ThouShaltNotSpecializestdFunctionTemplatesDocumentWG21P0551R1Date2017 - PDF document

1 ThouShaltNotSpecializestdFunctionTemplat
ThouShaltNotSpecializestdFunctionTemplates!Document#:WG21P0551R1Date:2017-10-14Project:JTC1.22.32ProgrammingLanguageC++Audience:LEWG)LWGReplyto:WalterE.Brownwebrown.cpp@gmail.com� Contents1Introduction.............12Discussion..............12.1Whatwesaytoday........12.2What'swrongwiththat?....22.3Whatshouldwedo?.......43Analternateapproach.......44Proposedwording..........65Acknowledgments..........76Bibliography.............77Documenthistory..........7 AbstractThispaperproposestomodifyclause[namespace.std]soas(a)toforbidusersfromspecializingstandardlibraryfunctiontemplatesand(b)toallowfunctionobjectsasimplementationsofstandardlibraryfacilitiesspeciedasfunctiontemplates.Tocreatearchitectureistoputinorder.Putwhatinorder?Functionandobjects.—LECORBUSIER,néCHARLES-ÉDOUARDJEANNERETSpecializationisforinsects.—ROBERTANSONHEINLEIN1IntroductionSpecializingfunctiontemplateshasprovenproblematicinpractice;specializingfunctiontemplatesinnamespacestdhasprovenevenmoreproblematic.Thispaper(a)willciteknowledgableandwell-respectedcolleaguesindescribingthecorelanguagecausesoftheissuesinvolved,and(b)willthenrecommendwordingadjustments(mostlytosubclause[namespace.std])toaddresstheseissuesinthecontextofthestandardlibrary.2Discussion2.1WhatwesaytodayQuotedverbatimfrom[N4687],hereistheentiretyofsubclause[namespace.std]: 1ThebehaviorofaC++programisundenedifitaddsdeclarationsordenitionstonamespacestdortoanamespacewithinnamespacestdunlessotherwisespecied.Aprogrammayaddatemplatespecializationforanystandardlibrarytemplatetonamespacestdonlyifthedeclarationdependsonauser-denedtypeandthespecializationmeetsthestandardlibrary Copyrightc 2017byWalterE.Brown.Allrightsreserved.1 2P0551R1:ThouShaltNotSpecializestdFunctionTemplates! requirementsfortheoriginaltemplateandisnotexplicitlyprohibited.[Footnote:Anylibrarycodethatinstantiatesotherlibrarytemplatesmustbepreparedtoworkadequatelywithanyuser-suppliedspecializationthatmeetstheminimumrequirementsofthisInternationalStandard.]2ThebehaviorofaC++programisundenedifitdeclaresanexplicitorpartialspecializationofanystandardlibraryvariabletemplate,exceptwhereexplicitlypermittedbythespecicationofthatvariabletemplate.3ThebehaviorofaC++programisundenedifitdeclares(3.1)—anexplicitspecializationofanymemberfunctionofastandardlibraryclasstemplate,or(3.2)—anexplicitspecializationofanymemberfunctiontemplateofastandardlibraryclassorclasstemplate,or(3.3)—anexplicitorpartialspecializationofanymemberclasstemplateofastandardlibraryclassorclasstemplate,or(3.4)—adeductionguideforanystandardlibraryclasstemplate.Aprogrammayexplicitlyinstantiateatemplatedenedinthestandardlibraryonlyifthedeclarationdependsonthenameofauser-denedtypeandtheinstantiationmeetsthestandardlibraryrequirementsfortheoriginaltempl

2 ate.4Atranslationunitshallnotdeclarename
