Googlec++编程规范.docx

Googlec++编程规范.docx

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Googlec++编程规范

GoogleC++StyleGuide

Revision3.199

BenjyWeinberger

CraigSilverstein

GregoryEitzmann

MarkMentovai

TashanaLandray

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TableofContents

HeaderFiles

The#defineGuard HeaderFileDependencies InlineFunctions The-inl.hFiles FunctionParameterOrdering NamesandOrderofIncludes

Scoping

Namespaces NestedClasses Nonmember,StaticMember,andGlobalFunctions LocalVariables StaticandGlobalVariables

Classes

DoingWorkinConstructors DefaultConstructors ExplicitConstructors CopyConstructors Structsvs.Classes Inheritance MultipleInheritance Interfaces OperatorOverloading AccessControlDeclarationOrder WriteShortFunctions

Google-SpecificMagic

SmartPointers cpplint

OtherC++Features

ReferenceArguments FunctionOverloading DefaultArguments Variable-LengthArraysandalloca（） Friends Exceptions Run-TimeTypeInformation（RTTI） Casting Streams PreincrementandPredecrementUseofconst IntegerTypes 64-bitPortability PreprocessorMacros 0andNULL sizeof Boost C++11

Naming

GeneralNamingRules FileNames TypeNames VariableNames ConstantNames FunctionNames NamespaceNames EnumeratorNames MacroNames ExceptionstoNamingRules

Comments

CommentStyle FileComments ClassComments FunctionComments VariableComments ImplementationComments Punctuation,SpellingandGrammar TODOComments DeprecationComments

Formatting

LineLength Non-ASCIICharacters Spacesvs.Tabs FunctionDeclarationsandDefinitions FunctionCalls Conditionals LoopsandSwitchStatements PointerandReferenceExpressions BooleanExpressionsReturnValues VariableandArrayInitialization PreprocessorDirectives ClassFormat ConstructorInitializerLists NamespaceFormatting HorizontalWhitespace VerticalWhitespace

ExceptionstotheRules

ExistingNon-conformantCode WindowsCode

ImportantNote

DisplayingHiddenDetailsinthisGuide

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Background

C++isthemaindevelopmentlanguageusedbymanyofGoogle'sopen-sourceprojects.AseveryC++programmerknows,thelanguagehasmanypowerfulfeatures,butthispowerbringswithitcomplexity,whichinturncanmakecodemorebug-proneandhardertoreadandmaintain.

Thegoalofthisguideistomanagethiscomplexitybydescribingindetailthedosanddon'tsofwritingC++code.TheserulesexisttokeepthecodebasemanageablewhilestillallowingcoderstouseC++languagefeaturesproductively.

Style,alsoknownasreadability,iswhatwecalltheconventionsthatgovernourC++code.ThetermStyleisabitofamisnomer,sincetheseconventionscoverfarmorethanjustsourcefileformatting.

Onewayinwhichwekeepthecodebasemanageableisbyenforcing consistency.Itisveryimportantthatanyprogrammerbeabletolookatanother'scodeandquicklyunderstandit.Maintainingauniformstyleandfollowingconventionsmeansthatwecanmoreeasilyuse"pattern-matching"toinferwhatvarioussymbolsareandwhatinvariantsaretrueaboutthem.Creatingcommon,requiredidiomsandpatternsmakescodemucheasiertounderstand.Insomecasestheremightbegoodargumentsforchangingcertainstylerules,butwenonethelesskeepthingsastheyareinordertopreserveconsistency.

AnotherissuethisguideaddressesisthatofC++featurebloat.C++isahugelanguagewithmanyadvancedfeatures.Insomecasesweconstrain,orevenban,useofcertainfeatures.Wedothistokeepcodesimpleandtoavoidthevariouscommonerrorsandproblemsthatthesefeaturescancause.Thisguideliststhesefeaturesandexplainswhytheiruseisrestricted.

Open-sourceprojectsdevelopedbyGoogleconformtotherequirementsinthisguide.

NotethatthisguideisnotaC++tutorial:

weassumethatthereaderisfamiliarwiththelanguage.

HeaderFiles

Ingeneral,every .cc fileshouldhaveanassociated .h file.Therearesomecommonexceptions,suchasunittestsandsmall .cc filescontainingjusta main（） function.

