instance signals an error. 

Initializationof aclass metaobj ect must bedone bycalling ma e- i nstance andallowing 

it tocall initialize-instance. Rei nitializationof aclass metaobj ect must bedone bycalling 

rei nitialize-instance. Portable programs must not cal l initialize-instance directly to 

initialize a class metaobj ect. Portable programs must not cal l shared-initialize directly 

to initialize or reinitialize a class metaobj ect. Portable programs must not cal l changecl 

ass to change the cl ass of any class metaobj ect or to turn a non-class object into a class 

metaobj ect. 

Since metaobj ect cl asses may not be redened, nobehavior is speci ed for the result 

of calls to update-instance-f or-redened-class on cl ass metaobj ects. Since the classof 

cl ass metaobj ects may not be changed, nobehavior is speci ed for the result of calls to 

update-instance-f or-di erent-cl ass on cl ass metaobj ects. 

Duringinitializationor reinitialization, each i nitializationargumentischeckedfor errors 

and then associ ated withtheclassmetaobj ect. The val ue canthen be accessed bycalling 

theappropriate accessor as shown inTabl e 6. 1. 

This section begi ns with a descri pti on of the error checkingandprocessing of each 

initializationargument. This is followed byatable showingthegeneri c functi ons that can 

be used to access the stored initialization arguments. Initialization behavior speci c to 

thedierent speci ed cl ass metaobj ect cl asses comes next. The secti on ends witha set of 

restrictionsonportablemethods aecti ngclass metaobj ect i nitializationandreinitialization. 

In these descri ptions, the phrase \thi s argument defaults to al ue" means that when 

that i nitializationargument isnot supplied, initializationor reinitializationis performed as 

if al ue had been supplied. For some initializationarguments this couldbedone bytheuse 

of default initializationarguments, but whether it is done this way is not speci ed. Implementations 

are free to dene default initializationarguments for speci ed cl ass metaobj ect 

classes. Portable programs are free to dene default initializationarguments for portable 

subclassesoftheclasscl ass. 

Unless thereisaspeci c note to the contrary, then duringreinitialization, if an initializationargument 

isnot supplied, thepreviously stored val ue isleftunchanged. 

The :di rect- def ault-initargs argument is a list of canonicalizeddefault initialization 

arguments. 

An error is signal ed i f this val ue isnot a proper list, or if anyelement ofthe list is not 

acanonicalizeddefault initializationargument. 

If theclassmetaobj ect i s bei nginitialized, thi s argument defaults totheemptylist.

58 Initializationof Class Metaobj ects Chapter 6 

The :direct-slots argument isalistofcanonicalizedslot speci cati ons. 

An errorissignal ed i f this val ue isnot a properlistorifanyelement ofthe list is not a 

canonicalizedslot speci cati on. 

After error checking, this val ue isconverted to a list of di rect sl ot denition metaobjects 

before i t i s associ ated withtheclassmetaobj ect. Conversi on of each canonicalized 

slot speci cation to a di rect sl ot denitionmetaobj ect i s a two-step process. First, the 

generi c functi on direct-slot-denition-class is called with theclassmetaobj ect and 

thecanonicalizedslot speci cationto determinethe class of thenewdi rect sl ot denition 

metaobj ect; this permits both the classmetaobj ect and thecanonicalizedslot speci - 

cation to control theresultingdi rect sl ot denition metaobj ect cl ass. Second, ma e- 

instance is appliedto thedi rect sl ot denitionmetaobj ect cl ass and thecanonicalized 

slot speci cati on. This conversioncouldbeimplementedas showninthe followingcode: 

e co er o irec slo e i i io class ca o icali e slo 

appl ma e i s a ce 

appl irec slo e i i io class 

class ca o icali e slo 

ca o icali e slo 

If theclassmetaobj ect i s bei nginitialized, thi s argument defaults totheemptylist. 

Once the di rect sl ot denitionmetaobj ects have been created, thespeci ed reader and 

writer methods are created. The generi c functi ons reader- method-class and wri termethod-class 

are cal l ed to determinethe classes of themethod metaobj ects created. 

The :direct-supercl asses argument is alist of class metaobj ects. Classes whichdonot 

support multiple inheri tance si gnal an error i f the list containsmore than one element. 

An error is signal ed i f this val ue isnot a proper list or if validate-supercl ass applied 

to cl ass and anyelement ofthis list returns f al se. 

When the classmetaobj ect i s bei nginitialized,andthi s argument iseither not supplied 

or i s the empty list, thi s argument defaults as follows: if the class is an instance of 

standard-class or one of its subclasses thedefault val ueis alist of theclassstandardobject;iftheclass 

is aninstance of funcallable-standard-class or one of its subclasses 

thedefault val ue is list of theclassfuncallable-standard-object. 

After anydefaultingof theval ue, the generi c functi on add-direct-subcl ass is called 

once f or each el ement ofthe list. 

When the class metaobj ect i s bei ng reinitialized andthi s argument is supplied, the 

generi c functi on remove-direct-subcl ass is called once f or each cl ass metaobj ect i n 

thepreviously stored val ue but not i n the newval ue; the generi c functi on add-directsubcl 

ass is calledonce f or each cl ass metaobj ect i n the newval ue but not i n the previously 

stored val ue.

Generi c Functi ons andMethods Initializationof Class Metaobj ects 59 

The :documentati on argument is a stringornil. 

An error is signal ed i f this val ue isnot a stringornil. 

If theclassmetaobj ect i s bei nginitialized, thi s argument defaults tonil. 

The :name argument i s an obj ect. 

If the class is bei nginitialized,thi s argument defaults tonil. 

After theprocessinganddefaultingof initializationarguments descri bed above, theval ue 

of each i nitializationargument i s associ ated with theclassmetaobj ect. These val ues can 

then be accessed bycallingthe correspondinggeneri c functi on. The correspondences are as 

follows: 

nitialization rgument Generi c Functi on 

:di rect- def ault-initargs 

:direct-slots 

:direct-supercl asses 

:documentation 

:name 

cl ass-di rect- default-initargs 

cl ass-direct-slots 

cl ass-direct-supercl asses 

documentati on 

cl ass-name 

a e .1 r r r . 

Instances of the classstandard-class support multiple inheri tance andreinitialization. 

Instances of the classfuncallable-standard-class support multiple inheri tance andreinitialization. 

For f orward referenced cl asses, al l of the initialization arguments default to 

nil. 

Since built-inclasses cannot be created or rei nitializedbytheuser, anerror is signal ed i f 

initialize-instance or rei nitialize-instance are calledtoinitializeor reinitializeaderi ved 

instance of the classbuilt-in-class. 

M 

It is not speci ed which methods provide the initialization andreinitialization behavior 

descri bed above. Instead, theinformati on needed to allowportable programs tospeci al i ze 

this behavior is presented as a set of restrictions on themethods aportable programcan 

dene. The model i s that portableinitializationmethods have access to theclassmetaobj ect 

when ei ther al l or none ofthespeci ed i nitializationhas taken eect. 

These restrictions governthemethods that a portable programcan dene onthegeneri c 

functi ons initialize-instance, rei nitialize-instance, and shared-initialize. These restrictions 

apply only to methods onthese generi c functi ons f or whichtherst speci al i zer 

is asubcl ass of the classcl ass. Other portable methods onthese generi c functi ons arenot 

aected bythese restrictions.

60 Initializationof Class Metaobj ects Chapter 6 

Portable programs must not dene methods onshared-initialize. 

For initialize-instance and rei nitialize-instance: 

Portable programs must not dene primary methods. 

Portable programs may dene around-methods, but these must beextending, not 

overridingmethods. 

Portablebefore-methods must assume that when they are run, none oftheinitializati 

on behavior descri bed above has been completed. 

Portable after-methods must assume that when they are run, all of theinitialization 

behavior descri bed above has been completed. 

The results are undened i f anyof these restrictionsareviolated.

Generi c Functi ons andMethods Initializationof Generi c Functi on Metaobj ects 61 

nitializati on o eneric uncti on etao ects 

Ageneri c functi on metaobj ect can be created bycalling ma e- i nstance. The initialization 

arguments establishthedenitionof thegeneri c functi on. Ageneri c functi onmetaobj ect can 

be redened bycalling rei nitialize-instance. Some classes of generi c functi on metaobj ect 

do not support redenition; in these cases, rei nitialize-instance signals an error. 

