课件文稿教案mastering the17stl make full use of standard library components

MasteringtheC++17MakefulluseofthestandardlibrarycomponentsinArthurBIRMINGHAM-MasteringtheC++17Copyright©2017PacktAllrights .Nopartofthisbookmaybereproduced,storedinaretrievalsystem,ortransmittedinanyformorbyanymeans,withoutthepriorwrittenpermissionofthepublisher,exceptinthecaseofbriefquotationsembeddedincriticalarticlesorreviews.Everyefforthasbeenmadeinthepreparationofthisbooktoensuretheaccuracyoftheinformationpresented.However,theinformationcontainedinthisbookissoldwithoutwarranty,eitherexpressorimplied.Neithertheauthor,norPacktPublishing,anditsdealersanddistributorswillbeheldliableforanydamagescausedorallegedtobecauseddirectlyorindirectlybythisbook.PacktPublishinghasendeavoredtoprovidetrademarkinformationaboutallofthecompaniesandproductsmentionedinthisbookbytheappropriateuseofcapitals.However,PacktPublishingcannotguaranteetheaccuracyofthisFirstpublished:SeptemberProductionPublishedbyPacktPublishing. 35LiveryStreetB32PB,ISBN978-1-78712-682-CopyArthurSafisArthurProjectWillPrajaktaWill MerintThomasSafisSandeep lContentDevelopmentLawrenceNileshTechnicalDhirajAbouttheArthurO'DwyerhasusedmodernC++inhisdayjobforabouttenyears--sincethedayswhen"modernC++"meant"classicC++."Between2006and2011heworkedontheGreenHillsC++compiler.Since2014hehasorganizedaweeklyC++meetupintheSanFranciscoBayArea,andhespeaksregularlyontopicssuchasthosetobefoundinthisbook.Laterthisyear,hewillattendanISOC++committeemeetingforthesecondtime.ThisishisfirstAbouttheWillBrennanisaC++/PythondeveloperbasedinLondonwithexperienceworkingonhighperformanceimageprocessingandmachinelearningapplications.Youcanvisithis linkat Forsupportfilesanddownloadsrelatedtoyourbook,pleaseDidyouknowthatPacktofferseBookversionsofeverybookpublished,withPDFandePubfilesavailable?YoucanupgradetotheeBookversionatwww.Pandasaprintbookcustomer,youareentitledtoadiscountontheeBookcopy.Getintouchwithusat formore youcanalsoreadacollectionoftechnicalarticles,signupforarangeofnewslettersandreceiveexclusivediscountsandoffersonPacktbooksandeBooks. Getthemostin-demandsoftwareskillswithMapt.MaptgivesyoufullaccesstoallPacktbooksand courses,aswellasindustry-leadingtoolstohelpyounyour aldevelopmentandadvanceyourcareer.WhyFullysearchableacrosseverybookpublishedbyPacktCopyandpaste,print,andbookmarkcontentOndemandandaccessibleviaawebCustomerThanksforpurchasingthisPacktbook.AtPackt,qualityisattheheartofoureditorialprocess.Tohelpusimprove,pleaseleaveusanhonestreviewonthisbook'sAmazonpageathttp X.Ifyou'dliketojoinourteamofregularreviewers,youcan usat .WeawardourregularreviewerswitheBooksandsinexchangefortheirvaluablefeedback.Helpusberelentlessinimprovingourproducts!TableofWhatthisbookcoversWhatyouneedforthisbookWhothisbookisforReaderfeedbackCustomerDownloadingtheexamplecodeClassicalPolymorphismandGenericConcretemonomorphicfunctionsClassicallypolymorphicfunctionsGenericprogrammingwithtemtes torsandTheproblemwithintegerindicesOnbeyondpointersConst Apairofi torsdefinesarange torcategoriesInputandoutputi PuttingitalltogetherThedeprecatedstd::i The tor-PairAnoteaboutRead-onlyrangealgorithmsShuntingdatawithVariationsonatheme-std::moveandstd::move_i