Google發(fā)布的C++編程規(guī)范

GoogleC++StyleGuideRevision3.180BenjyWeinbergerCraigSilversteinGregoryEitzmannMarkMentovaiTashanaLandrayEachstylepointhasasummaryforwhichadditionalinformationisavailablebytogglingtheaccompanyingarrowbuttonthatlooksthisway:▽.Youmaytoggleallsummarieswiththebigarrowbutton:7ToggleallsummariesTableofContentsHeaderFilesThe#defineGuardHeaderFileDependenciesInlineFunctionsThe-inLhFilesFunctionParameter〇rderinaNamesandOrderofIncludesScopi、。NamespacesNestedClassesNonmember,StaticMember,andGlobalFunctionsLocalVariablesStaticandGlobalVariablesClassesDoinaWorkinConstructorsDefaultConstructorsExplicitConstructorsCopyConstructorsStructsvs.ClassesInheritanceMu用pieInhehtacceInterfacesOperatorOverloadinqAccessControlDeclarationOrderWriteShortFunctionsGoocle-SpecificMagicSmartPointerscpplintOtherC++FeaturesReferenceArgumentsFunction〇verloadingDefaultArgumentsVariable-LenEhArraysandalloca()FriendsExceptionsRun-TimeTypeInformation(RTTI)CastingStreamsPreincrementandPredecrementUseofconstInteserTypes64-bitPortabilityPreprocessorMacros〇andNULLsizeofBoostC++0xNamiriqGeneralNaminqRulesFileNamesTypeNamesVariableNamesConstantNamesFunctionNamesNamespaceNamesEnumeratorNamesMacroNamesExceptionstoNaminqRulesCommentsCommentStyleFileCommentsClassCommentsFunctionCommentsVariableCommentsImplementationCommentsPunctuation,SpellingandGrammarTODOCommentsDeprecationCommentsFormattingUneLenqthNon?ASCIICharactersSpacesvs.TabsFunctionDeclarationsandDefinitionsFunctionCallsConditionalsLoopsand

SwitchStatementsPointerandReferenceExpressionsBooleanExpressionsReturnValuesVariableandArrayInitializationPreprocessorDirectivesClassFormatConstructorInitializerListsNamespaceFormattinqHorizontalWhitespaceVerticalWhitespaceExceptionstotheRulesExistingNon-conformantCodeWindowsCodeImportantNoteDisplayingHiddenDetailsinthisGuidelinkVThisstyleguidecontainsmanydetailsthatareinitiallyhiddenfromview.Theyaremarkedbythetriangleicon,whichyouseehereonyourleft.Clickitnow.Youshouldsee"Hooray"appearbelow.Hooray!Nowyouknowyoucanexpandpointstogetmoredetails.Alternatively,there*san"expandall"atthetopofthisdocument.BackgroundC++isthemaindevelopmentlanguageusedbymanyofGoogle'sopen-sourceprojects.AseveryC++programmerknows,thelanguagehasmanypowerfulfeatures,butthispowerbringswithitcomplexity,whichinturncanmakecodemorebug-proneandhardertoreadandmaintain.Thegoalofthisguideistomanagethiscomplexitybydescribingindetailthedosanddon'tsofwritingC++code.TheserulesexisttokeepthecodebasemanageablewhilestillallowingcoderstouseC++languagefeaturesproductively.Style,alsoknownasreadability,iswhatwecalltheconventionsthatgovernourC++code.ThetermStyleisabitofamisnomer,sincetheseconventionscoverfarmorethanjustsourcefileformatting.Onewayinwhichwekeepthecodebasemanageableisbyenforcingconsistency.Itisveryimportantthatanyprogrammerbeabletolookatanother'scodeandquicklyunderstandit.Maintainingauniformstyleandfollowingconventionsmeansthatwecanmoreeasilyuse"pattern-matching"toinferwhatvarioussymbolsareandwhatinvariantsaretrueaboutthem.Creatingcommon,requiredidiomsandpatternsmakescodemucheasiertounderstand.Insomecasestheremightbegoodargumentsforchangingcertainstylerules,butwenonethelesskeepthingsastheyareinordertopreserveconsistency.AnotherissuethisguideaddressesisthatofC++featurebloat.C++isahugelanguagewithmanyadvancedfeatures.Insomecasesweconstrain,orevenban,useofcertainfeatures