Initializationof ageneri c functi on metaobj ect must bedone bycalling ma e- i nstance 

andallowing i t to cal l initialize-instance. Rei nitialization of a generi c-functionmetaobj ect 

must bedone bycalling rei nitialize-instance. Portableprograms must not cal l initializeinstance 

directlyto initialize a generi c functi on metaobj ect. Portable programs must not 

cal l shared-initialize directly to initialize or reinitialize a generi c functi on metaobj ect. 

Portable programs must not cal l changecl ass to change the cl ass of any generi c functi on 

metaobj ect or to turnanon-generi c-functi on object i nto a generi c functi on metaobj ect. 

Since metaobj ect cl asses may not be redened, nobehavior is speci ed for the result of 

calls toupdate-instance-f or-redened-class on generi c functi on metaobj ects. Since the 

cl ass of a generi c functi on metaobj ect may not be changed, nobehavior is speci ed for the 

results of calls toupdate-instance-for-di erent-class on generi c functi on metaobj ects. 

Duringinitializationor reinitialization, each i nitializationargumentischeckedfor errors 

and then associ ated withthegeneri c functi on metaobj ect. The val ue canthen be accessed 

bycallingtheappropriate accessor as shown inTabl e 6. 2. 


initialization argument. This is followed by atable showingthe generi c functi ons that 

can be used to access the stored initializationarguments. The secti on ends with a set of 

restrictions on portable methods aecti ng generi c functi on metaobj ect i nitialization and 

reinitialization. 


that i nitializationargument isnot supplied, initializationor reinitializationis performed as 

if al ue had been supplied. For some initializationarguments this couldbedone bytheuse 

of default initializationarguments, but whether it is done this way is not speci ed. Implementations 

are free to dene default initializationarguments for speci ed generi c functi on 

metaobj ect cl asses. Portableprograms are free to dene default initializationarguments for 

portable subclassesoftheclassgeneri c- f uncti on. 

Unless thereisaspeci c note to the contrary, then duringreinitialization, if an initializationargument 

isnot supplied, thepreviously stored val ue isleftunchanged. 

The :argument-precedence-order argument isalistofsymbol s. 

An error is signal ed i f thi s argument appears but the :lambda-list argument does not 

appear. An error is signal ed i f this val ue isnot a proper list or if this val ue isnot 

apermutation of the symbol s f romthe requi red arguments part of the :lambda-list 

initializationargument.

62 Initializationof Generi c Functi on Metaobj ects Chapter 6 

When the generi c functi on i s bei nginitializedor reinitialized, andthi s argument isnot 

supplied, but the :lambda-list argument issupplied, this val ue defaults tothesymbol s 

fromthe requi red arguments part of the :lambda-list argument, in the order they 

appear i n that argument. If nei ther argument issupplied,nei ther are i nitialized(see the 

descri pti on of :lambda-list.) 

The :decl arations argument is alist of decl arati ons. 

An error is signal ed i f this val ue isnot a proper l i st or i f each of i ts el ements is not a 

l egal decl arati on. 

When the generi c functi on i s bei ng initialized, andthi s argument is not supplied, it 

defaults totheempty list. 

The :documentati on argument is a stringornil. 

An error is signal ed i f this val ue isnot a stringornil. 

If thegeneri c functi on i s bei nginitialized,thi s argument defaults tonil. 

The :lambda-list argument isalambda list. 

An error is signal ed i f this val ue isnot a proper generi c functi on l ambda list. 

When the generi c functi on i s bei ng initialized, andthi s argument isnot supplied, the 

generi c functi on's lambda listisnot i nitialized. The lambdalist will beinitializedlater, 

ei ther when the rst method i s added to thegeneri c functi on, or a later reinitialization 

of the generi c functi on. 

The :method-combination argument isamethod combinati on metaobj ect. 

The :method-class argument isaclassmetaobj ect. 

An error is signal ed i f this val ue isnot a subcl ass of the classmethod. 

When the generi c functi on i s bei ng initialized, andthi s argument is not supplied, it 

defaults totheclassstandard-method. 

The :name argument i s an obj ect. 

If thegeneri c functi on i s bei nginitialized,thi s argument defaults tonil. 

After the processing anddefaultingof initializationarguments descri bed above, the 

val ue of each i nitializationargument i s associ ated with the generi c functi on metaobj ect. 

These val ues can then be accessed by callingthe correspondinggeneri c functi on. The 

correspondences are as follows:

Generi c Functi ons andMethods Initializationof Generi c Functi on Metaobj ects 63 


:argument-precedence-order 

:decl arations 


:lambda-list 

:method-combination 

:method-class 

:name 

generi c- f uncti on-argument-precedence-order 

generi c- f uncti on-decl arations 

documentation 

generi c- f uncti on-lambda-list 

generi c- f uncti on-method-combinati on 

generi c- f uncti on-method-class 

generi c- f uncti on-name 

a e . r r r r . 

M 




dene. The model i s that portable initializationmethods have access to thegeneri c functi on 

metaobj ect when ei ther al l or none ofthespeci ed i nitializationhas taken eect. 


functi ons initialize-instance, rei nitialize-instance, and shared-initialize. These restrictions 

apply only to methods onthese generi c functi ons f or whichtherst speci al i zer 

is a subcl ass of the class generi c- f uncti on. Other portable methods on these generi c 

functi ons arenot aected bythese restrictions. 

Portable programs must not dene methods onshared-initialize. 

For initialize-instance and rei nitialize-instance: 









64 Initializationof Method Metaobj ects Chapter 6 

nitializati on o et o etao ects 

Amethod metaobj ect can be created bycalling ma e- i nstance. The initializationarguments 

establish thedenitionof themethod. Amethod metaobj ect cannot be redened; 

calling rei nitialize-instance signals an error. 

Initializationof amethod metaobj ect must bedone bycalling ma e- i nstance andallowingit 

tocall initialize-instance. Portable programs must not cal l initialize-instance directlytoinitialize 

amethod metaoject. Portable programs must not cal l shared-initialize 

directlytoinitialize amethod metaobj ect. Portable programs must not cal l changecl ass 

to change the cl ass of anymethod metaobj ect or to turna non-method obj ect i ntoa method 

metaobj ect. 

Since metaobj ect cl asses may not be redened, nobehavior is speci ed for the result of 

calls toupdate-instance-f or-redened-class on method metaobj ects. Since the classof 

amethod metaobj ect cannot be changed, nobehavior is speci ed for the result of calls to 

update-instance-f or-di erent-cl ass on method metaobj ects. 

Duringinitialization, each i nitializationargument ischecked for errors andthen associ - 

ated withthemethod metaobj ect. The val ue canthen be accessed bycallingtheappropri ate 

accessor as shown inTabl e 6. 3. 


initialization argument. This is followed by atable showingthe generi c functi ons that 

can be used to access the stored initializationarguments. The secti on ends with a set of 

restrictionsonportable methods aecti ngmethod metaobj ect i nitialization. 


that i nitializationargument isnot supplied, initializationis performed as i f al ue had been 

supplied. For some initializationarguments this couldbedone bytheuse of default initializationarguments, 

but whether it is done this wayis not speci ed. Implementationsarefree 

to dene default initializationarguments for speci ed method metaobj ect cl asses. Portable 

programs are free to dene default initializationarguments for portable subclasses of the 

cl ass method. 

The :qualiers argument is alist of method qualiers. Anerror is signal ed i f this val ue 

is not a proper list, or if anyelement ofthelistisnot a non-nul l atom. Thi s argument 


The :lambda-list argument istheunspeci al i zed l ambdalist of themethod. An error is 

signal ed i f this val ue isnot a proper l ambda list. If this val ue isnot supplied, an error 

is signal ed. 

The :speci alizers argument isalistofthespeci al i zer metaobj ects f or the method. An 

error is signal ed i f this val ue isnot a proper list, or if thelength of the list diers from 

thenumber of requi red arguments in the :lambda-list argument, or i f any element of 

the list is not a speci al i zer metaobj ect. I f this val ue isnot supplied, an error is signal ed.

Generi c Functi ons andMethods Initializationof Method Metaobj ects 65 

The :functi onargumentisamethod f uncti on. It must becompati blewiththemethods 

on compute-e ecti ve- method dened for this class of method and generi c functi on 

withwhichit will beused. That i s, i t must accept thesamenumber of arguments as 

al l uses of call-method that will call it supply. (See compute-e ecti ve- method for 

more i nformati on.) An error is signal ed i f thi s argument isnot supplied. 