Complicatedcopyingwithstd::transformWrite-onlyrangeAlgorithmsthataffectobjectlifetimeOurfirstpermutativealgorithm:std::sortSwap,reversing,andpartitioningRotationandpermutationHeapsandheapsortMergesandmergesortSearchingandinsertinginasortedarraywithstd::lower_boundDeletingfromasortedarraywithstd::remove_ifTheContainerThenotionofThesimplestcontainer:std::array<T,N>Theworkhorse:std::vector<T>ResizingaInsertinganderasinginastd::vectorPitfallswithvector<bool>Pitfallswithnon-noexceptmoveThespeedyhybrid:Aparticularsetofskills:WhatarethespecialskillsofRoughingitwithingwithstd::stack<T>andstd::queue<T>Theusefuladaptor:std::priority_queue<T>Thetrees:std::set<T>andstd::map<K,V>AnoteabouttransparentOddballs:std::multiset<T>andstd::multimap<K,MovingelementswithoutmovingThehashes:std::unordered_set<T>andstd::unordered_map<K,V>LoadfactorandbucketlistsWheredoesthememorycomefrom?VocabularyThestoryofTaggingreferencetypeswithreference_wrapperC++11andalgebraictypesWorkingwithstd::tupleManipulatingtuplevaluesAnoteaboutnamedclassesExpressingalternativeswithstd::variantVisitingWhataboutmake_variant?andanoteonvaluesemanticsDelayinginitializationwithstd::optionalRevisitingInfinitealternativeswithstd::anyversuspolymorphicclasstypesTypeerasureinanutsstd::anyandAgainwiththetypeerasure:std::functionstd::function,copyability,andallocationSmartTheoriginsofsmartSmartpointersneverAutomaticallymanagingmemorywithstd::unique_ptr<T>WhyC++doesn'thavethefinallykeywordCustomizingthedeletioncallbackManagingarrayswithstd::unique_ptr<T[]>Referencecountingwithstd::shared_ptr<T>Don'tdouble-HoldingnullablehandleswithTalkingaboutoneselfwithstd::enable_shared_from_thisTheCuriouslyRecurringTemtePatternAfinalDenotingun-special-nesswithobserver_ptr<T>TheproblemwithUsingstd::atomic<T>forthread-safeaccessesngcomplicatedoperationsatomicallyBigTakingturnswithstd::mutex"Takinglocks"therightwayAlwaysassociateamutexwithitscontrolleddataSpecial-purposemutextypesUpgradingaread-writeDowngradingaread-writelockWaitingforaconditionPromisesaboutfuturesPackaginguptasksforlaterThefutureoffuturesSpeakingofthreads...IdentifyingindividualthreadsandthecurrentThreadexhaustionandstd::asyncBuildingyourownthreadpoolImprovingourthreadpool'sperformanceAnallocatorisahandletoamemoryresourceRefresher-InterfacesversusconceptsDefiningaheapwithmemory_resourceUsingthestandardmemoryresourcesAllocatingfromapoolresourceThe500hatsofthestandardallocatorCarryingmetadatawithfancyStickingacontainertoasinglememoryresourceUsingthestandardallocatortypesSettingthedefaultmemoryMakingacontainerallocator-Propagatingdownwardswithscoped_allocator_adaptorPropagatingdifferentallocatorsThetroublewithI/OinC++BufferingversusUsingthePOSIXAPIUsingthestandardCAPIBufferinginthestandardCFormattingwithprintfandsnprintfTheclassicaliostreamshierarchyStreamingandmanipulatorsStreamingandwrappersSolvingthesticky-manipulatorproblemFormattingwithostringstreamAnoteonlocalesConvertingnumberstostringsConvertingstringstonumbersReadingalineorwordatatimeRegularWhatareregularexpressions?Anoteonbackslash-escaReifyingregularexpressionsintostd::regexobjectsMatchingandsearchingPullingsubmatchesoutofamatchConvertingsubmatchestodatavalues tingovermultipleUsingregularexpressionsforstringrecementAprimerontheECMAScriptregexgr