.Wedothistokeepcodesimpleandtoavoidthevariouscommonerrorsandproblemsthatthesefeaturescancause.Thisguideliststhesefeaturesandexplainswhytheiruseisrestricted.Open-sourceprojectsdevelopedbyGoogleconformtotherequirementsinthisguide.NotethatthisguideisnotaC++tutorial:weassumethatthereaderisfamiliarwiththelanguage.HeaderFilesIngeneral,every.ccfileshouldhaveanassociated.hfile.Therearesomecommonexceptions,suchasunittestsandsmall.ccfilescontainingjustamain()function.Correctuseofheaderfilescanmakeahugedifferencetothereadability,sizeandperformanceofyourcode.Thefollowingruleswillguideyouthroughthevariouspitfallsofusingheaderfiles.The#defineGuardlinkVAllheaderfilesshouldhave#defineguardstopreventmultipleinclusion.TheformatofthesymbolnameshouldbeくPROJECT>くPATH)<FILE〉!I.Toguaranteeuniqueness,theyshouldbebasedonthefullpathinaproject'ssourcetree.Forexample,thefilefoo/src/bar/baz.hinprojectfooshouldhavethefollowingguard:#ifndefFOOBAR_BAZH_4defineFOO_BAR_BAZ_H_#endif//FOO_BAR_BAZ_H_HeaderFileDependencieslinkVDon'tusean#includewhenaforwarddeclarationwouldsuffice.Whenyouincludeaheaderfileyouintroduceadependencythatwillcauseyourcodetoberecompiledwhenevertheheaderfilechanges.Ifyourheaderfileincludesotherheaderfiles,anychangetothosefileswillcauseanycodethatincludesyourheadertoberecompiled.Therefore,weprefertominimizeincludes,particularlyincludesofheaderfilesinotherheaderfiles.Youcansignificantlyminimizethenumberofheaderfilesyouneedtoincludeinyourownheaderfilesbyusingforwarddeclarations.Forexample,ifyourheaderfileusestheFileclassinwaysthatdonotrequireaccesstothedeclarationoftheFileclass,yourheaderfilecanjustforwarddeclareclassFile;insteadofhavingto#include/zfile/base/file.h”.HowcanweuseaclassFooinaheaderfilewithoutaccesstoitsdefinition?WecandeclaredatamembersoftypeFoo*orFoo&.Wecandeclare(butnotdefine)functionswitharguments,and/orreturnvalues,oftypeFoo.(OneexceptionisifanargumentFooorconstFoo&hasanon-explicit,one-argumentconstructor,inwhichcaseweneedthefulldefinitiontosupportautomatictypeconversion.)WecandeclarestaticdatamembersoftypeFoo.Thisisbecausestaticdatamembersaredefinedoutsidetheclassdefinition.Ontheotherhand,youmustincludetheheaderfileforFooifyourclasssubclassesFooorhasadatamemberoftypeFoo.Sometimesitmakessensetohavepointer(orbetter,scopedptr)membersinsteadofobjectmembers.However,thiscomplicatescodereadabilityandimposesaperformancepenalty,soavoiddoingthistransformationiftheonlypurposeistominimizeincludesinheaderfiles.Ofcourse,.ccfilestypicallydorequirethedefinitionsoftheclassestheyuse,andusuallyhavetoincludeseveralheaderfiles.Note：IfyouuseasymbolFooinyoursourcefile,youshouldbringinadefinitionforFooyourself,eitherviaan#includeorviaaforwarddeclaration.Donotdependonthesymbolbeingbroughtintransitivelyviaheadersnotdirectlyincluded.OneexceptionisifFooisusedinmyfile.cc,it'sokto#include(orforward-deciare)Fooinmyfile.h,insteadofmyfile.cc.InlineFunctionslinkVDefinefunctionsinlineonlywhentheyaresmall,say,10linesorless.Definition：Youcandeclarefunctionsinawaythatallowsthecompilertoexpandtheminlineratherthancallingthemthroughtheusualfunctioncallmechanism.Pros：Inliningafunctioncangeneratemoreefficientobjectcode,aslongastheinlinedfunctionissmall.Feelfreetoinlineaccessorsandmutators,andothershort,performance-criticalfunctions.Cons：Overuseofinliningcanactuallymakeprogramsslower.Dependingonafunction'ssize,inliningitcancausethecodesizetoincreaseordecrease.Inliningaverysmallaccessorfunctionwillusuallydecreasecodesizewhileinliningaverylargefunctioncandramaticallyincreasecodesize.Onmodernprocessorssmallercodeusuallyrunsfasterduetobetteruseoftheinstructioncache.Decision:Adecentruleofthumbistonotinlineafunctionifitismorethan10lineslong.Bewareofdestructors,whichareoftenlongerthantheyappearbecauseofimplicitmember-andbase-destructorcalls!Anotherusefulruleofthumb:it'stypicallynotcosteffectivetoinlinefunctionswithloopsorswitchstatements(unless,inthecommoncase,thelooporswitchstatementisneverexecuted).Itisimportanttoknowthatfunctionsarenotalwaysinlinedeveniftheyaredeclaredassuch;forexample,virtualandrecursivefunctionsarenotnormallyinlined.Usuallyrecursivefunctionsshouldnotbeinline.Themainreasonformakingavirtualfunctioninlineistoplaceitsdefinitionintheclass,eitherforconvenienceortodocumentitsbehavior,e.g.,foraccessorsandmutators.The-inl.hFileslinkVYoumayusefilenameswitha-inl.hsuffixtodefinecomplexinlinefunctionswhenneeded.Thedefinitionofaninlinefunctionneedstobeinaheaderfile,sothatthecompilerhasthedefinitionavailableforinliningatthecallsites.However,implementationcodeproperlybelongsin.ccfiles,andwedonotliketohavemuchactualcodein.hfilesunlessthereisareadabilityorperformanceadvantage.Ifaninlinefunctiondefinitionisshort,withverylittle,ifany,logicinit,youshouldputthecodeinyour.hfile.Forexample,accessorsandmutatorsshouldcertainlybeinsideaclassdefinition.Morecomplexinlinefunctionsmayalsobeputina.hfilefortheconvenienceoftheimplementerandcallers,thoughifthismakesthe.hfiletoounwieldyyoucaninsteadputthatcodeinaseparate-ini.hfile.Thisseparatestheimplementationfromtheclassdefinition,whilestillallowingtheimplementationtobeincludedwherenecessary.Anotheruseof-ini.hfilesisfordefinitionsoffunctiontemplates.Thiscanbeusedtokeepyourtemplatedefinitionseasytoread.Donotforgetthata-ini.hfilerequiresa[defineguardjustlikeanyotherheaderfile.FunctionParameterOrderinglinkVWhendefiningafunction,parameterorderis:inputs,thenoutputs.ParameterstoC/C++functionsareeitherinputtothefunction,outputfromthefunction,orboth.Inputparametersareusuallyvaluesorconstreferences,whileoutputandinput/outputparameterswillbenon-constpointers.Whenorderingfunctionparameters,putallinput-onlyparametersbeforeanyoutputparameters.Inparticular,donotaddnewparameterstotheendofthefunctionjustbecausetheyarenew;placenewinput-onlyparametersbeforetheoutputparameters.Thisisnotahard-and-fastrule.Parametersthatarebothinputandoutput(oftenclasses/structs)muddythewaters,and,asalways,consistencywithrelatedfunctionsmayrequireyoutobendtherule.NamesandOrderofIncludeslinkVUsestandardorderforreadabilityandtoavoidhiddendependencies:Clibrary,C++library,otherlibraries*.h,yourproject's.h.Allofaproject'sheaderfilesshouldbelistedasdescentantsoftheproject'ssourcedirectorywithoutuseofUNIXdirectoryshortcuts.(thecurrentdirectory)or..(theparentdirectory).Forexample,google-awesome-project/src/base/logging.hshouldbeincludedas#include*base/logging.h"Indir/foo.cc,whosemainpurposeistoimplementortestthestuffindir2/foo2.h,orderyourincludesasfollows:dir2/foo2.h(preferredlocation-seedetailsbelow).Csystemfiles.C++systemfiles.Otherlibraries'.hfiles.Yourproject's.hfiles.Thepreferredorderingreduceshiddendependencies.Wewanteveryheaderfiletobecompilableonitsown.Theeasiestwaytoachievethisistomakesurethateveryoneofthemisthefirst.hfilettincludedinsome.cc.

