C++面向对象程序设计双语教程（第3版）课件 ch08Templates

TemplatesC++面向对象程序设计双语教程(第3版)Chapter801Introduction

to

TemplatesManyC++programsusecommondatastructureslikestacks，queuesandlists.Aprogrammayrequireaqueueofcustomersandaqueueofmessages.Onecouldeasilyimplementaqueueofcustomers，thentaketheexistingcodeandimplementaqueueofmessages.IntroductiontoTemplatesTheprogramgrows，andnowthereisaneedforaqueueoforders.Sojusttakethequeueofmessagesandconvertthattoaqueueoforders（copy，paste，find，andreplace）.Needtomakesomechangestothequeueimplementation？IntroductiontoTemplatesNotaveryeasytask，sincethecodehasbeenduplicatedinmanyplaces.Reinventingsourcecodesisnotanintelligentapproachinanobject-orientedenvironmentwhichencouragesreusability.IntroductiontoTemplatesItseemstomakemoresensetoimplementaqueuethatcancontainanyarbitrarytyperatherthanduplicatingcode.Howdoesonedothat？Theansweristousetypeparameterization，ormorecommonlyreferredtoastemplates.IntroductiontoTemplatesToavoidrewritingcodethatwouldbeidenticalexceptfordifferenttypes.Sometimesyoudonotsimplyrelyonimplicittypeconversionorpromotionmatch，andyoucannotstuffeverythingintoaclasshierarchy.IntroductiontoTemplatesGenericprogrammingisastyleofprogramminginwhichalgorithmsarewrittenintermsoftypes

to-be-specified-later.Thenthesetypesworkingasparametersareinstantiatedwhenspecifictypesareneeded.Templatesprovidedirectsupportforgenericprogramming，thatis，programmingusingtypesasparameters.TheC++templatemechanismallowsatypetobeaparameterinthedefinitionofaclassorafunction.IntroductiontoTemplatesC++providestwokindsoftemplates：classtemplatesandfunctiontemplates.Usefunctiontemplatestowritegenericfunctionsthatcanbeusedwitharbitrarytypes.Forexample，onecanwritesearehingandsortingroutinesthatcanbeusedwithanyarbitrarytype.ThegenericalgorithmsintheStandardTemplateLibrary（i.e.STL）havebeenimplementedasfunctiontemplates，andthecontainershavebeenimplementedasclasstemplates.IntroductiontoTemplates02FunctionTemplatesLet'simaginethatwewanttowriteafunctiontocomparetwovaluesandindicatewhetherthefirstislessthan，equalto，orgreaterthanthesecondone.Inpractice，wewouldwanttodefineseveralsuchfunctions，eachofwhichcouldcomparevaluesofagiventype.DefinitionofFunctionTemplatesFunctionTemplatesThetwofunctionsarenearlyidentical.Theonlydifferencebetweenthemisthetypeoftheirparameters.Thefunetionbodyisthesameineachfunction.Havingtorepeatthebodyofthefunctionforeachtypethatwecompareistediousanderror-prone.FunctionTemplatesMoreimportantly，we

needtoknowinadvanceallthetypesthatwemighteverwanttocompare.Thisstrategy

cannotworkifwewanttobeabletousethefunctionontypesthatwedon'tknowabout.

Therefore，wethinkthatthesecomparefunctionsneedtobedefinedasagenericfunetion.FunctionTemplatesFunctionTemplateInstantiationWhenafunctiontemplateisfirstcalledforeachtype，thecompilercreatesaninstantiation.Eachinstantiationisaversionofthetemplatedfunctionspecializedforthetype.Thisinstantiationwillbeealledeverytimethefunctionisusedforthetype.FunctionTemplatesWhenthecompilerencountersthiscalltoatemplateinstantiation，itusesthetemplatetoautomaticallygenerateafunctionreplacingeachappearanceofTbythetypepassedastheactualtemplateparameter（intinthiscase）andthencallsit.Thisprocessisautomaticallyperformedbythecompilerandisinvisibletotheprogrammer.FunctionTemplatesFunctionTemplatewithDifferentParameterTypesSincethefunctiontemplateinExample8-1inchidesonlyonetemplateparameter(typenameT)andthefunctiontemplateitselfacceptstwoparameters，bothofthesameTtypes，wecannotcallourfunctiontemplatewithtwoobjectsofdifferenttypesasarguments.FunctionTemplatesFunctionTemplateOverloadingFunctiontemplatesandoverloadingareintimatelyrelated.Onecandeclareseveralfunctiontemplateswiththesamenameandevendeclareacombinationoffunctiontemplatesandordinaryfunctionswiththesamename.Whenanoverloadedfunctioniscalled，overloadingresolutionisnecessarytofindtherightfunctionortemplatefunction