Non-consumingObscureECMAScriptfeaturesandpitfallsRandomRandomnumbersversuspseudo-randomnumbersTheproblemwithrand()Solvingproblemswith<random>DealingwithgeneratorsTrulyrandombitswithstd::random_devicePseudo-randombitswithstd::mt19937FilteringgeneratoroutputswithadaptorsDealingwithdistributionsRollingdicewithuniform_int_distributionGeneratingpopulationswithnormal_distributionMakingweightedchoiceswithdiscrete_distributionShufflingcardswithstd::shuffleAnoteaboutAverylongnoteonerror-reportingUsing<system_error>ErrorcodesanderrorThrowingerrorswithstd::system_errorFilesystemsandpathsRepresentingpathsinC++OperationsonpathsStattingfileswithdirectory_entryWalkingdirectorieswithdirectory_i RecursivedirectorywalkingModifyingthefilesystemReportingdiskusageTheC++languagehasalonghistory,datingbacktothe1980s.Recentlyithasundergonearenaissance,withmajornewfeaturesbeingintroducedin2011and2014.Atpresstime,theC++17standardisjustaroundthecorner.C++11practicallydoubledthesizeofthestandardlibrary,addingsuchheadersas<tuple>,<type_traits>,and<regex>.C++17doublesthelibraryagain,withadditionssuchas<optional>,<any>,and<filesystem>.Aprogrammerwho’sbeenspendingtimewritingcodeinsteadofwatchingthestandardizationprocessmightfairlyfeelthatthestandardlibraryhasgottenawayfromhim--thatthere’ssomanynewthingsinthelibrarythathe'llneverbeabletomasterthewholething,oreventosortthewheatfromthechaff.Afterall,whowantstospendamonthreadingtechnical ationonstd::localeandstd::ratio,justtofindoutthattheyaren'tusefulinyourdailywork?Inthisbook,I'llteachyouthemostimportantfeaturesoftheC++17standardlibrary.Intheinterestofbrevity,Iomitsomeparts,suchastheaforementioned<type_traits>;butwe'llcovertheentiremodernSTL(everystandardcontainerandeverystandardalgorithm),plussuchimportanttopicsassmartpointers,randomnumbers,regularexpressions,andthenew-in-C++17<filesystem>library.I'llteachbyexample.You'lllearntobuildyourownitortype;yourownmemoryallocatorusingstd::pmr::memory_resource;yourownthreadpoolusingI'llteachconceptsbeyondwhatyou'dfindinareferencemanual.You'lllearnthedifferencebetweenmonomorphic,polymorphic,andgenericalgorithms(Chapter1,ClassicalPolymorphismandGenericProgramming);whatitmeansforstd::stringorstd::anytobetermeda"vocabularytype"(Chapter5,VocabularyTypes);andwhatwemightexpectfromfutureC++standardsin2020andbeyond.IassumethatyouarealreadyreasonablyfamiliarwiththecorelanguageofC++11;forexample,thatyoualreadyunderstandhowtowriteclassandfunctiontemtes,thedifferencebetweenlvalueandrvaluereferences,andsoon.WhatthisbookChapter1,ClassicalPolymorphismandGenericProgramming,coversclassicalpolymorphism(virtualmemberfunctions)andgenericprogrammingChapter2,ItorsandRanges,exinstheconceptofitorasageneralizationofpointer,andtheutilityofhalf-openrangesexpressedasapairofitors.Chapter3,TheItor-PairAlgorithms,exploresthevastvarietyofstandardgenericalgorithmsthatoperateonrangesexpressedasitor-pairs.Chapter4,TheContainerZoo,exploresthealmostequallyvastvarietyofstandardcontainerclasstemtes,andwhichcontainersaresuitableforwhichjobs.Chapter5,VocabularyTypes,walksyouthroughalgebraictypessuchstd::optional.andABI-friendlytype-erasedtypessuchasChapter6,SmartPointers,teachesthepurposeanduseofsmartChapter7,Concurrency,coversatomics,mutexes,conditionvariables,threads,futures,andpromises.Chapter8,Allocators,exinsthenewfeaturesofC++17'sChapter9,Iostreams,explorestheevolutionoftheC++I/Omodel,<unistd.h>to<stdio.h>toChapter10,RegularExpressions,teachesregularexpressionsinChapter11,RandomNumbers,walksyouthroughC++'ssupportforpseudo-randomnumbergeneration.Chapter12,Filesystem,coversthenew-in-C++17<filesystem>WhatyouneedforthisAsthisbookisnotareferencemanual,itmightbeusefulforyoutohaveareferencemanual,suchascppreference(e /w/cpp),atyoursidetoclarifyanyconfusingpoints.ItwilldefiniyhelptohaveaC++17compilerhandy.Atpresstime,thereareseveralmoreorless pleteC++17implementations,includingGCC,Clang,and VisualStudio.Youcanrunthemlocallyorviamanyonlinecompilerservices,suchasWandbox(),Godbolt(),andRextester WhothisbookisThisbookisfordeveloperswhowouldliketomastertheC++17STLandmakefulluseofitscomponents.PriorC++knowledgeisassumed.Inthisbook,youwillfindanumberoftextstylesthatdistinguishbetweendifferentkindsofinformation.Herearesomeexamplesofthesestylesandanexnationoftheirmeaning.Codewordsintext,databasetablenames,foldernames,filenames,fileextensions,pathnames,dummyURLs,userinput,and handlesareshownasfollows:"Thebuffer()functionacceptsargumentsoftypeint."Ablockofcodeissetastry}catch(conststd::future_error&{assert(ex.code()==}NewtermsandimportantwordsareshowninWarningsorimportantnotesappearlikeTipsandnotesappearlikeReaderFeedbackfromourreadersisalways 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,andwewilldoourbesttoaddresstheClassicalPolymorphismandGenericProgrammingTheC++standardlibraryhastwodistinct,yetequallyimportant,missions.Oneofthesemissionsistoproviderock-solidimplementationsofcertainconcretedatatypesorfunctionsthathavetendedtobeusefulinmanydifferentprograms,yetaren'tbuiltintothecorelanguagesyntax.Thisiswhythestandardlibrarycontainsstd::string,std::regex,std::filesystem::exists,andsoon.Theothermissionofthestandardlibraryistoproviderock-solidimplementationsofwidelyused algorithmssuchassorting,searching,reversing,collating,andsoon.Inthisfirstchapter,wewillnaildownexactlywhatwemeanwhenwesaythataparticularpieceofcodeis ,"anddescribethetwoapproachesthatthestandardlibraryusesto ion:classicalpolymorphismandgenericprogramming.Wewilllookatthefollowingtopicsinthis Classicalpolymorphismbymeansofbaseclasses,virtualmemberfunctions,andinheritanceGenericprogrammingbymeansofconcepts,requirements,andmodelsThepracticaladvantagesanddisadvantagesofeachapproachConcretemonomorphicWhatdistinguishesan algorithmfromaconcretefunction?Thisisbestshownbyexample.Let'swriteafunctiontomultiplyeachoftheelementsinanarrayby2:class{intdata[10]={};intsize()const{return10;}int&at(inti){returndata[i];}{for(inti=0;i<arr.size();{arr.at(i)*=}}Ourfunctiondouble_each_elementworksonlywithobjectsoftypearray_of_int;passinginanobjectofadifferenttypewon'twork(norevencompile).Werefertofunctionslikethisversionofdouble_each_elementasconcreteormonomorphicfunctions.Wecallthemconcretebecausetheyare forourpurposes.JustimaginehowpainfulitwouldbeiftheC++standardlibraryprovidedaconcretesortroutinethatworkedonlyononespecificdatatype!ClassicallypolymorphicWecanincreasethe ionlevelofouralgorithmsviathetechniquesofclassicalobject-oriented(OO)programming,asseeninlanguagessuchasJavaandC#.TheOOapproachistodecideexactlywhichbehaviorswe'dliketobecustomizable,andthendeclarethemasthepublicvirtualmemberfunctionsofan