When the method bei nginitializedis aninstance of a subcl ass of standard-accessormethod, 

the :slot-denitioninitializationargumentmust beprovided. Its val ue isthe 

di rect sl ot denitionmetaobj ect whichdenes thi s accessor method. Anerror is signal ed 

if theval ue isnot an i nstance of a subcl ass of direct-slot-denition. 

The :documentation argument is a stringor nil. An error is signal ed i f this val ue is 

not a stringornil. Thi s argument defaults tonil. 


of each i nitializationargument i s associ ated withthemethod metaobj ect. These val ues can 

then be accessed bycallingthe correspondinggeneri c functi on. The correspondences are as 

follows: 


:qualiers 

:lambda-list 

:speci alizers 

:functi on 

:slot-denition 


method-qualiers 

method-lambda-list 

method-speci alizers 

method-functi on 

accessor-method-slot-denition 

documentati on 

a e . r r r . 

M 

It is not speci ed whichmethods providetheinitializationbehavior descri bed above. Instead, 

the informati on needed to allowportable programs to speci al i ze this behavior is 

presented in as a set of restrictions on themethods a portable programcan dene. The 

model i s that portable initializationmethods have access to the method metaobj ect when 

ei ther al l or none ofthespeci ed i nitializationhas taken eect. 


functi ons initialize-instance, rei nitialize-instance, and shared-initialize. These restrictionsapplyonlytomethods 

onthese generi c functi ons f or whichtherst speci al i zer i s 

asubcl ass of the classmethod. Other portable methods onthese generi c functi ons arenot 

aected bythese restrictions.

66 Initializationof Method Metaobj ects Chapter 6 

Portable programs must not dene methods on shared-initialize or rei nitializeinstance. 

For initialize-instance: 









Generi c Functi ons andMethods Initializationof Slot DenitionMetaobj ects 67 

nitializati on o Slot e nition etao ects 

Aslot denitionmetaobj ect can be created bycalling ma e- i nstance. The initialization 

arguments establishthedenitionof theslotdenition. Asl ot denitionmetaobj ect cannot 

be redened; calling rei nitialize-instance signals an error. 

Initializationof a slot denition metaobj ect must bedone by calling ma e- i nstance 

and allowing i t to cal l initialize-instance. Portable programs must not cal l initializeinstance 

directly to initialize a slot denition metaobj ect. Portable programs must not 

cal l shared-initializedirectlytoinitialize aslot denitionmetaobj ect. Portable programs 

must not cal l change-class to change the cl ass of any slot denitionmetaobj ect or to turn 

anon-slot-denitionobject into a slot denitionmetaobj ect. 

Since metaobj ect cl asses may not be redened, nobehavior is speci ed for the result 

of calls toupdate-instance-f or-redened-class on sl ot denitionmetaobj ects. Since the 

cl ass of a sl ot denitionmetaobj ect cannot be changed, nobehavioris speci ed for theresult 

of calls toupdate-instance-for-di erent-class on sl ot denitionmetaobj ects. 

Duringinitialization, each i nitializationargument ischecked for errors andthen associ 

ated withtheslotdenitionmetaobj ect. The val ue canthen be accessed bycallingthe 

appropriate accessor as showninTabl e 6. 4. 


initializationargument. This is followed byatable showingthegeneri c functi ons that can 

be used to access the stored initializationarguments. 


that i nitializationargument isnot supplied, initializationis performed as i f al ue had been 

supplied. For some initializationarguments this couldbedone bytheuse of default initializationarguments, 

but whether it is done this way is not speci ed. Implementationsare 

free todene default initializationarguments for speci ed sl ot denitionmetaobj ect cl asses. 

Portable programs are free to dene default initializationarguments for portable subclasses 

of the classslot-denition. 

The :name argument i s a sl ot name. An error is signal ed i f thi s argument isnot a 

symbol whi ch can be used as a vari able name. An error is signal ed i f thi s argument is 

not supplied. 

The :initformargumentisaform. The :initformargumentdefaults tonil. Anerror is 

signal ed i f the :initformargument issupplied, but the :initfuncti on argument isnot 

supplied. 

The :initfuncti on argument isafuncti on of zero arguments which,when cal l ed, eval u- 

ates the :initformintheappropriate lexical environment. The :initfuncti onargument 

defaults tofalse. Anerrorissignal ed i f the :initfuncti onargumentissupplied,but the 

:initformargument isnot supplied.

68 Initializationof Slot DenitionMetaobj ects Chapter 6 

The :type argumentisatype speci er name. Anerror is signal ed otherwise. The :type 

argument defaults tothesymbol t. 

The :allocationargumentisasymbol . Anerror is signal ed otherwise. The :allocation 

argument defaults tothesymbol :instance. 

The :initargs argument is a list of symbol s. An error is signal ed i f thi s argument is 

not a proper list, or if anyelement ofthis list is not a symbol . The :initargs argument 


The :readers argument is a list of functi on names. An error is signaledif it is not a 

proper list, or if anyelement isnot a validfuncti on name. It defaults totheempty list. 

An error is signal ed i f thi s argument issuppliedandthemetaobj ect i s not a di rect sl ot 

denition. 

The :wri ters argument is a list of functi on names. An error is signaledif it is not a 

proper list, or if anyelement isnot a validfuncti on name. It defaults totheempty list. 

An error is signal ed i f thi s argument issuppliedandthemetaobj ect i s not a di rect sl ot 

denition. 

The :documentation argument is a stringornil. An error is signal ed otherwise. The 

:documentation argument defaults tonil. 


of eachi nitializationargument i s associ atedwiththeslotdenitionmetaobj ect. These val ues 

can then be accessed bycallingthe correspondinggeneri c functi on. The correspondences 

are as f ol l ows:

Generi c Functi ons andMethods Initializationof Slot DenitionMetaobj ects 69 


:name 

:initform 

:initfuncti on 

:type 

:allocation 

:initargs 

:readers 

:wri ters 


slot-denition-name 

slot-denition-initform 

slot-denition-initfuncti on 

slot-denition-type 

slot-denition-allocation 

slot-denition-initargs 

slot-denition-readers 

slot-denition-writers 

documentation 

a e . r r r 

. 

M 




dene. The model i s that portable initializationmethods have access to the slotdenition 

metaobj ect when ei ther al l or none ofthespeci ed i nitializationhas taken eect. 


functi ons initialize-instance, rei nitialize-instance, and shared-initialize. These restrictionsapplyonlytomethods 

onthese generi c functi ons f or whichtherst speci al i zer i s 

asubcl ass of the classslot-denition. Other portable methods onthese generi c functi ons 

are not aected bythese restrictions. 

Portable programs must not dene methods on shared-initialize or rei nitializeinstance. 

For initialize-instance: 









70 make-instance Chapter 6 

intern e l s ecializer unct i on 

S 

intern-eql-speci alizer 

obj ect 

A 

The obj ect argument isanyLi sp obj ect. 

The result is the eql speci al i zer metaobj ect f or obj ect . 

P 

This functi on returnstheunique eql speci al i zer metaobj ect f or obj ect , creatingoneifnecessary. 

Twocalls tointern-eql-speci alizer witheql arguments will return thesame (i.e., 

eq) val ue. 

R 

The result of calling eql-speci alizer-object on the resul t of a cal l to intern-eql-speci alizer 

is onlyguaranteed to be eql to the ori gi nal obj ect argument, not necessarily eq. 

a e instance eneri c unct i on 

S 

ma e- i nstance 


A 

The cl ass argument isaclassmetaobj ect or a cl ass name. 

The ini t args argument is alist of alternati ng initializationargument names andval ues. 

The result is anewlyallocatedandinitializedinstance of cl ass. 

P 

The generi c functi on ma e- i nstance creates andreturnsanewi nstance of the given cl ass. 

Its behavior anduse is descri bed i n the CLOS speci cati on. 

M 

ma e- i nstance ri mar et hod 

cl ass s m ol res ini t args 

This method si mplyinvokes ma e- i nstance recursivel y on the arguments i class 

cl ass and ini t args.

Generi c Functi ons andMethods make-method-lambda 71 


cl ass s a ar class res ini t args 


cl ass calla le s a ar class res ini t args 

These methods implementthebehavior of ma e- i nstance descri bed i n theCLOS specicati 

on secti on named \Object Creati on and Initialization." 

a e et o la a eneri c unct i on 

S 

ma e- method-lambda 

generi c- unct i on met hod lambda- e 

ressi on en ironment 

A 


The met hod argument isa(possiblyuninitialized)method metaobj ect. 

The lambda- e ression argument isalambda expression. 