ate.4Atranslationunitshallnotdeclarenamespacestdtobeaninlinenamespace(10.3.1).(Notethatparagraph2andbullet3.4arerelativelyrecentadditionstothisspecication.)2.2What'swrongwiththat?Let'sstartwithasimpleexampleinwhichthenamegisobviouslyoverloaded: 1templateTlas;&#xs-60;�voidg(Tconst&);//functiontemplate 2�templatevoidg&#xint0;(int);//explicitspecialization 3voidg(double);//functionGiventhesedeclarationsandconsideringsuchcallsasg(42),g(4.2),g('4'),org("4.2"),what'sthemaximumsizeofthesetofcandidatefunctionsthatwouldbeconsideredduringoverloadresolution?ManyC++programmersseemsurprisedbytheanswer.STOP!Pleasehaveananswerinmindbeforereadingfurther. P0551R1:ThouShaltNotSpecializestdFunctionTemplates!3 SPOILERALERT!,Answer:Theoverloadsetdescribedabovewouldconsistofatmostonlytwocandidates:thefunctiong(double)(declaredinline3above),and&#xT000;g(const&),acompiler-synthesizedfunctiontemplatespecializationoftheprimarytemplate(line1above),withthetypeTdeducedfromthetypeofthecall'sargument.Mostprogrammersseemtounderstandthattheordinaryfunction(line3)shouldparticipateinoverloadresolution.However,experiencesuggeststhatmanyC++programmersexpectthesetofcandidatesforoverloadresolutiontoincludetheprimarytemplate(line1),theexplicitspecialization(line2),orboth.Reline1:Giventhatthecompileristryingtondafunctiontocall,noprimarytemplatecanbeacandidate:overloadresolutionconsidersonlyfunctiondeclarations,andaprimarytemplateis,afterall,notafunction.However,afunctioncanbecompiler-synthesizedfromaprimaryfunctiontemplate—ultimately,that'sthepurposeofdeclaringatemplate.Reline2:Explicitspecializationsarenevercandidatestoparticipateinoverloadresolution.However,ifoverloadresolutionselectsthesynthesizedspecializationasthebettermatch,thenpartialorderingwilldeterminewhetheritis(a)theexplicittemplatespecializationor(b)thesynthesizedtemplatespecializationthatistobe(instantiated,ifneeded,andthen)called.AsproclaimedbyseveralC++cognoscenti,itisforthesereasonsapoorC++programmingpracticetospecializeafunctiontemplate,especiallysoinnamespacestd.Herearerepresentativesummaryexplanationsandadvice:HerbSutter:“specializationsdon'tparticipateinoverloading....Ifyouwanttocustomizeafunctionbasetemplateandwantthatcustomizationtoparticipateinoverloadresolution(or,toalwaysbeusedinthecaseofexactmatch),makeitaplainoldfunction,notaspecialization.And,ifyoudoprovideoverloads,avoidalsoprovidingspecializations.”1DavidAbrahams:“it'swrongtousefunctiontemplatespecialization[because]itinteractsinbadwayswithoverloads....Forexample,ifyouspecializetheregularstd::swapfor&#xmyty;&#xpe00;std::vector,yourspecializationwon'tgetchosenoverthestandard'svector-specicswap,becausespecializationsaren'tconsideredduringoverloadresolution.”2HowardHinnant:“thisissuehasbeensett

3 ledforalongtime....DisregardDave'sexpert
ledforalongtime....DisregardDave'sexpertopinion/answerinthisareaatyourownperil.”3EricNiebler:“[becauseof]thedecidedlywonkywayC++resolvesfunctioncallsintem-plates...,[w]emakeanunqualiedcalltoswapinordertondanoverloadthatmightbedenedin...associatednamespaces...,andwedousingstd::swapsothat,ontheoff-chancethatthereisnosuchoverload,wendthedefaultversiondenedinthestdnamespace.”4HighIntegrityC++CodingStandard:“Overloadresolutiondoesnottakeintoaccountexplicitspecializationsoffunctiontemplates.Onlyafteroverloadresolutionhaschosenafunctiontemplatewillanyexplicitspecializationsbeconsidered.”5 