baseclass:class{virtualintsize()const=0;virtualint&at(int)=0;classarray_of_ints:public{intdata[10]={};intsize()constoverride{return10;}int&at(inti)override{returndata[i];}classlist_of_ints:public{structnode{intdata;node*next;node*head_=nullptr;intsize_=0;intsize()constoverride{returnsize_;}int&at(inti)override{if(i>=size_)throwstd::out_of_range("at");node*p=head_;for(intj=0;j<i;{p=p-}returnp-}{for(inti=0;i<arr.size();{arr.at(i)*=}}void{array_of_intsarr;list_of_intslst;}Insidetest,thetwodifferentcallstodouble_each_elementcompilebecauseinclassicalOOterminology,anarray_of_intsIS-Acontainer_of_ints(thatis,itinheritsfromcontainer_of_intsandimplementstherelevantvirtualmemberfunctions),andalist_of_intsIS-Acontainer_of_intsaswell.However,thebehaviorofanygivencontainer_of_intsobjectisparameterizedbyitsdynamictype;thatis,bythetableoffunctionpointersassociatedwiththisparticularSincewecannowparameterizethebehaviorofthedouble_each_elementfunctionwithouteditingitssourcecodedirectly--simplybypassinginobjectsofdifferentdynamictypes--wesaythatthefunctionhas epolymorphic.Butstill,thispolymorphicfunctioncanhandleonlythosetypeswhicharedescendantsofthebaseclasscontainer_of_ints.Forexample,youcouldn'tpassastd::vector<int>tothisfunction;you'dgetacompileerrorifyoutried.Classicalpolymorphismisuseful,butitdoesnotgetusallthewaytofullAnadvantageofclassical(object-oriented)polymorphismisthatthesourcecodestillbearsaone-to-onecorrespondencewiththemachinecodethatisgeneratedbythecompiler.Atthemachine-codelevel,westillhavejustonedouble_each_elementfunction,withonesignatureandonewell-definedentrypoint.Forexample,wecantaketheaddressofdouble_each_elementasafunctionGenericprogrammingwithInmodernC++,thetypicalwaytowriteafullygenericalgorithmistoimplementthealgorithmasatemte.We'restillgoingtoimplementthefunctiontemteintermsofthepublicmemberfunctions.size()and.at(),butwe'renolongergoingtorequirethattheargumentarrbeofanyparticulartype.Becauseournewfunctionwillbeatemte,we'llbelingthecompiler"Idon'tcarewhattypearris.Whatevertypeitis,justgenerateabrand-newfunction(thatis,atemteinstantiation)withthattypeasitsparametertype." te<class{for(inti=0;i<arr.size();{arr.at(i)*=}}void{array_of_intsarr;list_of_intslst;std::vector<int>vec={1,2,3};}Inmostcases,ithelpsusdesignbetterprogramsifwecanputdowninwordsexactlywhatoperationsmustbesupportedbyourtemtetypeparameterContainerModel.Thatsetofoperations,takentogether,constituteswhat'sknowninC++asaconcept;inthisexamplewemightsaythattheconceptContainerconsistsof"havingamemberfunctionnamedsizethatreturnsthesizeofthecontainerasanint(orsomethingcomparabletoint);andhavingamemberfunctionnamedatthattakesanintindex(orsomethingimplicitlyconvertiblefromint)andproducesanon-constreferencetotheindex'thelementofthecontainer."Wheneversomeclassarray_of_intscorrectlystheoperationsrequiredbytheconceptContainer,suchthatarray_of_intsisusablewithdouble_each_element,wesaythattheconcreteclassarray_of_intsisamodeloftheContainerconcept.ThisiswhyIgavethenameContainerModeltothetemtetypeparameterintheprecedingexample.ItwouldbemoretraditionaltousethenameContainerforthetemtetypeparameteritself,andIwilldothatfromnowon;Ijustdidn'twanttostartusoffonthewrongfootbymuddyingthedistinctionbetweentheContainerconceptandtheparticulartemtetypeparametertothisparticularfunctiontemtethathappenstodesireasitsargumentaconcreteclassthatmodelstheContainerconcept.Whenweimplementan