ThouShaltNotSpecializestdFunctionTemplatesDocumentWG21P0551R0Date2017 - PDF document

1 ThouShaltNotSpecializestdFunctionTemplat
ThouShaltNotSpecializestdFunctionTemplates!Document#:WG21P0551R0Date:2017-02-01Project:JTC1.22.32ProgrammingLanguageC++Audience:LEWG)LWGReplyto:WalterE.Brownwebrown.cpp@gmail.com� Contents1Introduction.............12Discussion..............12.1Whatwesaytoday........12.2What'swrongwiththat?....22.3Whatshouldwedo?.......33Analternateapproach.......34Proposedwording..........55Acknowledgments..........66Bibliography.............67Documenthistory..........6 AbstractThispaperproposestomodifyclause[namespace.std]soas(a)toforbidusersfromspecializingstandardlibraryfunctiontemplatesand(b)toallowfunctionobjectsasimplementationsofstandardlibraryfacilitiesspeciedasfunctiontemplates.Tocreatearchitectureistoputinorder.Putwhatinorder?Functionandobjects.—LECORBUSIER,néCHARLES-ÉDOUARDJEANNERET1IntroductionSpecializingfunctiontemplateshasprovenproblematicinpractice;specializingfunctiontemplatesinnamespacestdhasprovenevenmoreproblematic.Thispaper(a)willciteknowledgableandwell-respectedcolleaguesindescribingthecorelanguagecausesoftheissuesinvolved,and(b)willthenrecommendwordingadjustments(tosubclause[namespace.std])toaddresstheseissuesinthecontextofthestandardlibrary.2Discussion2.1WhatwesaytodayQuotedverbatimfrom[N4618],herearethethreeparagraphsthatconstitutetheentiretyof[namespace.std]: 1ThebehaviorofaC++programisundenedifitaddsdeclarationsordenitionstonamespacestdortoanamespacewithinnamespacestdunlessotherwisespecied.Aprogrammayaddatemplatespecializationforanystandardlibrarytemplatetonamespacestdonlyifthedeclarationdependsonauser-denedtypeandthespecializationmeetsthestandardlibraryrequirementsfortheoriginaltemplateandisnotexplicitlyprohibited. Copyrightc 2017byWalterE.Brown.Allrightsreserved.1 2P0551R0:ThouShaltNotSpecializestdFunctionTemplates! [Footnote:Anylibrarycodethatinstantiatesotherlibrarytemplatesmustbepreparedtoworkadequatelywithanyuser-suppliedspecializationthatmeetstheminimumrequirementsoftheStandard.]2ThebehaviorofaC++programisundenedifitdeclares(2.1)—anexplicitspecializationofanymemberfunctionofastandardlibraryclasstemplate,or(2.2)—anexplicitspecializationofanymemberfunctiontemplateofastandardlibraryclassorclasstemplate,or(2.2)—anexplicitorpartialspecializationofanymemberclasstemplateofastandardlibraryclassorclasstemplate.Aprogrammayexplicitlyinstantiateatemplatedenedinthestandardlibraryonlyifthedeclarationdependsonthenameofauser-denedtypeandtheinstantiationmeetsthestandardlibraryrequirementsfortheoriginaltemplate.3Atranslationunitshallnotdeclarenamespacestdtobeaninlinenamespace(7.3.1).2.2What'swrongwiththat?AccordingtoseveralC++cognoscenti,itisa

2 poorC++programmingpracticetospecializeaf
poorC++programmingpracticetospecializeafunctiontemplate,especiallyoneinnamespacestd.Herearerepresentativeexplanationsandadvice: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:“thisissuehasbeensettledforalongtime....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/. P0551R0:ThouShaltNotSpecializestdFunctionTemplates!3 Whilethisissueh