1HerbSutter:“WhyNotSpecializeFunctionTemplates?”http://www.gotw.ca/publications/mill17.htm,2009(re-trieved2016–10–17).OriginallypublishedinC/C++UsersJournal,19(7),July2001.2DavidAbrahams:Replyto“Howtooverloadstd::swap().”http://stackoverow.com/questions/11562/how-to-overload-stdswap#comment-5729583,2011–02–24(retrieved2016–10–17).3HowardHinnant:Replyto“Howtooverloadstd::swap().”http://stackoverow.com/questions/11562/how-to-overload-stdswap#8439357,2011–12–08(retrieved2016–10–17).4EricNiebler:“CustomizationPointDesigninC++11andBeyond.”http://ericniebler.com/2014/10/21/customization-point-design-in-c11-and-beyond,2014–10–21(retrieved2016–10–17).5ProgrammingResearchLtd.:“Donotexplicitlyspecializeafunctiontemplatethatisoverloadedwithothertemplates,”InHighIntegrityC++CodingStandard,version4.0,2013–10–03(retrieved2016–10–20).http://www.codingstandard.com/rule/14-2-2-do-not-explicitly-specialize-a-function-template-that-is-overloaded-with-other-templates/. 4P0551R1:ThouShaltNotSpecializestdFunctionTemplates! Whilethisissuehasbeenknownforover15years,itseemsnotparticularywellknownamongthegeneralpopulationofC++programmers.Moreover,thewordingin[namespace.std]isstill,eventoday,beingtweaked,6andtherearepapers(e.g.,[N4381])consideringfurtherrenementsincustomizinglibrary-providedfunctiontemplates.2.3Whatshouldwedo?Weproposemeasurestoaddressthepresentuncomfortablesituationregardingusercustomizationoffunctiontemplatesinthestandardlibrary.Inthefollowingsummaryofourproposal,letFdenoteanarbitrarystandardlibraryfacilitythatisspeciedasanon-memberfunctiontemplate.(ProminentexamplesofsuchanFincludebegin,swap,andforward.)1.First,let'sprohibitprogramsfromspecializinganysuchF.(Thisisinadditiontothelong-standingprohibitionagainstoverloadinganysuchFinnamespacestd.)2.Then,despiteF'sspecicationasafunctiontemplate,let'spermitimplementationstocodeFintheformofaninstantiatedfunctionobjectthathasthespeciedtemplateparameters,functionparameters,andreturntype.Together,theseadjustmentswillnotonlyreduceprogrammer

4 uncertaintyregardingcustomizationofstand
uncertaintyregardingcustomizationofstandardlibraryfacilities(“shouldIspecializeorshouldIoverload?”),theywillalsoallowstandardlibraryimplementorstoprovidecustomizationpoints7thatwillsmoothlyinteroperatewithoverloadsprovidedbyusersintheirownnamespaces,thusavoidingsurprisesdueto(common)misunderstandingsofinteractionsofspecializationandoverloading.Finally,wenotethat[P0684R0]hasalreadyproposed“rulesforwell-behavedusercode.”Amongtheserules(whichwerediscussedandgenerallyendorsedduringWG21'srecentTorontomeeting),wendthefollowingtobeofspecialrelevancewithrespecttothepresentproposal: Donotdenenamesinnamespacestd...,exceptasspecicallydirectedforlibraryextensionpoints.Donotforwarddeclaresymbolsfromnamespacestd.Donottaketheaddressoffunctionsormemberfunctionsinnamespacestd.Moregenerally,donotdependonthesignatureofstandardAPIs—assumeonlythatitiscallableasspecied.DonotdependonthepresenceorabsenceofAPIs....Webelievethatthepresentproposalisentirelyconsistentwiththisrecommendedapproach.3AnalternateapproachTheRangesTS[N4684]speciescustomizationpointfunctionalitythatoverlapswhatweproposeabove,butthatalsoappearstogointofargreaterdetailaboutimplementationtechniques.Weareuncertainthatalltheseimplementationdetailsaretrulynecessarytoitsspecicationofcustomizationpoints.Forexample,thefollowingextensivedetails(cross-referenceselided)areprovidedasa“conven-tion”intheTSsubclause“CustomizationPointObjects”[customization.point.object]: 