The en ironment argument is the same as the environment argument to macro 

expansion functi ons. 

Thi s generi c functi on returnstwoval ues. The rst is a lambda expression, the secondisa 

list of initializationarguments andval ues. 

P 

Thi s generi c functionis calledto produce a l ambda expression whichcanitself beused to 

produce a method f uncti on for a method and generi c functi on with thespeci ed classes. 

The generi c functi on andmethod the method f uncti on will beused withare not required 

to bethegiven ones. Moreover, the method metaobj ect may beuninitialized. 

Either the functi on compile,thespeci al formfuncti onor the functi on coerce must be 

usedtoconvert the lambda expression a method f uncti on. The method f unctionitself can 

be appliedtoarguments withapplyor funcall. 

When a method i s actually calledbyaneecti ve method, its rst argument will bea 

list of thearguments to thegeneri c functi on. Its remai ningarguments will beallbut the 

rst argument passed to call-method. Bydefault, all method f uncti ons must accept two 

arguments: the list of arguments to thegeneri c functi on andthe list of next methods. 

For a gi ven generi c functi on and method class, the applicable methods on ma e- 

method-lambda and compute-e ecti ve-method must be consistent in the following 

way: each use of call-method returned bythemethod on compute-e ecti ve-method 

must have thesamenumber of arguments, andthemethod l ambdareturned bythemethod 

on ma e- method-lambda must accept a correspondingnumber of arguments.

72 make-method-lambda Chapter 6 

ote that the system-suppliedimplementationof call-ne t-method is not requiredto 

handle extra arguments to themethod f uncti on. Users who dene additional arguments to 

themethod f uncti on must either redene or f orego call-ne t-method. (See the example 

bel ow.) 

When the method metaobj ect i s created with ma e- i nstance, the method f uncti on 

must betheval ue ofthe :functi on initializationargument. The additional i nitialization 

arguments, returned as the secondval ue of thi s generi c functi on, must also bepassedinthis 

cal l to ma e- i nstance. 

M 

ma e- method-lambda ri mar et hod 



lambda- e ression 

en ironment 

This method returns a method l ambda whi ch accepts twoarguments, the list of arguments 

to the generi c functi on, andthe list of next methods. What i nitialization 

arguments may be returned i n the secondval ue areunspeci ed. 


ample: 

This example shows howto dene akindof method which,fromwithinthebody 

of the method, has access to the actual method metaobj ect f or the method. This 

simplied code overrides whatever method combination is speci ed for the generi c 

functi on, implementingasimplemethod combinationsupportingonlyprimary methods, 

call-ne t-method and ne t-method-p. (In addition, its a simplied version 

of call-ne t-method whichdoes no error checking. ) 

oti ce that the extra lexical functi onbindings get wrapped aroundthebodybef ore 

call-ne t-method is called. In this way, theuser's denitionof call-ne t-method 

and ne t-method-pare sure to overridethesystem's denitions. 

e class m e eric c io s a ar e eric c io 

e a l i i ar s me o class i class m me o 

e class m me o s a ar me o 

e me o ma e me o lam a m e eric c io 

me o m me o 

lam a expressio 

e iro me 

eclare i ore e iro me

Generi c Functi ons andMethods map-dependents 73 

lam a ar s ex me o s is me o 

call ex me o me o 

lam a ca r lam a expressio 

le is me o is me o 

call ex me o res c m ar s 

call me o c io car ex me o s 

or c m ar s ar s 

c r ex me o s 

car ex me o s 

ex me o p 

o ll ex me o s 

c r lam a expressio 

e iro me 

ar s ex me o s 

e me o comp e e ec i e me o m e eric c io 

me o com i a io 

me o s 

call me o car me o s c r me o s car me o s 

a e en ents eneri c unct i on 

S 

map-dependents 

met aobj ect 

unct i on 

A 


The unct i on argument isafuncti on whi ch accepts oneargument. 

The val ue returned i s unspeci ed. 

P 

Thi s generi c functionapplies unct i on to each of thedependents of met aobj ect . The order i n 

whichthedependents are processed is not speci ed, but unct i on is appliedtoeachdependent 

once andonlyonce. If, duringthemapping, add-dependent or remove- dependent 

is calledtoalter thedependents of met aobj ect , it is not speci ed whether the newlyadded 

or removed dependent will have unct i on appliedtoit.

74 method- ::: Chapter 6 

M 

map-dependents ri mar et hod 

met aobj ect s a ar class unct i on 

This method has no speci ed behavior beyond that which is speci ed for the generi c 

functi on. 


well: 

add-dependent (standard-class t) 

remove-dependent (standard-class t) 


met aobj ect calla le s a ar class unct i on 


functi on. 


well: 

add-dependent (funcallable-standard-class t) 

remove-dependent (funcallable-standard-class t) 


met aobj ect s a ar e eric c io unct i on 


functi on. 


well: 

add-dependent (standard-generi c- f uncti ont) 

remove-dependent (standard-generic-functi ont) 

R 

See the\Dependent Mai ntenance Protocol" sectionfor remarks about theuse of this facility. 

et o eneri c unct i on 

The followinggeneri c functi ons aredescri bed together under \Readers f or Method Metaobjects" 

(page 81): method-functi on, method-generic-functi on, method-lambda-list, 

method-speci alizers, method-qualiers and accessor-method-slot-denition.

Generi c Functi ons andMethods Readers f or Class Metaobj ects 75 

ea ers or lass etao ects 

Inthis andtheimmediatelyfollowing secti ons, the\reader" generi c functi ons whichsimply 

return informati on associ ated withaparticular kindof metaobj ect are presented together. 

General i nformationis presented rst, followed byadescri pti on of the purpose of each, and 

endingwiththespeci ed methods forthese generi c functi ons. 

The reader generi c functi onswhichsimplyreturninformati on associ ated withclass metaobj 

ects are presented together i n this section. 

Each of the reader generi c functi ons f or cl ass metaobj ects has the samesyntax, accepting 

one requi red argument calledcl ass , whichmust beanclassmetaobj ect; otherwi se, an error 

is signal ed. Anerror is alsosignal ed i f the classmetaobj ect has not been i nitialized. 

These generi c functi ons can be calledbytheuserortheimplementation. 

For anyof these generi c functi ons whichreturns a list, such lists will not be mutatedby 

theimplementation. The results are undened i f a portable programallows sucha list to 

be mutated. 

cl ass-def ault-initargs eneri c unct i on 

cl ass 

Returns alist of thedefault initializationarguments for cl ass. Each el ement ofthis list 

is acanonicalizeddefault initializationargument. The emptylist is returned i f cl ass has 

no default initializationarguments. 

Duringnalizationnalize-inheri tance calls compute-def ault-initargs to compute 

thedefault initializationarguments for the class. That val ue i s associ ated withthe 

cl ass metaobj ect and is returned by cl ass-def ault-initargs. 

Thi s generi c functionsignals an error if cl ass has not been nal i zed. 

cl ass-di rect- def ault-initargs eneri c unct i on 

cl ass 

Returns a list of thedi rect default initializationarguments for cl ass. Each el ement of 

this list is a canonicalized default initializationargument. The empty list is returned 

if cl ass has no di rect default initializationarguments. This is the defaulted val ue of 

the :di rect- def ault-initargs initializationargument that was associ ated withtheclass 

duringinitializationor reinitialization. 

cl ass-di rect- sl ots eneri c unct i on 

cl ass 

Returnsasetofthedi rect sl ots of cl ass. The elements of this set are di rect sl ot denition 

metaobj ects. If the class has no direct slots, the empty set is returned. This is the

76 Readers f or Class Metaobj ects Chapter 6 

defaultedval ue of the :direct-slots initializationargumentthat was associ ated withthe 

cl ass duringinitializationandreinitialization. 

cl ass-direct-subcl asses eneri c unct i on 

cl ass 

Returnsasetofthedi rect subclasses of cl ass. The elements of this set are class metaobj 

ects that al l mention thi s cl ass among thei r di rect superclasses. The empty set is 

returned i f cl ass has no di rect subclasses. This val ue ismai ntained bythegeneri c functions 

add-direct-subcl ass and remove- direct-subcl ass. 

cl ass-direct-supercl asses eneri c unct i on 

cl ass 

Returns a list of the di rect supercl asses of cl ass. The elements of this list are class 

metaobj ects. The emptylist is returned i f cl ass has no di rect superclasses. This is the 

defaultedval ue of the :direct-supercl asses initializationargumentthat was associ ated 

withtheclassduringinitializationor reinitialization. 

cl ass-nalized-p eneri c unct i on 

cl ass 

Returns trueif cl ass has been nal i zed. Returns f al se otherwise. Also returns false if 

theclasshas not been i nitialized. 

cl ass-name eneri c unct i on 

cl ass 

Returnsthename of cl ass. This val ue can be anyLi sp obj ect, but isusuallyasymbol , 

or nil if theclasshas no name. This is thedefaultedval ue ofthe :name initialization 

argumentthat was associ ated withtheclassduringinitializationorreinitialization. (Also 

see (setf class-name).) 

cl ass-precedence-l i st eneri c unct i on 

cl ass 

Returnstheclassprecedence list of cl ass. The elements of this list are class metaobj ects. 