toinvoke.FunctionTemplates03ClassTemplatesJustaswecandefinefunctiontemplates，wecanalsodefineclasstemplates.Forexample，a"stack³isadatastruetureintowhichweinsertitemsatthetopandretrievethoseitemsinlast-in，first-outorder.Itisindependentofthetypeoftheitemsbeingplacedinthestack.Normally，aStackclasscanbedefinedbyaspecifieddatatype.DefinitionofClassTemplatesClassTemplatesIfyouwanttocreatea

wonderfulopportunityforsoftwarereusability，youneedtodefineagenericStaekclassandtoinstantiatetheclassthatisthetype-specificversionsofthegenericStackclass.C++providesthiscapabilitythroughtheclasstemplate.ClassTemplatesAclasstemplateiscalledaparameterizedtype，becauseitrequiresoneormoretypeparameterstospecify

howtocustomizea"genericclass"templatetoformaclass-templatespecialization.类模板称为参数化类型，因为它们需要一个或多个类型参数来指定如何自定义“通用类”模板以形成类模板专用化。ClassTemplatesClassTemplateInstantiationItiseasytouseaclasstemplate.Createtherequiredclassesbypluggingintheactualtypeforthetypeparameters.Thisprocessiscommonlyknownas

"Instantiatingaelass".HereisasampledriverclassthatusesclasstemplateStack.ClassTemplatesTheprocessofgeneratingaclassdeclarationfromatemplateclassandatemplateargumentisoftencalledclasstemplateinstantiation.从模板类和模板参数生成类声明的过程通常称为类模板实例化。ClassTemplatesNow，letusconsidertheuserprogramthatexercisesclasstemplateStack.Althoughtemplatesoffersoftware-reusabilitybenefits，rememberthatmultipleclass-templatespecializationsareinstantiatedinaprogram（atcompiletime），eventhoughthetemplateiswrittenonlyonce.ClassTemplatesTheprogrambeginsbyinstantiatinganobjeetdoubleStaekofsize5inLine5.ThisobjectisdeclaredtobeofclassStack<double>（pronounced

"Stackofdouble"）.ThecompilerassociatestypedoublewithtypeparameterTintheclasstemplatetoproducethesourcecode

foraStackclassoftypedouble.ClassTemplatesClassStackprovidestheisFullfunction，whichtheprogrammercanusetodeterminewhetherthestackis

fullbeforeattemptingapushoperation.Thiswouldavoidthepotentialerrorofpushingontoa

fullstack.ClassTemplates04Non-TypeParametersforTemplatesBesidesthetemplateargumentsthatareprecededbythetypenamekeywords，which

representtypes，templatescanalsohaveregulartypedparameters，likethosefoundinfunctions.Asanexample，takealookatthisclasstemplatethatisusedtocontainsequencesofelements.Non-TypeParametersforTemplatesInthisexample，integerNisanon-typetemplateparameter.Whentheobjectsofthe

templatearecreatedinLines24and25，aninteger5isdirectlypassedintoN.Itisalsopossibletosetdefaultvaluesortypesforclasstemplateparameters.Non-TypeParametersforTemplates05AbstractBaseClassesClasstemplatescaninheritorbeinheritedfrom.Formanypurposes，thereisnothing

significantlydifferentbetweenthetemplateandnon-templatescenarios.DerivationandClassTemplatesHowever，thereisoneimportantsubtletywhenderivingaclasstemplatefromabaseclassreferredtobyadependentname.Let'sfirstlookatthesomewhatsimplercaseofnondependentbaseclasses.DerivationandClassTemplatesInaclasstemplate，anondependentbaseclassisonewithacompletetypethatcanbe

determinedwithoutknowingthetemplatearguments.Inotherwords，thenameofthisbaseis

denotedbyusinganondependentname.DerivationandClassTemplatesNondependentbasesintemplatesbehaveverymuchlikebasesinordinarynontemplate

classes，butthereisaslightlyunfortunatesurprise：Whenanunqualifiednameislookedupinthetemplatedderivation，thenondependentbasesareconsideredbeforetheIistoftemplateparameters.DerivationandClassTemplatesThismeansthatinthepreviousexample，thestrangememberoftheD2classtemplatealwayshastheTtypecorrespondingtoBase<double>∴T（inotherwords，int）.DerivationandClassTemplatesThisiscounterintuitiveandrequiresthewriterofthederivedtemplatetobeawareofnamesinthenondependentbasesfromwhichitderivesevenwhenthatderivationisindirectorthenamesareprivate.Itwouldprobablyhavebeenpreferabletoplacetemplateparametersinthescopeoftheentitythey“templatize".DerivationandClassTemplates