dir/foo.ccanddir2/foo2.hareofteninthesamedirectory(e.g.base/basictypes_test.ccandbase/basictypes.h),butcanbeindifferentdirectoriestoo.Withineachsectionitisnicetoordertheincludesalphabetically.Forexample,theincludesingoogle-awesome-project/src/foo/internal/fooserver.ccmightlooklikethis:#include*foo/public/fooserver.h*//Preferredlocation.#include<sys/types.h>#include<unistd.h>#include<hash_map>4include<vector>#include"base/basictypes.h"#include"base/commandlineflags,h"#include"foo/public/bar.h"ScopingNamespaceslink▽Unnamednamespacesin.ccfilesareencouraged.Withnamednamespaces,choosethenamebasedontheproject,andpossiblyitspath.Donotuseausing-directive.Definition：Namespacessubdividetheglobalscopeintodistinct,namedscopes,andsoareusefulforpreventingnamecollisionsintheglobalscope.Pros:Namespacesprovidea(hierarchical)axisofnaming,inadditiontothe(alsohierarchical)nameaxisprovidedbyclasses.Forexample,iftwodifferentprojectshaveaclassFoointheglobalscope,thesesymbolsmaycollideatcompiletimeoratruntime.Ifeachprojectplacestheircodeinanamespace,projectl::Fooandproject2::Fooarenowdistinctsymbolsthatdonotcollide.Cons:

Namespacescanbeconfusing,becausetheyprovideanadditional(hierarchical)axisofnaming,inadditiontothe(alsohierarchical)nameaxisprovidedbyclasses.UseofunnamedspacesinheaderfilescaneasilycauseviolationsoftheC++OneDefinitionRule(ODR).Decision:Usenamespacesaccordingtothepolicydescribedbelow.UnnamedNamespacesUnnamednamespacesareallowedandevenencouragedin.ccfiles,toavoidruntimenamingconflicts:namespace{ //Thisisina.ccfile.?//Thecontentofanamespaceisnotindentedenum{kUnused,kEOF,kError}; //Commonlyusedtokens.boolAtEof()(returnpos==kEOF;}//Usesournamespace//Alldeclarationsarewithinthenamespacescope.//Alldeclarationsarewithinthenamespacescope.*}//namespaceHowever,file-scopedeclarationsthatareassociatedwithaparticularclassmaybedeclaredinthatclassastypes,staticdatamembersorstaticmemberfunctionsratherthanasmembersofanunnamednamespace.Terminatetheunnamednamespaceasshown,withacomment//namespace.Donotuseunnamednamespacesin.hfiles.NamedNamespacesNamednamespacesshouldbeusedasfollows:Namespaceswraptheentiresourcefileafterincludes,qflagsdefinitions/declarations,andforwarddeclarationsofclassesfromothernamespaces://Inthe.hfile? namespacemynamespace{

//Noticethelackofindentation,classMyClass{public:voidFoo();}；}//namespacemynamespace//Inthe.ccfilenamespacemynamespace{//Definitionoffunctionsiswithinscopeofthenamespace,voidMyClass::Foo(){}//namespacemynamespaceThetypical.ccfilemighthavemorecomplexdetail,includingtheneedtoreferenceclassesinothernamespaces.#include"a.h*DEFINE_bool(someflag,false,*dummyflag");classC;//ForwarddeclarationofclassCintheglobalspacea{classA;)//Forwarddeclarationofa::A.namespaceb{...codeforb... //Codegoesagainsttheleftmargin.}//namespacebDonotdeclareanythinginnamespacestd,notevenforwarddeclarationsofstandardlibraryclasses.Declaringentitiesinnamespacestdisundefinedbehavior,i.e.,notportable.Todeclareentitiesfromthestandardlibrary,includetheappropriateheaderfile.Youmaynotuseausing-directivetomakeallnamesfromanamespaceavailable.//ForbiddenーーThispollutesthenamespace.

