外文翻译-委托

毕业设计（论文）外文文献原文及译文学院计算机科学与工程学院专业网络工程原文DELEGATEDELEGATESENABLESCENARIOSTHATOTHERLANGUAGESSUCHASC,PASCAL,ANDMODULAHAVEADDRESSEDWITHFUNCTIONPOINTERSUNLIKECFUNCTIONPOINTERS,HOWEVER,DELEGATESAREFULLYOBJECTORIENTED,ANDUNLIKECPOINTERSTOMEMBERFUNCTIONS,DELEGATESENCAPSULATEBOTHANOBJECTINSTANCEANDAMETHODADELEGATEDECLARATIONDEFINESACLASSTHATISDERIVEDFROMTHECLASSSYSTEMDELEGATEADELEGATEINSTANCEENCAPSULATESANINVOCATIONLIST,WHICHISALISTOFONEORMOREMETHODS,EACHOFWHICHISREFERREDTOASACALLABLEENTITYFORINSTANCEMETHODS,ACALLABLEENTITYCONSISTSOFANINSTANCEANDAMETHODONTHATINSTANCEFORSTATICMETHODS,ACALLABLEENTITYCONSISTSOFJUSTAMETHODINVOKINGADELEGATEINSTANCEWITHANAPPROPRIATESETOFARGUMENTSCAUSESEACHOFTHEDELEGATESCALLABLEENTITIESTOBEINVOKEDWITHTHEGIVENSETOFARGUMENTSANINTERESTINGANDUSEFULPROPERTYOFADELEGATEINSTANCEISTHATITDOESNOTKNOWORCAREABOUTTHECLASSESOFTHEMETHODSITENCAPSULATESALLTHATMATTERSISTHATTHOSEMETHODSBECOMPATIBLEWITHTHEDELEGATESTYPETHISMAKESDELEGATESPERFECTLYSUITEDFOR“ANONYMOUS”INVOCATION11DELEGATEDECLARATIONSADELEGATEDECLARATIONISATYPEDECLARATIONTHATDECLARESANEWDELEGATETYPEDELEGATEDECLARATIONATTRIBUTESOPTDELEGATEMODIFIERSOPTDELEGATERETURNTYPEIDENTIFIERVARIANTTYPEPARAMETERLISTOPTFORMALPARAMETERLISTOPTTYPEPARAMETERCONSTRAINTSCLAUSESOPTDELEGATEMODIFIERSDELEGATEMODIFIERDELEGATEMODIFIERSDELEGATEMODIFIERDELEGATEMODIFIERNEWPUBLICPROTECTEDINTERNALPRIVATEITISACOMPILETIMEERRORFORTHESAMEMODIFIERTOAPPEARMULTIPLETIMESINADELEGATEDECLARATIONTHENEWMODIFIERISONLYPERMITTEDONDELEGATESDECLAREDWITHINANOTHERTYPE,INWHICHCASEITSPECIFIESTHATSUCHADELEGATEHIDESANINHERITEDMEMBERBYTHESAMENAMETHEPUBLIC,PROTECTED,INTERNAL,ANDPRIVATEMODIFIERSCONTROLTHEACCESSIBILITYOFTHEDELEGATETYPEDEPENDINGONTHECONTEXTINWHICHTHEDELEGATEDECLARATIONOCCURS,SOMEOFTHESEMODIFIERSMAYNOTBEPERMITTEDTHEDELEGATESTYPENAMEISIDENTIFIERTHEOPTIONALFORMALPARAMETERLISTSPECIFIESTHEPARAMETERSOFTHEDELEGATE,ANDRETURNTYPEINDICATESTHERETURNTYPEOFTHEDELEGATETHEOPTIONALVARIANTTYPEPARAMETERLISTSPECIFIESTHETYPEPARAMETERSTOTHEDELEGATEITSELFTHERETURNTYPEOFADELEGATETYPEMUSTBEEITHERVOID,OROUTPUTSAFEALLTHEFORMALPARAMETERTYPESOFADELEGATETYPEMUSTBEINPUTSAFEADDITIONALLY,ANYOUTORREFPARAMETERTYPESMUSTALSOBEOUTPUTSAFENOTETHATEVENOUTPARAMETERSAREREQUIREDTOBEINPUTSAFE,DUETOALIMITIATIONOFTHEUNDERLYINGEXECUTIONPLATFORMDELEGATETYPESINCARENAMEEQUIVALENT,NOTSTRUCTURALLYEQUIVALENTSPECIFICALLY,TWODIFFERENTDELEGATETYPESTHATHAVETHESAMEPARAMETERLISTSANDRETURNTYPEARECONSIDEREDDIFFERENTDELEGATETYPESHOWEVER,INSTANCESOFTWODISTINCTBUTSTRUCTURALLYEQUIVALENTDELEGATETYPESMAYCOMPAREASEQUALFOREXAMPLEDELEGATEINTD1INTI,DOUBLEDCLASSAPUBLICSTATICINTM1INTA,DOUBLEBCLASSBDELEGATEINTD2INTC,DOUBLEDPUBLICSTATICINTM1INTF,DOUBLEGPUBLICSTATICVOIDM2INTK,DOUBLELPUBLICSTATICINTM3INTGPUBLICSTATICVOIDM4INTGTHEDELEGATETYPESD1ANDD2AREBOTHCOMPATIBLEWITHTHEMETHODSAM1ANDBM1,SINCETHEYHAVETHESAMERETURNTYPEANDPARAMETERLISTHOWEVER,THESEDELEGATETYPESARETWODIFFERENTTYPES,SOTHEYARENOTINTERCHANGEABLETHEDELEGATETYPESD1ANDD2AREINCOMPATIBLEWITHTHEMETHODSBM2,BM3,ANDBM4,SINCETHEYHAVEDIFFERENTRETURNTYPESORPARAMETERLISTSLIKEOTHERGENERICTYPEDECLARATIONS,TYPEARGUMENTSMUSTBEGIVENTOCREATEACONSTRUCTEDDELEGATETYPETHEPARAMETERTYPESANDRETURNTYPEOFACONSTRUCTEDDELEGATETYPEARECREATEDBYSUBSTITUTING,FOREACHTYPEPARAMETERINTHEDELEGATEDECLARATION,THECORRESPONDINGTYPEARGUMENTOFTHECONSTRUCTEDDELEGATETYPETHERESULTINGRETURNTYPEANDPARAMETERTYPESAREUSEDINDETERMININGWHATMETHODSARECOMPATIBLEWITHACONSTRUCTEDDELEGATETYPEFOREXAMPLEDELEGATEBOOLPREDICATETVALUECLASSXSTATICBOOLFINTISTATICBOOLGSTRINGSTHEDELEGATETYPEPREDICATEISCOMPATIBLEWITHTHEMETHODXFANDTHEDELEGATETYPEPREDICATEISCOMPATIBLEWITHTHEMETHODXGTHEONLYWAYTODECLAREADELEGATETYPEISVIAADELEGATEDECLARATIONADELEGATETYPEISACLASSTYPETHATISDERIVEDFROMSYSTEMDELEGATEDELEGATETYPESAREIMPLICITLYSEALED,SOITISNOTPERMISSIBLETODERIVEANYTYPEFROMADELEGATETYPEITISALSONOTPERMISSIBLETODERIVEANONDELEGATECLASSTYPEFROMSYSTEMDELEGATENOTETHATSYSTEMDELEGATEISNOTITSELFADELEGATETYPEITISACLASSTYPEFROMWHICHALLDELEGATETYPESAREDERIVEDCPROVIDESSPECIALSYNTAXFORDELEGATEINSTANTIATIONANDINVOCATIONEXCEPTFORINSTANTIATION,ANYOPERATIONTHATCANBEAPPLIEDTOACLASSORCLASSINSTANCECANALSOBEAPPLIEDTOADELEGATECLASSORINSTANCE,RESPECTIVELYINPARTICULAR,ITISPOSSIBLETOACCESSMEMBERSOFTHESYSTEMDELEGATETYPEVIATHEUSUALMEMBERACCESSSYNTAXTHESETOFMETHODSENCAPSULATEDBYADELEGATEINSTANCEISCALLEDANINVOCATIONLISTWHENADELEGATEINSTANCEISCREATEDFROMASINGLEMETHOD,ITENCAPSULATESTHATMETHOD,ANDITSINVOCATIONLISTCONTAINSONLYONEENTRYHOWEVER,WHENTWONONNULLDELEGATEINSTANCESARECOMBINED,THEIRINVOCATIONLISTSARECONCATENATEDINTHEORDERLEFTOPERANDTHENRIGHTOPERANDTOFORMANEWINVOCATIONLIST,WHICHCONTAINSTWOORMOREENTRIESDELEGATESARECOMBINEDUSINGTHEBINARYANDOPERATORSADELEGATECANBEREMOVEDFROMACOMBINATIONOFDELEGATES,USINGTHEBINARYANDOPERATORSDELEGATESCANBECOMPAREDFOREQUALITYTHEFOLLOWINGEXAMPLESHOWSTHEINSTANTIATIONOFANUMBEROFDELEGATES,ANDTHEIRCORRESPONDINGINVOCATIONLISTSDELEGATEVOIDDINTXCLASSCPUBLICSTATICVOIDM1INTIPUBLICSTATICVOIDM2INTICLASSTESTSTATICVOIDMAINDCD1NEWDCM1/M1DCD2NEWDCM2/M2DCD3CD1CD2/M1M2DCD4CD3CD1/M1M2M1DCD5CD