Duringclass nalization nalize-inheri tance calls compute-class-precedencelist 

to compute theclassprecedence list of the class. That val ue i s associ ated withthe 

cl ass metaobj ect and is returned by cl ass-precedence-l i st. 

Thi s generi c functionsignals an error if cl ass has not been nal i zed.

Generi c Functi ons andMethods Readers f or Class Metaobj ects 77 

cl ass-prototype eneri c unct i on 

cl ass 

Returnsaprototype instance of cl ass. Whether theinstanceis initializedis not speci ed. 

The results are undened i f a portable programmodies the bindingof anyslot of 

prototype instance. 


cl ass-slots eneri c unct i on 

cl ass 

Returnsapossiblyempty set of the slots accessible ininstances of cl ass . The elements 

of this set are eecti ve slotdenitionmetaobj ects. 

Duringclass nalizationnalize-inheritance calls compute-slots to compute the 

sl ots of the class. That val ue i s associ ated withtheclassmetaobj ect and is returned by 

cl ass-slots. 


M 

The speci ed methods fortheclassmetaobj ect reader generi c functi ons arepresentedbel ow. 

Each entry in the table indicates a method on one of the reader generi c functi ons, 

speci al i zed to a speci ed cl ass. The number i neachentry i s a reference tothefull descri ption 

of the method. The full descri ptionsappear after the table.

78 Readers f or Class Metaobj ects Chapter 6 

standard-class forward- built-inand 

referenced- cl ass 

funcallable- cl ass 

standard-class 

cl ass-def ault-initargs 2 3 4 

cl ass-direct-def ault-initargs 1 4 4 

cl ass-direct-sl ots 1 4 4 

cl ass-direct-subcl asses 9 9 7 

cl ass-direct-supercl asses 1 4 7 

cl ass-nalized-p 2 6 5 

cl ass-name 1 1 8 

cl ass-precedence-l i st 2 3 7 

cl ass-prototype 10 10 10 

cl ass-slots 2 3 4 

1. This method returns theval ue whichwas associ ated withtheclassmetaobj ect during 

initializationor reinitialization. 

2. This method returns the val ue associ ated with the class metaobj ect by nalizeinheri 

tance (standard-class) or nalize-inheri tance (funcallable-standardcl 

ass). 

3. This method si gnals an error. 

4. This method returnstheempty list. 

5. This method returns true. 

6. This method returns f al se. 

7. This method returns a val ue deri ved fromthe informati on i n Tabl e 5. 1, except that 

implementation-speci cmodicati ons arepermitted as descri bed i n secti on \Implementation 

andUser Specialization." 

8. This method returnsthename ofthebuilt-inclass. 

9. This methods returnsaval ue whichis mai ntainedby add-direct-subcl ass (class class) 

and remove-direct-subcl ass (class class). This method can be overridden only if 

those methods areoverridden as well. 

10. obehaviorisspeci ed for this method beyond that speci ed for the generi c functi on.

Generi c Functi ons andMethods Readers f or Generi c Functi on Metaobj ects 79 

ea ers or eneric uncti on etao ects 

The reader generi c functi ons whichsimplyreturn informati on associ ated withgeneri c functi 

on metaobj ects are presented together i n this section. 

Each of the reader generi c functi ons f or generi c functi on metaobj ects has the samesyntax, 

acceptingonerequi red argument calledgeneri c- unct i on, whichmust be a generi c functi 

on metaobj ect; otherwise, an error is signal ed. An error is also signal ed i f the generi c 

functi on metaobj ect has not been i nitialized. 




functi on. 

generi c- f uncti on-argument-precedence-order eneri c unct i on 


Returns the argument precedence order of the generi c functi on. This val ue is a list 

of symbol s, a permutation of therequiredparameters i n the lambdalist of thegeneri c 

functi on. This isthedefaultedval ue of the :argument-precedence-order initialization 

argument that was associ ated withthegeneri c functi on metaobj ect duringinitialization 

or rei nitialization. 

generi c- f uncti on-decl arations eneri c unct i on 


Returnsapossiblyemptylist of thedecl arati ons ofthegeneri c functi on. The elements 

of this list are decl arati ons. This list is thedefaulted val ue of the :decl arations initializationargument 

that was associ ated with the generi c functi on metaobj ect during 

initializationor reinitialization. 

generi c- f uncti on-lambda-list eneri c unct i on 


Returns the lambda list of the generi c functi on. This is the defaulted val ue of the 

:lambda-listinitializationargumentthat was associ ated withthegeneri c functi onmetaobj 

ect duringinitializationor reinitialization. An error is signal ed i f the lambdalist has 

yet to be supplied.

80 Readers f or Generi c Functi on Metaobj ects Chapter 6 

generi c- f uncti on-method-class eneri c unct i on 


Returnsthedefault method cl ass of the generi c functi on. This class must beasubcl ass 

of the classmethod. This is thedefaulted val ue ofthe :method-class initialization 



generi c- f uncti on-method-combination eneri c unct i on 


Returnsthemethod combinati on of the generi c functi on. This is amethod combination 

metaobj ect. This is the defaulted val ue of the :method-combination initialization 



generi c- f uncti on-methods eneri c unct i on 


Returnsthe set of methods currently connected to the generi c functi on. This is aset of 

method metaobj ects. This val ue ismai ntained bythegeneri c functi ons add-method 

and remove-method. 

generi c- f uncti on-name eneri c unct i on 


Returns thename ofthegeneri c functi on, or nil if thegeneri c functi on has no name. 

This is thedefaulted val ue ofthe :name initializationargument that was associ ated 

withthegeneri c functi on metaobj ect duringinitializationor reinitialization. (Also see 

(setf generic-functi on-name).) 

M 

The speci ed methods for the generi c functi on metaobj ect reader generi c functi ons are 

presented bel ow.

Generi c Functi ons andMethods Readers f or Method Metaobj ects 81 

generi c- f uncti on-argument-precedence-order ri mar et hod 


generi c- f uncti on-decl arations ri mar et hod 


generi c- f uncti on-lambda-list ri mar et hod 


generi c- f uncti on-method-class ri mar et hod 


generi c- f uncti on-method-combination ri mar et hod 


generi c- f uncti on-name ri mar et hod 


obehavior is speci ed for these methods beyond that whichis speci ed for thei r respecti 

ve generi c functi ons. 

generi c- f uncti on-methods ri mar et hod 


obehaviorisspeci ed for this method beyondthat whichis speci ed for thei r respecti ve 

generi c functi ons. 

The val ue returned by this method i s mai ntained by add-method (standardgeneri 

c- f uncti on standard-method) and remove- method (standard-generi c- 

functi onstandard-method). 

ea ers or et o etao ects 

The reader generi c functi ons whichsimplyreturninformati on associ ated withmethod metaobj 

ects are presented together here i n the f ormat descri bed under \Readers f or Class Metaobj 

ects. " 

Each of these reader generi c functi ons have the samesyntax, acceptingone required 

argumentcalledmet hod, whichmust beamethodmetaobj ect; otherwise,anerrorissignal ed. 

Anerror is alsosignal ed i f the method metaobj ect has not been i nitialized. 




be mutated.

82 Readers f or Method Metaobj ects Chapter 6 

method-functi on eneri c unct i on 

met hod 

Returns themethod f uncti on of met hod. This is thedefaulted val ue ofthe :functi on 

initializationargument that was associ ated withthemethod duringinitialization. 

method-generi c- f uncti on eneri c unct i on 

met hod 

Returns thegeneri c functi on that met hod is currently connected to, or nil if it is not 

currently connected to any generi c functi on. This val ue iseither a generi c functi on 

metaobj ect or nil. When a method i s rst created it is not connected to anygeneri c 

functi on. This connecti on i s mai ntained by the generi c functi ons add-method and 

remove- method. 

method-lambda-list eneri c unct i on 

met hod 

Returnsthe(unspeci al i zed) lambdalist of met hod. This val ue isaCommon Lisplambda 

list. This is thedefaulted val ue of the :lambda-list initializationargument that was 

associ ated withthemethod duringinitialization. 

method-speci alizers eneri c unct i on 

met hod 

Returns a list of thespeci al i zers of met hod. This val ue isalistofspeci al i zer metaobjects. 