usingnamespacefoo;Youmayuseausing-declarationanywhereina.ccfile,andinfunctions,methodsorclassesin.hfiles.//OKin.ccfiles.//Mustbeinafunction,methodorclassin.hfiles.using::foo::bar;Namespacealiasesareallowedanywhereina.ccfile,anywhereinsidethenamednamespacethatwrapsanentire.hfile,andinfunctionsandmethods.//Shortenaccesstosomecommonlyusednamesin.ccspacefbz=::foo::bar::baz;//Shortenaccesstosomecommon1yusednames(ina.hfile).namespacelibrarian{//Thefollowingaliasisavailabletoal1filesincluding//thisheader(innamespacelibrarian)://aliasnamesshouldthereforebechosenconsistently//withinaspacepd_s=::pipeline_diagnostics::sidetable;inlinevoidmy_inline_function(){//namespacealiaslocaltoafunction(ormethod).namespacefbz=::foo::bar::baz;}//namespacelibrarianNotethatanaliasina.hfileisvisibletoeveryone/includingthatfile,sopublicheaders(thoseavailableoutsideaproject)andheaderstransitively#includedbythem,shouldavoiddefiningaliases,aspartofthegeneralgoalofkeepingpublicAPIsassmallaspossible.NestedClasseslinkVAlthoughyoumayusepublicnestedclasseswhentheyarepartofaninterface,consideranamespacetokeepdeclarationsoutoftheglobalscope.Definition：Aclasscandefineanotherclasswithinit;thisisalsocalledamemberclass.

classFoo{private://Barisamemberclass,nestedwithinFoo.classBar{)；}；Pros：Thisisusefulwhenthenested(ormember)classisonlyusedbytheenclosingclass;makingitamemberputsitintheenclosingclassscoperatherthanpollutingtheouterscopewiththeclassname.Nestedclassescanbeforwarddeclaredwithintheenclosingclassandthendefinedinthe.ccfiletoavoidincludingthenestedclassdefinitionintheenclosingclassdeclaration,sincethenestedclassdefinitionisusuallyonlyrelevanttotheimplementation.Cons：Nestedclassescanbeforward-declaredonlywithinthedefinitionoftheenclosingclass.Thus,anyheaderfilemanipulatingaFoo::Bar*pointerwillhavetoincludethefullclassdeclarationforFoo.Decision：Donotmakenestedclassespublicunlesstheyareactuallypartoftheinterface,e.g.,aclassthatholdsasetofoptionsforsomemethod.Nonmember,StaticMember,andGlobalFunctionslinkVPrefernonmemberfunctionswithinanamespaceorstaticmemberfunctionstoglobalfunctions;usecompletelyglobalfunctionsrarely.Pros：Nonmemberandstaticmemberfunctionscanbeusefulinsomesituations.Puttingnonmemberfunctionsinanamespaceavoidspollutingtheglobalnamespace.Cons：Nonmemberandstaticmemberfunctionsmaymakemoresenseasmembersofanewclass,especiallyiftheyaccessexternalresourcesorhavesignificantdependencies.Decision:Sometimesitisuseful,orevennecessary,todefineafunctionnotboundtoaclassinstance.Suchafunctioncanbeeitherastaticmemberoranonmemberfunction.Nonmemberfunctionsshouldnotdependonexternalvariables,andshouldnearlyalwaysexistinanamespace.Ratherthancreatingclassesonlytogroupstaticmemberfunctionswhichdonotsharestaticdata,usenamespacesinstead.