6See,forexample,LWGIssue2139,“Whatisauser-denedtype?”InC++StandardLibraryActiveIssuesList(RevisionD012),revised2016–12–18at14:12:32UTC(retrieved2016–12–20),http://wg21.link/lwg2139.7Accordingto[N4381],“Acustomizationpoint...isafunctionusedbytheStandardLibrarythatcanbeoverloadedonuser-denedtypesintheuser'snamespaceandthatisfoundbyargument-dependentlookup.”Lessformally,EricNieblerdenescustomizationpointsas“hooksusedbygenericcodethatend-userscanspecializetocustomizethebehaviorfortheirtypes.”See“CustomizationPointDesigninC++11andBeyond.”2014–10–21(retrieved2017–01–26).http://ericniebler.com/2014/10/21/customization-point-design-in-c11-and-beyond/. P0551R1:ThouShaltNotSpecializestdFunctionTemplates!5 1Acustomizationpointobjectisafunctionobjectwithaliteralclasstypethatinteractswithuser-denedtypeswhileenforcingsemanticrequirementsonthatinteraction.2ThetypeofacustomizationpointobjectshallsatisfySemiregular.3Allinstancesofaspeciccustomizationpointobjecttypeshallbeequal.4ThetypeofacustomizationpointobjectTshallsatisfyInvocableons;&#xt-60;�T,Args...()whenthetypesofArgs...meettherequirementsspeciedinthatcustomizationpointobject'sdenition.Otherwise,Tshallnothaveafunctioncallo

5 peratorthatparticipatesinoverloadresolut
peratorthatparticipatesinoverloadresolution.5Eachcustomizationpointobjecttypeconstrainsitsreturntypetosatisfyaparticularconcept.6Thelibrarydenesseveralnamedcustomizationpointobjects.Ineverytranslationunitwheresuchanameisdened,itshallrefertothesameinstanceofthecustomizationpointobject.7[Note:Manyofthecustomizationpointobjectsinthelibraryevaluatefunctioncallexpres-sionswithanunqualiednamewhichresultsinacalltoauser-denedfunctionfoundbyargumentdependentnamelookup.Toprecludesuchanexpressionresultinginacalltounconstrainedfunctionswiththesamenameinnamespacestd,customizationpointobjectsspecifythatlookupfortheseexpressionsisperformedinacontextthatincludesdeletedoverloadsmatchingthesignaturesofoverloadsdenedinnamespacestd.Whenthedeletedoverloadsareviable,user-denedoverloadsmustbemorespecializedormoreconstrainedtobeusedbyacustomizationpointobject.—endnote]Inadditiontotheabove“convention,”eachcustomizationpointspecicationcarriessignicantadditionalverbiageregardingitsimplementation.Forexample,hereisthefurtherspecication(cross-referencesagainelided)ofswapfromtheTS's[utility.swap]: 1Thenameswapdenotesacustomizationpointobject.Theeffectoftheexpressionranges::swap(E1,E2)forsomeexpressionsE1andE2isequivalentto:(1.1)—(void)swap(E1,E2),ifthatexpressionisvalid,withoverloadresolutionperformedinacontextthatincludesthedeclarations templateTlas;&#xs-60;�voidswap(T&,T&)=delete; templateT,size_tNlas;&#xs-60;�voidswap(T(&)[N],T(&)[N])=delete;anddoesnotincludeadeclarationofranges::swap.IfthefunctionselectedbyoverloadresolutiondoesnotexchangethevaluesdenotedbyE1andE2,theprogramisill-formedwithnodiagnosticrequired.(1.2)—Otherwise,(void)swap_ranges(E1,E2)ifE1andE2arelvaluesofarraytypesofequalextentandranges::swap(*(E1),*(E2))isavalidexpression,exceptthatnoexcept(ranges::swap(E1,E2))isequaltonoexcept(ranges::swap(*(E1),*(E2))).