This is thedefaultedval ue of the :speci alizers initializationargument that was 


method-qualiers eneri c unct i on 

met hod 

Returnsa(possiblyempty) list of thequaliers of met hod. This val ue isalistofnon- nil 

atoms. This is thedefaultedval ue of the :qualiers initializationargument that was 


accessor-method-slot-denition eneri c unct i on 

met hod 

Thi s accessor can only be calledonaccessor methods. It returns thedi rect sl ot denitionmetaobj 

ect that dened this method. This is theval ue of the :slot-denition 

initializationargument associ ated withthemethod duringinitialization. 

M 

The speci ed methods forthemethod metaobj ect readers are presented bel ow.

Generi c Functi ons andMethods Readers f or Slot DenitionMetaobj ects 83 

method-functi on ri mar et hod 


method-lambda-list ri mar et hod 


method-speci alizers ri mar et hod 


method-qualiers ri mar et hod 




method-generi c- f uncti on ri mar et hod 


obehavior is speci ed for this method beyond that whichis speci ed f or i ts generi c 

functi on. 

The val ue returned by this method i s mai ntained by add-method (standardgeneri 

c- f uncti on standard-method) and remove- method (standard-generi c- 

functi onstandard-method). 

accessor-method-slot-denition ri mar et hod 

met hod s a ar accessor me o 

obehavior is speci ed for this method beyond that whichis speci ed f or i ts generi c 

functi on. 

ea ers or Slot e nition etao ects 

The reader generi c functi ons whichsimplyreturninformati on associ ated withslot denition 

metaobj ects are presented together here i n the f ormat descri bed under \Readers f or Class 

Metaobj ects. " 

Each of the reader generi c functi ons f or sl ot denitionmetaobj ects has the samesyntax, 

acceptingonerequi red argument called sl ot, which must be a sl ot denition metaobj ect; 

otherwise, anerror is signal ed. Anerror is alsosignal ed i f the slotdenitionmetaobj ect has 

not been i nitialized. 

These generi c functi ons can be calledbytheuserortheimplementation.

84 Readers f or Slot DenitionMetaobj ects Chapter 6 



be mutated. 

G F 

slot-denition-allocation eneri c unct i on 

sl ot 

Returnsthe al l ocati on of sl ot. This is asymbol . This is thedefaultedval ue of the :allocation 

initializationargument that was associ ated withtheslotdenitionmetaobj ect 

duringinitialization. 

slot-denition-initargs eneri c unct i on 

sl ot 

Returnsthesetofinitializationargument keywords forsl ot. This is thedefaultedval ue 

of the :initargs initialization argument that was associ ated with the slot denition 

metaobj ect duringinitialization. 

slot-denition-initform eneri c unct i on 

sl ot 

Returnstheinitializationformof sl ot. This canbeany f orm. This is thedefaultedval ue 

of the :initforminitialization argument that was associ ated with the slot denition 

metaobj ect duringinitialization. When sl ot has noinitialization f orm, theval ue returned 

is unspeci ed (however, slot-denition-initfuncti on is guaranteed to return nil). 

slot-denition-initfuncti on eneri c unct i on 

sl ot 

Returnstheinitializationfuncti onof sl ot. This val ue iseither a functi onof noarguments, 

or nil,indicatingthat the slothas no initializationfuncti on. This is thedefaultedval ue 

of the :initfuncti on initializationargumentthat was associ ated withtheslotdenition 


slot-denition-name eneri c unct i on 

sl ot 

Returnsthename of sl ot. This val ue isasymbol that can be used as a vari able name. 

This is theval ue of the :name initializationargument that was associ ated withtheslot 

denitionmetaobj ect duringinitialization.

Generi c Functi ons andMethods Readers f or Slot DenitionMetaobj ects 85 

slot-denition-type eneri c unct i on 

sl ot 

Returns the al l ocati on of sl ot. This is a type speci er name. This is the defaulted 

val ue ofthe :name initializationargument that was associ ated withtheslotdenition 


M 

The speci ed methods fortheslotdenitionmetaobj ect readers are presented bel ow. 

slot-denition-allocation ri mar et hod 

sl ot - de nition s a ar slo e i i io 

slot-denition-initargs ri mar et hod 


slot-denition-initform ri mar et hod 


slot-denition-initfuncti on ri mar et hod 


slot-denition-name ri mar et hod 


slot-denition-type ri mar et hod 




D S D M 

The followingadditional reader generi c functi ons aredened for di rect sl ot denitionmetaobj 

ects. 

slot-denition-readers eneri c unct i on 

di rect -sl ot 

Returns a (possibly empty) set of readers of the di rect sl ot. This val ue is a list of 

functi on names. This is thedefaultedval ue of the :readers initializationargument that 

was associ ated withthedi rect sl ot denitionmetaobj ect duringinitialization.

86 reader-method-class Chapter 6 

slot-denition-wri ters eneri c unct i on 

di rect -sl ot 

Returns a (possibly empty) set of writers of the di rect sl ot. This val ue is a list of 

functi on names. This is thedefaultedval ue of the :wri ters initializationargument that 

was associ ated withthedi rect sl ot denitionmetaobj ect duringinitialization. 

slot-denition-readers ri mar et hod 

di rect -sl ot -de nition s a ar irec slo e i i io 

slot-denition-wri ters ri mar et hod 

di rect -sl ot -de nition s a ar irec slo e i i io 

obehavior is speci ed for these methods beyond what i s speci ed for thei r generi c 

functi ons. 

E S D M 

The followingreader generi c functi on i s dened for eecti ve slotdenitionmetaobj ects. 

slot-denition-location eneri c unct i on 

e ect i e- sl ot - de nition 

Returnsthe l ocati on of e ect i e- sl ot - de nition. The meaningandinterpretation of this 

val ue isdescri bed i n the sectioncalled\Instance Structure Protocol." 

slot-denition-location ri mar et hod 

e ect i e- sl ot - de nition s a ar e ec i e slo e i i io 

This method returns theval ue stored by compute-slots : around (standard-class) 

and compute-slots : around(funcallable-standard-class). 

rea er et o class eneri c unct i on 

S 

reader- method-class 

class direct-slot res ini t args 

A 


The di rect -sl ot argument isadi rect sl ot denitionmetaobj ect. 

The ini t args argument consists of alternati ng initializationargument names andval ues.

Generi c Functi ons andMethods remove-dependent 87 

The val ue returnedis aclass metaobj ect. 

P 

Thi s generi c function is called to determine the class of reader methods created during 

cl ass i nitializationandreinitialization. The result must beasubcl ass of standard-readermethod. 

The ini t args argument must bethesameaswill bepassed to ma e- i nstance to create 

thereader method. The ini t args must incl ude :slot-denition with sl ot - de nition as i ts 

val ue. 

M 

reader- method-class ri mar et hod 


di rect -sl ot s a ar irec slo e i i io 


reader- method-class ri mar et hod 




These methods return the class standard-reader-method. 

overridden. 

These methods can be 

re o e e en ent eneri c unct i on 

S 

remove- dependent 

met aobj ect de endent 

A 


The de endent argument i s an obj ect. 


P 

Thi s generi c functi on removes de endent fromthedependents of met aobj ect. If de endent 

is not one ofthedependents of met aobj ect, noerrorissignal ed. 

The generi c functi on map-dependents can be cal l ed to access the set of dependents 

of a cl ass or generi c functi on. The generi c functi on add-dependent can be calledto add 

an obj ect f romthe set of dependents of a cl ass or generi c functi on. The eect of calling

88 remove-dependent Chapter 6 

add-dependent or remove-dependent while a call to map-dependents on the same 

classorgeneri c functionis inprogress i s unspeci ed. 

The situati ons inwhichremove- dependent is calledare not speci ed. 