algorithmusingtemtes,sothatthebehaviorofthealgorithmcanbeparameterizedatcompiletimebyanytypesmodelingtheappropriateconcepts,wesaywearenggenericNoticethatourdescriptionoftheContainerconceptdidn'tmentionthatweexpectthetypeofthecontainedelementstobeint;andnotco ally,wefindthatwecannowuseourgenericdouble_each_elementfunctionevenwithcontainersthatdon'tholdint!std::vector<double>vecd={1.0,2.0,3.0};ThisextralevelofgenericityisoneofthebigbenefitsofusingC++temtesforgenericprogramming,asopposedtoclassicalpolymorphism.Classicalpolymorphismhidesthevaryingbehaviorofdifferentclassesbehindastableinterfacesignature(forexample,.at(i)alwaysreturnsint&),butonceyoustartmessingwithvaryingsignatures,classicalpolymorphismisnolongeragoodtoolforthejob.Anotheradvantageofgenericprogrammingisthatitoffersblazingspeedthroughincreasedopportunitiesforinlining.Theclassicallypolymorphicexamplemustrepeatedlyquerythecontainer_of_intobject'svirtualtabletofindtheaddressofitsparticularvirtualatmethod,andgenerallycannotseethroughthevirtualdispatchatcompiletime.Thetemtefunctiondouble_each_element<array_of_int>cancompileinadirectcalltoarray_of_int::atoreveninlinethecallcompley.Becausegenericprogrammingwithtemtescansoeasilydealwithcomplicatedrequirementsandissoflexibleindealingwithtypes--evenprimitivetypeslikeint,whereclassicalpolymorphismfails--thestandardlibraryusestemtesforallitsalgorithmsandthecontainersonwhichtheyoperate.Forthisreason,thealgorithms-and-containerspartofthestandardlibraryisoftenreferredtoastheStandardTemteLibraryorSTL.That'sright--technically,theSTLisonlyasmallpartoftheC++standardlibrary!However,inthisbook,asinreallife,wemayoccasionallyslipupandusethetermSTLwhenwemeanstandardlibrary,orviceversa.Let'slookatacouplemorehand-writtengenericalgorithms,beforewediveintothestandardgenericalgorithmsprovidedbytheSTL.Hereisafunctiontemtecount,returningthetotalnumberofelementsinacontainer: te<classintcount(constContainer&{intsum=for(auto&&elt:{sum+=}return}Andhereiscount_if,whichreturnsthenumberofelementssatisfyingauser-dpredicatefunction: te<classContainer,classintcount_if(constContainer&container,Predicate{intsum=for(auto&&elt:{if(pred(elt)){sum+=1;}}return}Thesefunctionswouldbeusedlikestd::vector<int>v={3,1,4,1,5,9,2,assert(count(v)==8);intnumber_above=count_if(v,[](inte){returne>5;});intnumber_below=count_if(v,[](inte){returne<5;assert(number_above==assert(number_below==Thereissomuchpowerpackedintothatlittleexpressionpred(elt)!Iencourageyoutotryre-implementingthecount_iffunctionintermsofclassicalpolymorphism,justtogetasenseofwherethewholethingbreaksdown.TherearealotofvaryingsignatureshiddenunderthesyntacticsugarofmodernC++.Forexample,therangedfor-loopsyntaxinourcount_iffunctionisconverted(orlowered")bythecompilerintoafor-loopintermsofcontainer.begin()andcontainer.end(),eachofwhichneedstoreturnanitorwhosetypeisdependentonthetypeofcontaineritself.Foranotherexample,inthegeneric-programmingversion,weneverspecify--weneverneedtospecify--whetherpredtakesitsparametereltbyvalueorbyreference.ngthatwithavirtualboolSpeakingofitors:youmayhavenoticedtha