3 asbeenknownforover15years,itseemsnotpart
asbeenknownforover15years,itseemsnotparticularywellknown.Moreover,thewordingin[namespace.std]isstill,eventoday,beingtweaked,6andtherearepapers(e.g.,[N4381])consideringfurtherrenementsincustomizinglibrary-providedfunctiontemplates.2.3Whatshouldwedo?Weproposeafewrelatedmeasurestoaddressthepresentuncomfortablesituationregardingusercustomizationoffunctiontemplatesinthestandardlibrary.LetFdenoteanarbitrarystandardlibraryfacilitythatisspeciedasanon-memberfunctiontemplate.(ProminentexamplesofsuchanFincludebegin,swap,andforward.)Then:1.ProhibitprogramsfromspecializinganysuchF.(ThisisinadditiontotheexistingprohibitionagainstoverloadinganysuchFinnamespacestd.)2.DespiteF'sspecicationasafunctiontemplate,grantimplementationslicensetoimplementFintheformofaninstantiatedfunctionobjectthathasthespeciedtemplateparameters,functionparameters,andreturntype.Thesechangeswillallowstandardlibraryimplementorstoprovidecustomizationpoints7thatwillsmoothlyinteroperatewithoverloadsprovidedbyusersintheirownnamespaces,thusavoidingsurprisesdueto(common)misunderstandingsofinteractionsofspecializationandoverloading.3AnalternateapproachTheRangesTS[N4620]speciescustomizationpointfunctionalitythatoverlapswhatweproposeabove,butthatalsoappearstogointofargreaterdetailaboutimplementationtechniques.Weareuncertainthatalltheseimplementationdetailsaretrulynecessarytoitsspecicationofcustomizationpoints.Forexample,thefollowingextensivedetails(cross-referenceselided)areprovidedasa“conven-tion”intheTSsubclause“CustomizationPointObjects”[customization.point.object]: 1Acustomizationpointobjectisafunctionobjectwithaliteralclasstypethatinteractswithuser-denedtypeswhileenforcingsemanticrequirementsonthatinteraction.2ThetypeofacustomizationpointobjectshallsatisfySemiregular.3Allinstancesofaspeciccustomizationpointobjecttypeshallbeequal.4ThetypeofacustomizationpointobjectTshallsatisfyInvocableons;&#xt-60;�T,Args...()whenthetypesofArgs...meettherequirementsspeciedinthatcustomizationpointobject'sdenition.Otherwise,Tshallnothaveafunctioncalloperatorthatparticipatesinoverloadresolution.5Eachcustomizationpointobjecttypeconstrainsitsreturntypetosatisfyaparticularconcept.6Thelibrarydenesseveralnamedcustomizationpointobjects.Ineverytranslationunitwheresuchanameisdened,itshallrefertothesameinstanceofthecustomizationpointobject. 6See,forexample,LWGIssue2139,“Whatisauser-denedtype?”.InC++StandardLibraryActiveIssuesList(RevisionD012),revised2016–12–18at14:12:32UTC(retrieved2016–12–20),http://cplusplus.github.io/LWG/lwg-active.html#2139.7Accordingto[N4381],

4 47;Acustomizationpoint...isafunctionused
47;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/. 4P0551R0:ThouShaltNotSpecializestdFunctionTemplates! 7[Note:Manyofthecustomizationpointobjectsinthelibraryevaluatefunctioncallexpres-sionswithanunqualiednamewhichresultsinacalltoauser-denedfunctionfoundbyargumentdependentnamelookup.Toprecludesuchanexpressionresultinginacalltounconstrainedfunctionswiththesamenameinnamespacestd,customizationpointobjectsspecifythatlookupfortheseexpressionsisperformedinacontextthatincludesdeletedoverloadsmatchingthesignaturesofoverloadsdenedinnamespacestd.Whenthedeletedoverloadsareviable,user-denedoverloadsmustbemorespecializedormoreconstrainedtobeusedbyacustomizationpointobject.—endnote]Inadditiontotheabove“convention,”eachcustomizationpointspecicationcarriessignicantadditionalverbiageregardingitsimplementation.Forexample,hereisthefurtherspecicationofswapfromtheTS'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(),exchangesthede-notedvalues.ranges::swap(E1,E2)isaconstantexpressioniftheconstructorselectedbyoverloadresolutionforT{std::move(E1)}isaconstexprconstructorandtheexpres-sionE1=std::move(E2)canappearinaconstexprfunction.noexcept(ranges::swap(E1,E2))isequalto&#xT000;is_nothrow_move_constructible&&is_no&#xT000;throw_move_assignable.IfeitherMoveConstructibleorAssign