(1.3)—Otherwise,ifE1andE2arelvaluesofthesametypeTwhichmeetsthesyntacticre-quirementsof&#xT000;MoveConstructible()and&#xT&,T;Assignable(),exchangesthedenotedvalues.ranges::swap(E1,E2)isaconstantexpressioniftheconstructorselectedbyoverloadresolutionforT{std::move(E1)}isaconstexprconstructorandtheexpressionE1=std::move(E2)canappearinaconstexprfunction.noexcept(ranges::swap(E1,E2))isequalto&#xT000;is_nothrow_move_constructible&&&#xT000;is_nothrow_move_assignableT::value.IfeitherMoveConstructibleorAssignableisnotsatised,theprogramisill-formedwithnodiagnosticrequired.(1.4)—Otherwise,ranges::swap(E1,E2)isill-formed. 6P0551R1:ThouShaltNotSpecializestdFunctionTemplates! 2Remark:Wheneverranges::swap(E1,E2)isavalidexpression,itexchangesthevaluesdenotedbyE1andE2andhastypevoid.Thereisasimilarlydetailedamountofadditionalspecicationforeachoftheothercus-tomizationpointsintheTS:iter_move,iter

6 _swap,begin,end,cbegin,cend,rbegin,rend,
_swap,begin,end,cbegin,cend,rbegin,rend,crbegin,crend,size,empty,data,andcdata.Despiteallthisbulk,theTSdoesnotspeaktothefundamentalissueweseektoaddress,namelytoforbidusercodefromprovidinginconsistentreinterpretationsofstandardlibraryfeatures.Itseemsplausiblethatsomehybridofthetwoapproachesmayprovebenecial,butthispaperproposesaminimalistapproachinordertoprovideacontrastingviewpoint.4Proposedwording8 4.1Adjust[namespace.std]asshown:1 The Unless otherwise specied, thebehaviorofaC++programisundenedifitaddsdeclarationsordenitionstonamespacestdortoanamespacewithinnamespacestd un less oth er wise spec i - ed. 2 A Unless explicitly prohibited, aprogrammayaddatemplatespecializationforanystandardlibrary classtemplatetonamespacestd only if provided that (a)the addeddeclarationdependson a at least oneuser-denedtypeand (b)thespecializationmeetsthestandardlibraryrequirementsfortheoriginaltemplate and is not ex plic itly pro hib ited.[Footnote:Anylibrarycodethatinstantiatesotherlibrarytemplatesmustbepreparedtoworkadequatelywithanyuser-suppliedspecializationthatmeetstheminimumrequirementsoftheStandard.] 2 3ThebehaviorofaC++programisundenedifitdeclaresanexplicitorpartialspecializationofanystandardlibraryvariabletemplate,exceptwhereexplicitlypermittedbythespecicationofthatvariabletemplate. 3 4ThebehaviorofaC++programisundenedifitdeclares( 3 4.1)—anexplicitspecializationofanymemberfunctionofastandardlibraryclasstemplate,or( 3 4.2)—anexplicitspecializationofanymemberfunctiontemplateofastandardlibraryclassorclasstemplate,or( 3 4.3)—anexplicitorpartialspecializationofanymemberclasstemplateofastandardlibraryclassorclasstemplate,or( 3 4.4)—adeductionguideforanystandardlibraryclasstemplate. 5Aprogrammayexplicitlyinstantiatea classtemplatedenedinthestandardlibraryonlyifthedeclaration (a)dependsonthenameof a at least oneuser-denedtypeand (b)theinstantiationmeetsthestandardlibraryrequirementsfortheoriginaltemplate. 