Functionsdefinedinthesamecompilationunitasproductionclassesmayintroduceunnecessarycouplingandlink-timedependencieswhendirectlycalledfromothercompilationunits;staticmemberfunctionsareparticularlysusceptibletothis.Considerextractinganewclass,orplacingthefunctionsinanamespacepossiblyinaseparatelibrary.Ifyoumustdefineanonmemberfunctionanditisonlyneededinits.ccfile,useanunnamednamespaceorstaticlinkage(egstaticintFoo(){...})tolimititsscope.LocalVariableslinkVPlaceafunction'svariablesinthenarrowestscopepossible,andinitializevariablesinthedeclaration.C++allowsyoutodeclarevariablesanywhereinafunction.Weencourageyoutodeclaretheminaslocalascopeaspossible,andasclosetothefirstuseaspossible.Thismakesiteasierforthereadertofindthedeclarationandseewhattypethevariableisandwhatitwasinitializedto.Inparticular,initializationshouldbeusedinsteadofdeclarationandassignment,i;i=f(); //Bad-initializationseparatefromj=g();//Good-declarationhasinitialization.Notethatgccimplementsfor(inti=0;i<10;++i)correctly(thescopeofiisonlythescopeoftheforloop),soyoucanthenreuseiinanotherforloopinthesamescope.Italsocorrectlyscopesdeclarationsinifandwhilestatements,e.g.while(constchar*p=strchr(str,'/'))str=p+1;Thereisonecaveat:ifthevariableisanobject,itsconstructorisinvokedeverytimeitentersscopeandiscreated,anditsdestructorisinvokedeverytimeitgoesoutofscope.//Inefficientimplementation:for(inti=0;i<1000000;++i){Foof;//Myctoranddtorgetcalled1000000timeseach.f.DoSomething(i);}Itmaybemoreefficienttodeclaresuchavariableusedinaloopoutsidethatloop:Foof;//Myctoranddtorgetcalledonceeach,for(inti=0;i<1000000;++i){

f.DoSomething(i);StaticandGlobalVariableslinkVStaticorglobalvariablesofclasstypeareforbidden:theycausehard-to-findbugsduetoindeterminateorderofconstructionanddestruction.Objectswithstaticstorageduration,includingglobalvariables,staticvariables,staticclassmembervariables,andfunctionstaticvariables,mustbePlainOldData(POD):onlyints,chars,floats,orpointers,orarrays/structsofPOD.TheorderinwhichclassconstructorsandinitializersforstaticvariablesarecalledisonlypartiallyspecifiedinC++andcanevenchangefrombuildtobuild,whichcancausebugsthataredifficulttofind.Thereforeinadditiontobanningglobalsofclasstype,wedonotallowstaticPODvariablestobeinitializedwiththeresultofafunction,unlessthatfunction(suchasgetenv(),orgetpid())doesnotitselfdependonanyotherglobals.Likewise,theorderinwhichdestructorsarecalledisdefinedtobethereverseoftheorderinwhichtheconstructorswerecalled.Sinceconstructororderisindeterminate,soisdestructororder.Forexample,atprogram-endtimeastaticvariablemighthavebeendestroyed,butcodestillrunning-perhapsinanotherthread-triestoaccessitandfails.Orthedestructorforastatic'string'variablemightberunpriortothedestructorforanothervariablethatcontainsareferencetothatstring.AsaresultweonlyallowstaticvariablestocontainPODdata.Thisrulecompletelydisallowsvector(useCarraysinstead),orstring(useconstchar[]).Ifyouneedastaticorglobalvariableofaclasstype,considerinitializingapointer(whichwillneverbefreed),fromeitheryourmain()functionorfrompthread_once().Notethatthismustbearawpointer,nota"smart"pointer,sincethesmartpointer'sdestructorwillhavetheorder-of-destructorissuethatwearetryingtoavoid.ClassesClassesarethefundamentalunitofcodeinC++.Naturally,weusethemextensively.Thissectionliststhemaindosanddon'tsyoushouldfollowwhenwritingaclass.DoingWorkinConstructorslinkVIngeneral,constructorsshouldmerelysetmembervariablestotheirinitialvalues.AnycomplexinitializationshouldgoinanexplicitInit()method.Definition:Itispossibletoperforminitializationinthebodyoftheconstructor.Pros：Convenienceintyping.Noneedtoworryaboutwhethertheclasshasbeeninitializedornot.Cons：Theproblemswithdoingworkinconstructorsare:Thereisnoeasywayforconstructorstosignalerrors,shortofusingexceptions(whichareforbidden).Iftheworkfails,wenowhaveanobjectwhoseinitializationcodefailed,soitmaybeanindeterminatestate.Iftheworkcallsvirtualfunctions,thesecallswillnotgetdispatchedtothesubclassimplementations.Futuremodificationtoyourclasscanquietlyintroducethisproblemevenifyourclassisnotcurrentlysubclassed,causingmuchconfusion.Ifsomeonecreatesaglobalvariableofthistype(whichisagainsttherules,butstill),theconstructorcodewillbecalledbeforemain(),possiblybreakingsomeimplicitassumptionsintheconstructorcode.Forinstance,gflaqswillnotyethavebeeninitialized.Decision：Ifyourobjectrequiresnon-trivialinitialization,considerhavinganexplicitInit()method.Inparticular,constructorsshouldnotcallvirtualfunctions,attempttoraiseerrors,accesspotentiallyuninitializedglobalvariables,etc.DefaultConstructorslinkVYoumustdefineadefaultconstructorifyourclassdefinesmembervariablesandhasnootherconstructors.Otherwisethecompilerwilldoitforyou,badly.Definition：Thedefaultconstructoriscalledwhenwenewaclassobjectwithnoarguments.Itisalwayscalledwhencallingnew[](forarrays).Pros：Initializingstructuresbydefault,tohold"impossible'*values,makesdebuggingmucheasier.Cons：Extraworkforyou,thecodewriter.Decision:Ifyourclassdefinesmembervariablesandhasnootherconstructorsyoumustdefineadefaultconstructor(onethattakesnoarguments).Itshouldpreferablyinitializetheobjectinsuchawaythatitsinternalstateisconsistentandvalid.Thereasonforthisisthatifyouhavenootherconstructorsanddonotdefineadefaultconstructor,thecompilerwillgenerateoneforyou.Thiscompilergeneratedconstructormaynotinitializeyourobjectsensibly.Ifyourclassinheritsfromanexistingclassbutyouaddnonewmembervariables,youarenotrequiredtohaveadefaultconstructor.ExplicitConstructorslinkVUsetheC++keywordexplicitforconstructorswithoneargument.Definition：Normally,ifaconstructortakesoneargument,itcanbeusedasaconversion.Forinstance,ifyoudefineFoo::Foo(stringname)andthenpassastringtoafunctionthatexpectsaFoo,theconstructorwillbecalledtoconvertthestringintoaFooandwillpasstheFootoyourfunctionforyou.Thiscanbeconvenientbutisalsoasourceoftroublewhenthingsgetconvertedandnewobjectscreatedwithoutyoumeaningthemto.Declaringaconstructorexplicitpreventsitfrombeinginvokedimplicitlyasaconversion.Pros：Avoidsundesirableconversions.Cons：None.Decision:Werequireallsingleargumentconstructorstobeexplicit.Alwaysputexplicitinfrontofone-argumentconstructorsintheclassdefinition:explicitFoo(stringname);Theexceptioniscopyconstructors,which,intherarecaseswhenweallowthem,shouldprobablynotbeexplicit.Classesthatareintendedtobetransparentwrappersaroundotherclassesarealsoexceptions.Suchexceptionsshouldbeclearlymarkedwithcomments.CopyConstructorslink▽Provideacopyconstructorandassignmentoperatoronlywhennecessary.Otherwise,disablethemwithDISALLOWCOPY_AND_ASSIGN.Definition：Thecopyconstructorandassignmentoperatorareusedtocreatecopiesofobjects.Thecopyconstructorisimplicitlyinvokedbythecompilerinsomesituations,e.g.passingobjectsbyvalue.

Pros：Copyconstructorsmakeiteasytocopyobjects.STLcontainersrequirethatallcontentsbecopyableandassignable.CopyconstructorscanbemoreefficientthanCopyFrom()-styleworkaroundsbecausetheycombineconstructionwithcopying,thecompilercanelidetheminsomecontexts,andtheymakeiteas