5 ableisnotsatised,theprogramisill-for
ableisnotsatised,theprogramisill-formedwithnodiagnosticrequired.(1.4)—Otherwise,ranges::swap(E1,E2)isill-formed.2Remark:Wheneverranges::swap(E1,E2)isavalidexpression,itexchangesthevaluesdenotedbyE1andE2andhastypevoid.Thereisasimilarlydetailedamountofadditionalspecicationforeachoftheothercus-tomizationpointsintheTS:iter_move,iter_swap,begin,end,cbegin,cend,rbegin,rend,crbegin,crend,size,empty,data,andcdata.Despiteallthisbulk,theTSdoesnotspeaktothefundamentalissueweseektoaddress,namelytoforbidusercodefromprovidinginconsistentreinterpretationsofstandardlibraryfeatures.Itseemsplausiblethatsomehybridofthetwoapproachesmayprovebenecial,butthispaperproposesaminimalistapproachinordertoprovideacontrastingviewpoint. P0551R0:ThouShaltNotSpecializestdFunctionTemplates!5 4Proposedwording8 4.1Adjust[namespace.std]asshown:1 Unless otherwise specied, T thebehaviorofaC++programisundenedifitaddsdeclarationsordenitionstonamespacestdortoanamespacewithinnamespacestd un less oth er wise spec i ed. 2 Unless explicitly prohibited, A aprogrammayaddatemplatespecializationforanystandardlibrary class template or variabletemplatetonamespacestd only if provided that (a)the addeddeclarationdependsonauser-denedtypeand (b)thespecializationmeetsthestandardlibraryrequirementsfortheoriginaltemplate and is not ex plic itly pro hib ited.[Footnote:Anylibrarycodethatinstantiatesotherlibrarytemplatesmustbepreparedtoworkadequatelywithanyuser-suppliedspecializationthatmeetstheminimumrequirementsoftheStandard.] 2 3ThebehaviorofaC++programisundenedifitdeclares( 2 3.1)—anexplicitspecializationofanymemberfunctionofastandardlibraryclasstemplate,or( 2 3.2)—anexplicitspecializationofanymemberfunctiontemplateofastandardlibraryclassorclasstemplate,or( 2 3.2)—anexplicitorpartialspecializationofanymemberclasstemplateofastandardlibraryclassorclasstemplate. 4Aprogrammayexplicitlyinstantiatea classtemplatedenedinthestandardlibraryonlyifthedeclarationdependsonthenameofauser-denedtypeandtheinstantiationmeetsthestandardlibraryrequirementsfortheoriginaltemplate. 5 A program may provide (in a namespace of its own) an overload for any library function template designated as a customizationpoint, provided that (a) the overload's declaration depends on a 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-su

6 pplied overload that meets the minimum r
pplied overload that meets the minimum requirements of the Standard. Therefore an implementation may elect, under the as-if rule ([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 of its operator() must match those of the corresponding customization point's specication.] 3 6Atranslationunitshallnotdeclarenamespacestdtobeaninlinenamespace(7.3.1). 4.2Whereandasshown,designatethefollowingstandardlibraryfunctionalityascustomiza-tionpoints:(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]]. 8Allproposed additionsand dele tionsarerelativetothepost-IssaquahWorkingDraft[N4618].Editorialnotesaredisplayedagainsta graybackground. 6P0551R0:ThouShaltNotSpecializestdFunctionTemplates! [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/WG21doc-umentN4381(pre-Lenexamailing),2015–03–11.http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2015/n4381.html.[N4618]RichardSmith:“WorkingDraft,StandardforProgrammingLanguageC++,”ISO/IECJTC1/SC22/WG21documentN4618(post-Issaquahmailing),2016–11–28.http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4618.pdf.[N4620]EricNieblerandCaseyCarter:“WorkingDraft,C++ExtensionsforRanges.”ISO/IECJTC1/SC22/WG21documentN4620(post-Issaquahmailing),2016–11–27.http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2016/n4620.pdf.7DocumenthistoryVersionDateChanges02017-02-01PublishedasP055