6 In its own namespace, a program may provide an overload for any library function template designated as a customizationpoint, provided that (a) the overload's declaration depends on at least one user-dened type and (b) the overload meets the standard library requirements for the customization point. [Note: this permits a (qualied or unqualied) call to the customization point to invoke the most appropriate overload for the given arguments.] [Footnote: Any library customization point must be prepared to work adequately with any user-dened overload that meets the minimum requirements of the Standard. Therefore an implementation may elect, under the as-if rule 8Allproposed additionsand dele tionsarerelativetothepost-TorontoWorkingDraft[N4687].Editorialnotesaredisplayedagainsta graybackground. P0551R1:Thou

7 ShaltNotSpecializestdFunctionTemplates!7
ShaltNotSpecializestdFunctionTemplates!7 ([intro.execution]), to provide any customization point in the form of an instantiated function object ([function.objects]) even though the customization point's specication is in the form of a function template. The template parameters of each such function object and the function parameters and return type of the object's operator() must match those of the corresponding customization point's specication.] 4 7Atranslationunitshallnotdeclarenamespacestdtobeaninlinenamespace(10.3.1). 4.2Whereandasshown,designatethefollowingstandardlibrarycomponentsascustomizationpoints:(a)swap;(b)therangeaccessalgorithmsbegin,end,andtheirvariants;and(c)thecontaineraccessalgorithmssize,empty,anddata. [utility.swap]1Remarks:Thisfunction is a designated customization point ([namespace.std]] andshallnotparticipateinoverloadresolutionunless.... [iterator.range]1Inadditiontobeingavailableviainclusionofthe&#xiter; tor;header,thefunctiontemplatesin24.7areavailablewhenanyofthefollowingheadersareincluded: rra;&#xy000;,Þqu;,orw; rd_;&#xlist;,...,and&#xvect;&#xor00;. Each of these templates is a designated cus tom- i za tion point ([namespace.std]]. [iterator.container]1Inadditiontobeingavailableviainclusionofthe&#xiter; tor;header,thefunctiontemplatesin24.8areavailablewhenanyofthefollowingheadersareincluded: rra;&#xy000;,Þqu;,orw; rd_;&#xlist;,...,and&#xvect;&#xor00;. Each of these templates is a designated cus tom- i za tion point ([namespace.std]].5AcknowledgmentsManythanks,fortheirthoughtfulcomments,toAndreySemashevandtheotherreadersofprepublicationdraftsofthispaper.6Bibliography[N4381]EricNiebler:“SuggestedDesignforCustomizationPoints.”ISO/IECJTC1/SC22/WG21docu-mentN4381(pre-Lenexamailing),2015–03–11.http://wg21.link/n4381.[N4684]EricNieblerandCaseyCarter:“WorkingDraft,C++ExtensionsforRanges.”ISO/IECJTC1/SC22/WG21documentN4684(post-Torontomailing),2017–07–31.http://wg21.link/n4684.[N4687]RichardSmith:“WorkingDraft,StandardforProgrammingLanguageC++.”ISO/IECJTC1/SC22/WG21documentN4687(post-Torontomailing),2017–07–30.http://wg21.link/n4687.[P0684R0]TitusWinters,BjarneStroustrup,etal.:“C++Stability,Velocity,andDeploymentPlans.”ISO/IECJTC1/SC22/WG21documentP0684R0(pre-Torontomailing),2017–06–19.http://wg21.link/p0684r0.7DocumenthistoryRev.DateChanges02017-02-01PublishedasP0551R0,pre-Kona.12017-10-14Updatedcontentandbibliographytoreectthepost-TorontoWorkingDraftsandotherrecentdevelopments.Added(§2.2)examplewithextendedexplanations.Remarked(§2.3)onconsistencywith[P0684R0].Adjustedcitationstousehttp://wg21.link.Madenumerousminoreditorialandformattingimprovements.PublishedasP0551R1,pre-Albuqu