M 

remove- dependent ri mar et hod 

cl ass s a ar class de endent 


functi on. 


well: 

add-dependent (standard-class t) 

map-dependents (standard-class t) 


cl ass calla le s a ar class de endent 


functi on. 


well: 

add-dependent (funcallable-standard-class t) 

map-dependents (funcallable-standard-class t) 


generi c- unct i on s a ar e eric c io de endent 


functi on. 


well: 

add-dependent (standard-generi c- f uncti ont) 

map-dependents (standard-generic-functi ont) 

R 

See the\Dependent Mai ntenance Protocol" sectionfor remarks about theuse of this facility.

Generi c Functi ons andMethods remove-di rect-method 89 

re o e irect et o eneri c unct i on 

S 

remove- di rect- method 

s eci al i er met hod 

A 



The val ue returned by remove-di rect- method is unspeci ed. 

P 

Thi s generi c functionis calledto mai ntai n a set of backpoi nters froma speci al i zer to the 

set of methods speci al i zed to i t. If met hod is inthe set it is removed. If it is not, no error 

is signal ed. 

Thi s set can be accessed as a list by callingthegeneri c functi on speci alizer-directmethods. 

Methods areadded to thesetby add-di rect- method. 

The generi c functi on remove- di rect- method is calledby remove- method whenever 

amethod i s removed fromageneri c functi on. Itiscalledonce f or each of the speci al i zers of 

themethod. ote that i n cases where a speci al i zer appears more than once i nthespeci al i zers 

of a method, thi s generi c functi on will be calledmore than once withthesamespeci al i zer 

as argument. 

The results are undened i f the s eci al i er argument isnot one ofthespeci al i zers of the 

met hod argument. 

M 

remove- di rect- method ri mar et hod 


met hod me o 

This method i mplements thebehavior of thegeneri c functi on f or cl ass speci al i zers. o 


functi on. 


well: 

add-di rect- method(class method) 


speci alizer-di rect- methods (class)

90 remove-di rect-subcl ass Chapter 6 

remove- di rect- method ri mar et hod 


met hod me o 

This method i mplements thebehavior of thegeneri c functi on for eql speci al i zers. o 


functi on. 

re o e irect su class eneri c unct i on 

S 

remove- direct-subcl ass 

su ercl ass subcl ass 

A 

The su ercl ass argument isaclassmetaobj ect. 

The subcl ass argument isaclassmetaobj ect. 


P 

Thi s generi c functionis calledto mai ntai n a set of backpoi nters froma class to its di rect 

subclasses. It removes subcl ass fromthe set of di rect subclasses of su ercl ass. o error is 

signal ed i f subcl ass is not i n this set. 

Whenever a class is reinitialized, thi s generi c functionis calledonce witheachdel eted 

di rect supercl ass of the class. 

M 

remove- direct-subcl ass ri mar et hod 


subcl ass class 


functi on. 


well: 

add-direct-subcl ass (class class) 

cl ass-direct-subcl asses (class)

Generi c Functi ons andMethods remove-method 91 

re o e et o eneri c unct i on 

S 

remove- method 

generi c- unct i on met hod 

A 



The generi c- unct i on argument isreturned. 

P 

Thi s generi c functionbreaksthe associationbetween a generi c functi on andoneofitsmethods. 

o error is signal ed i f the method i s not among themethods ofthegeneri c functi on. 

Breakingthe association between the method and thegeneri c functi on proceeds infour 

steps: (i) remove met hod fromthe set returned by generic-functi on-methods andarrange 

for method-generic-functi on to return nil; (ii) call remove-di rect- method f or each 

of the method's specializers; (iii) call compute-di scri minating-functi on and install its 

result with set-funcallable-instance-f uncti on; and(iv) update the dependents of the 

generi c functi on. 

The generi c functi on remove- methodcan be calledbytheuser or theimplementation. 

M 

remove- method ri mar et hod 




functi on. 

set uncalla le instance uncti on unct i on 

S 

set-funcallable-instance-f uncti on 

uncal l abl e- i nstance unct i on 

A 

The uncal l abl e- i nstance argument isafuncallable instance (i t must have been returned by 

allocate-instance (f uncallable-standard-class)). 

The unct i on argument isafuncti on.

92 (setf generi c-functi on-name) Chapter 6 

The val ue returned bythis functi on i s unspeci ed. 

P 

This functi on i s cal l ed to set or to change the functi on of a funcallable instance. After 

set-funcallable-instance-f uncti on is called, anysubsequent calls to uncal l abl e- i nstance 

will runthenewfuncti on. 

set class na e unct i on 

S 

(setf class-name) eneri c unct i on 

ne -name class 

A 


The ne -name argument isanyLi sp obj ect. 

R 

This functi on returnsits ne 

-name argument. 

P 

This functi on changes the name of cl ass to ne -name. This val ue isusuallya symbol , or 

nil if theclasshas no name. 

This functi on works bycalling rei nitialize-instance with cl ass as i ts rst argument, 

thesymbol :name as i ts secondargument and ne -name as i ts thirdargument. 

set eneric uncti on na e unct i on 

S 

(setf generi c- f uncti on-name) eneri c unct i on 

ne -name generi c- unct i on 

A 


The ne -name argument isafuncti on name ornil. 

R 

This functi on returnsits ne 

-name argument.

Generi c Functi ons andMethods (setf slot-val ue-using- cl ass) 93 

P 

This functi on changes the name of generi c- unct i on to ne -name. This val ue isusually 

afuncti on name (i.e., a symbol or a l i st of the f orm(setf s mbol )) ornil, if thegeneri c 

functi on i s to have noname. 

This functi on works bycalling rei nitialize-instance withgeneri c- unct i on as i ts rst 

argument, thesymbol :name as i ts secondargument and ne -name as i ts thirdargument. 

set sl ot alue usi n class eneri c unct i on 

S 

(setf slot-value- usi ng-class) 

ne - al ue cl ass object sl ot 

A 

The ne 

- al ue argument i s an obj ect. 

The cl ass argument isaclassmetaobj ect. I t i s the class of the obj ect argument. 

The obj ect argument i s an obj ect. 

The sl ot argument isaneecti ve slotdenitionmetaobj ect. 

Thi s generi c functi on returnsthe ne 

- al ue argument. 

P 

The generi c functi on (setf slot-value-using-class) implements thebehavior of the (setf 

slot-value) functi on. It is calledby (setf slot-value) withtheclassofobj ect as i ts second 

argument andthepertinent eecti ve slotdenitionmetaobj ect as i ts f ourth argument. 

The generi c functi on(setf slot-value- using-class) sets theval ue containedi nthegiven 

sl ot of the given object to the given newval ue; anypreviousval ue is lost. 

The results are undened i f the cl ass argument isnot the class of the obj ect argument, 

or i f the sl ot argument does not appear among the set of eecti ve slots associated withthe 

cl ass argument.

94 slot-boundp-using- cl ass Chapter 6 

M 

(setf slot-value- usi ng-class) ri mar et hod 

ne - al ue 


obj ect 

sl ot s a ar e ec i e slo e i i io 


ne - al ue 


obj ect 


These methods implementthefull behavior of thi s generi c functi on f or sl ots withallocation 

:instance and :class. If thesuppliedslot has an al l ocati on other than :instance 

or :class an error is signal ed. 

Overridingthese methods ispermitted,but mayrequire overridingother methods in 

the standard i mplementationof the sl ot access protocol. 


ne - al ue 

cl ass il i class 

obj ect 

sl ot 


sl ot oun usi n class eneri c unct i on 

S 

slot-boundp-usi ng-class 

cl ass obj ect sl ot 

A 




Thi s generi c functi on returns true or f al se.

Generi c Functi ons andMethods slot-denition- ::: 95 

P 

Thi s generi c functi on i mplements thebehavior of the slot-boundpfuncti on. It is calledby 

slot-boundp withtheclassof obj ect as i ts rst argument andthepertinent eecti ve slot 

denitionmetaobj ect as i ts thirdargument. 

The generi c functi on slot-boundp-using-class tests whether a speci c slotinaninstance 

i s bound. 



cl ass argument. 

M 

slot-boundp-usi ng-class ri mar et hod 


obj ect 




obj ect 









obj ect 

sl ot 


R 

In cases where the classmetaobj ect cl ass does not distinguishunbound slots, trueshould 

be returned. 

sl ot e nition eneri c unct i on 

The following generi c functi ons are descri bed together under \Readers f or Slot Definition 

Metaobj ects" (page 83): slot-denition-allocation, slot-denition-initargs,

96 slot-makunbound-using- cl ass Chapter 6 

slot-denition-initform, slot-denition-initfuncti on, slot-denition-location, slotdenition-name, 

slot-denition-readers, slot-denition-writers and slot-denitiontype. 

sl ot a un oun usi n class eneri c unct i on 

S 

slot-ma unbound-using-class 


A 




Thi s generi c functi on returnsits obj ect argument. 