Correctuseofheaderfilescanmakeahugedifferencetothereadability,sizeandperformanceofyourcode.

Thefollowingruleswillguideyouthroughthevariouspitfallsofusingheaderfiles.

The#defineGuard

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Allheaderfilesshouldhave #define guardstopreventmultipleinclusion.Theformatofthesymbolnameshouldbe ___H_.

Toguaranteeuniqueness,theyshouldbebasedonthefullpathinaproject'ssourcetree.Forexample,thefile foo/src/bar/baz.h inproject foo shouldhavethefollowingguard:

#ifndefFOO_BAR_BAZ_H_

#defineFOO_BAR_BAZ_H_

...

#endif//FOO_BAR_BAZ_H_

HeaderFileDependencies

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Don'tusean #include whenaforwarddeclarationwouldsuffice.

Whenyouincludeaheaderfileyouintroduceadependencythatwillcauseyourcodetoberecompiledwhenevertheheaderfilechanges.Ifyourheaderfileincludesotherheaderfiles,anychangetothosefileswillcauseanycodethatincludesyourheadertoberecompiled.Therefore,weprefertominimizeincludes,particularlyincludesofheaderfilesinotherheaderfiles.

Youcansignificantlyreducethenumberofheaderfilesyouneedtoincludeinyourownheaderfilesbyusingforwarddeclarations.Forexample,ifyourheaderfileusesthe File classinwaysthatdonotrequireaccesstothedeclarationofthe File class,yourheaderfilecanjustforwarddeclare classFile; insteadofhavingto #include"file/base/file.h".

Howcanweuseaclass Foo inaheaderfilewithoutaccesstoitsdefinition?

∙Wecandeclaredatamembersoftype Foo* or Foo&.

∙Wecandeclare（butnotdefine）functionswitharguments,and/orreturnvalues,oftype Foo.（Oneexceptionisifanargument Foo or constFoo& hasanon-explicit,one-argumentconstructor,inwhichcaseweneedthefulldefinitiontosupportautomatictypeconversion.）

∙Wecandeclarestaticdatamembersoftype Foo.Thisisbecausestaticdatamembersaredefinedoutsidetheclassdefinition.

Ontheotherhand,youmustincludetheheaderfilefor Foo ifyourclasssubclasses Foo orhasadatamemberoftype Foo.

Sometimesitmakessensetohavepointer（orbetter, scoped_ptr）membersinsteadofobjectmembers.However,thiscomplicatescodereadabilityandimposesaperformancepenalty,soavoiddoingthistransformationiftheonlypurposeistominimizeincludesinheaderfiles.

Ofcourse, .cc filestypicallydorequirethedefinitionsoftheclassestheyuse,andusuallyhavetoincludeseveralheaderfiles.

Note:

 Ifyouuseasymbol Foo inyoursourcefile,youshouldbringinadefinitionfor Foo yourself,eitherviaan#includeorviaaforwarddeclaration.Donotdependonthesymbolbeingbroughtintransitivelyviaheadersnotdirectlyincluded.Oneexceptionisif Foo isusedin myfile.cc,it'sokto#include（orforward-declare） Foo in myfile.h,insteadof myfile.cc.

InlineFunctions

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Definefunctionsinlineonlywhentheyaresmall,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.hFiles

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Youmayusefilenameswitha -inl.h suffixtodefinecomplexinlinefunctionswhenneeded.

Thedefinitionofaninlinefunctionneedstobeinaheaderfile,sothatthecompilerhasthedefinitionavailableforinliningatthecallsites.However,implementationcodeproperlybelongsin .cc files,andwedonotliketohavemuchactualcodein .h filesunlessthereisareadabilityorperformanceadvantage.

Ifaninlinefunctiondefinitionisshort,withverylittle,ifany,logicinit,youshouldputthecodeinyour .h file.Forexample,accessorsandmutatorsshouldcertainlybeinsideaclassdefinition.Morecomplexinlinefunctionsmayalsobeputina .h filefortheconvenienceoftheimplementerandcallers,thoughifthismakesthe .h filetoounwieldyyoucaninsteadputthatcodeinaseparate -inl.h file.Thisseparatestheimp