P 

Thi s generi c functi on i mplements thebehavior of the slot-ma unbound functi on. It is 

calledby slot-ma unboundwiththeclassofobj ect as i ts rst argumentandthepertinent 

eecti ve slotdenitionmetaobj ect as i ts thirdargument. 

The generi c functi on slot-ma unbound-usi ng-class restores a slot in an object to its 

unboundstate. The interpretationof \restoringaslottoitsunboundstate" depends onthe 

cl ass metaobj ect cl ass. 



cl ass argument. 

M 

slot-ma unbound-using-class ri mar et hod 


obj ect 




obj ect 




or :class an error is signal ed.

Generi c Functi ons andMethods slot-val ue-using- cl ass 97 





obj ect 

sl ot 


sl ot alue usi n class eneri c unct i on 

S 

slot-value- using-class 


A 




The val ue returned bythi s generi c functi on i s an object. 

P 

Thi s generi c functi on i mplements thebehavior of the slot-value functi on. It is calledby 

slot-value with the class of obj ect as i ts rst argument andthe pertinent eecti ve slot 

denitionmetaobj ect as i ts thirdargument. 

The generi c functi on slot-value- using-class returns theval ue contained i n the given 

sl ot of the given object. If the slotisunbound slot-unbound is called. 



cl ass argument.

98 speci al i zer-di rect-generi c-functi ons Chapter 6 

M 

slot-value- using-class ri mar et hod 


obj ect 




obj ect 









obj ect 

sl ot 


s ecializer irect eneric uncti ons eneri c unct i on 

S 

speci alizer-di rect-generi c- f uncti ons 

s eci al i er 

A 


The result of thi s generi c functi on i s a possiblyemptylist of generi c functi on metaobj ects. 

P 

Thi s generi c functi on returnsthepossiblyempty set of those generi c functi ons whichhave 

amethod with s eci al i er as a speci al i zer. The elements of this set are generi c functi on 

metaobj ects. This val ue ismai ntained bythegeneri c functi ons add-di rect- method and 

remove- di rect- method.

Generi c Functi ons andMethods speci al i zer-di rect-methods 99 

M 

speci alizer-di rect-generi c- f uncti ons ri mar et hod 



functi on. 


well: 



speci alizer-di rect- methods (class) 

speci alizer-di rect-generi c- f uncti ons ri mar et hod 



functi on. 

s ecializer irect et o s eneri c unct i on 

S 

speci alizer-di rect-methods 

s eci al i er 

A 


The result of thi s generi c functi on i s a possiblyemptylist of method metaobj ects. 

P 

Thi s generi c functi on returnsthepossiblyemptyset of those methods, connected to generi c 

functi ons, which have s eci al i er as a speci al i zer. The elements of this set are method 

metaobj ects. This val ue ismai ntained bythegeneri c functi ons add-di rect- method and 

remove- di rect- method. 

M 

speci alizer-di rect-methods ri mar et hod 



functi on. 


well:

100 standard-instance-access Chapter 6 




speci alizer-di rect-methods ri mar et hod 



functi on. 

stan ar instance access unct i on 

S 

standard-instance-access 

instance l ocation 

A 

The instance argument i s an obj ect. 

The locat i on argument is a slot location. 

The result of this functi on i s an object. 

P 

This functionis calledto providedi rect access to a sl ot i n an i nstance. By usurpingthe 

normal sl ot l ookupprotocol, this functi on i s i ntended to providehighlyoptimi zed access to 

the slots associated withaninstance. 

The following restrictionsapply to theuse of this functi on: 

The instance argument must be a standard i nstance (i t must have been returned by 

allocate-instance (standard-class)). 

The instance argument cannot be annon-updated obsolete instance. 

The locat i on argument must be a l ocati on of one ofthedi rectl y accessi bl e sl ots of the 

instance' s cl ass. 

The slotmust bebound. 

The results are undened i f anyof these restrictionsarenot met.

Generi c Functi ons andMethods update- dependent 101 

u ate e en ent eneri c unct i on 

S 

update-dependent 

met aobj ect de endent res ini t args 

A 

The met aobj ect argument is a class or generi c functi on metaobj ect. It is themetaobj ect 

bei ngreinitializedor otherwise modied. 

The de endent argument isanobject. Itisthedependent bei ngupdated. 

The ini t args argument isalistoftheinitializationarguments for themetaobj ect redenition. 

The val ue returned by update-dependent is unspeci ed. 

P 

Thi s generi c functionis calledtoupdate a dependent of met aobj ect. 

When a cl ass or a generi c functi on i s rei nitializedeachof its dependents is updated. The 

ini t args argument toupdate-dependent is thesetofinitializationarguments recei ved by 

rei nitialize-instance. 

When a method i s added to a generi c functi on, eachofthegeneri c functi on's dependents 

is updated. The ini t args argument isalistoftwoelements: thesymbol add-method, and 

themethod that was added. 

When a method i s removed f roma generi c functi on, each of the generi c functi on's dependents 

is updated. The ini t args argument isalistoftwoelements: thesymbol removemethod, 

andthemethod that was removed. 

In each case, map-dependents is used to cal l update-dependent on each of the 

dependents. So, f or example, theupdate of a generi c functi on's dependents when a method 

is added couldbeperformed bythe followingcode: 

map epe e s generi c- unct i on 

lam a ep 

p a e epe e generi c- unct i on 

ep 

a me o 

ne -met hod 

M 

There are nospeci ed methods onthi s generi c functi on. 

R 

See the\Dependent Mai ntenance Protocol" sectionfor remarks about theuse of this facility.

102 validate- supercl ass Chapter 6 

ali ate su erclass eneri c unct i on 

S 

validate-supercl ass 

cl ass su ercl ass 

A 


The su ercl ass argument isaclassmetaobj ect. 

Thi s generi c functi on returns true or f al se. 

P 

Thi s generi c functionis calledtodeterminewhether the classsu ercl ass is suitable for use 

as a supercl ass of cl ass. 

Thi s generi c functi on can be be calledbytheimplementationor user code. It is called 

duringclass metaobj ect i nitializationandreinitialization,bef ore the di rect superclasses are 

stored. If thi s generi c functi on returns f al se, the initializationor reinitializationwill signal 

an error. 

M 

validate-supercl ass ri mar et hod 

cl ass class 


This method returns trueinthree situati ons: 

(i) If the su ercl ass argument istheclassnamed , 

(ii) if the class of the cl ass argument isthesameasthe class of the su ercl ass 

argument or 

(iii) if the classes oneofthearguments is standard-class andthe class of the 

other i s funcallable-standard-class. 

In all other cases, this method returns f al se. 


R 

Deningamethod on validate-supercl ass requires detailedknowledge of of the internal 

protocol followed by each of the twoclass metaobj ect cl asses. Amethod on validatesupercl 

ass whichreturns truefortwodierent classmetaobj ect cl asses decl ares that they 

are compati ble.

Generi c Functi ons andMethods writer-method-class 103 

riter et o class eneri c unct i on 

S 

wri ter-method-class 

class direct-slot res ini t args 

A 


The di rect -sl ot argument isadi rect sl ot denitionmetaobj ect. 

The ini t args argument isalistofinitializationarguments andval ues. 

The val ue returnedis aclass metaobj ect. 

P 

Thi s generi c function is called to determine the cl ass of writer methods created during 

cl ass i nitializationandreinitialization. The result must beasubcl ass of standard-writermethod. 

The ini t args argument must bethesameaswill bepassed to ma e- i nstance to create 

thereader method. The ini t args must incl ude :slot-denition with sl ot - de nition as i ts 

val ue. 

M 

wri ter-method-class ri mar et hod 




wri ter-method-class ri mar et hod 




These methods returns the class standard-writer-method. 

overridden. 

These methods can be

104

o 

[CLtLII] Steel e, Guy ommon is he anguage SecondEdition,Digital Press, 1990. 

[X3J13] Bobrow, Daniel G. Linda G. Demichiel, Richard P. Gabriel, Sonya E. Keene, 

Gregor Ki czal es, andDavidA. Moon ommon is bj ect stem eci cat i on 

X3J13 Document 88- 002R, June 1988; appears i n is and mbol i c om utation 

1, 3 4, January 1989, 245 394, and as Chapter 28 of [CLtLII ] , 770 864. 

105

The Art of the Metaobject Protocol Chapters 5 and 6

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