4CD3/M1M2M1M1M2WHENCD1ANDCD2AREINSTANTIATED,THEYEACHENCAPSULATEONEMETHODWHENCD3ISINSTANTIATED,ITHASANINVOCATIONLISTOFTWOMETHODS,M1ANDM2,INTHATORDERCD4SINVOCATIONLISTCONTAINSM1,M2,ANDM1,INTHATORDERFINALLY,CD5SINVOCATIONLISTCONTAINSM1,M2,M1,M1,ANDM2,INTHATORDERFORMOREEXAMPLESOFCOMBININGASWELLASREMOVINGDELEGATES12DELEGATECOMPATIBILITYAMETHODORDELEGATEMISCOMPATIBLEWITHADELEGATETYPEDIFALLOFTHEFOLLOWINGARETRUEDANDMHAVETHESAMENUMBEROFPARAMETERS,ANDEACHPARAMETERINDHASTHESAMEREFOROUTMODIFIERSASTHECORRESPONDINGPARAMETERINMFOREACHVALUEPARAMETERAPARAMETERWITHNOREFOROUTMODIFIER,ANIDENTITYCONVERSIONORIMPLICITREFERENCECONVERSIONEXISTSFROMTHEPARAMETERTYPEINDTOTHECORRESPONDINGPARAMETERTYPEINMFOREACHREFOROUTPARAMETER,THEPARAMETERTYPEINDISTHESAMEASTHEPARAMETERTYPEINMANIDENTITYORIMPLICITREFERENCECONVERSIONEXISTSFROMTHERETURNTYPEOFMTOTHERETURNTYPEOFD13DELEGATEINSTANTIATIONANINSTANCEOFADELEGATEISCREATEDBYADELEGATECREATIONEXPRESSIONORACONVERSIONTOADELEGATETYPETHENEWLYCREATEDDELEGATEINSTANCETHENREFERSTOEITHERTHESTATICMETHODREFERENCEDINTHEDELEGATECREATIONEXPRESSION,ORTHETARGETOBJECTWHICHCANNOTBENULLANDINSTANCEMETHODREFERENCEDINTHEDELEGATECREATIONEXPRESSION,ORANOTHERDELEGATEFOREXAMPLEDELEGATEVOIDDINTXCLASSCPUBLICSTATICVOIDM1INTIPUBLICVOIDM2INTICLASSTESTSTATICVOIDMAINDCD1NEWDCM1/STATICMETHODCTNEWCDCD2NEWDTM2/INSTANCEMETHODDCD3NEWDCD2/ANOTHERDELEGATEONCEINSTANTIATED,DELEGATEINSTANCESALWAYSREFERTOTHESAMETARGETOBJECTANDMETHODREMEMBER,WHENTWODELEGATESARECOMBINED,ORONEISREMOVEDFROMANOTHER,ANEWDELEGATERESULTSWITHITSOWNINVOCATIONLISTTHEINVOCATIONLISTSOFTHEDELEGATESCOMBINEDORREMOVEDREMAINUNCHANGED14DELEGATEINVOCATIONCPROVIDESSPECIALSYNTAXFORINVOKINGADELEGATEWHENANONNULLDELEGATEINSTANCEWHOSEINVOCATIONLISTCONTAINSONEENTRYISINVOKED,ITINVOKESTHEONEMETHODWITHTHESAMEARGUMENTSITWASGIVEN,ANDRETURNSTHESAMEVALUEASTHEREFERREDTOMETHODIFANEXCEPTIONOCCURSDURINGTHEINVOCATIONOFSUCHADELEGATE,ANDTHATEXCEPTIONISNOTCAUGHTWITHINTHEMETHODTHATWASINVOKED,THESEARCHFORANEXCEPTIONCATCHCLAUSECONTINUESINTHEMETHODTHATCALLEDTHEDELEGATE,ASIFTHATMETHODHADDIRECTLYCALLEDTHEMETHODTOWHICHTHATDELEGATEREFERREDINVOCATIONOFADELEGATEINSTANCEWHOSEINVOCATIONLISTCONTAINSMULTIPLEENTRIESPROCEEDSBYINVOKINGEACHOFTHEMETHODSINTHEINVOCATIONLIST,SYNCHRONOUSLY,INORDEREACHMETHODSOCALLEDISPASSEDTHESAMESETOFARGUMENTSASWASGIVENTOTHEDELEGATEINSTANCEIFSUCHADELEGATEINVOCATIONINCLUDESREFERENCEPARAMETERS,EACHMETHODINVOCATIONWILLOCCURWITHAREFERENCETOTHESAMEVARIABLECHANGESTOTHATVARIABLEBYONEMETHODINTHEINVOCATIONLISTWILLBEVISIBLETOMETHODSFURTHERDOWNTHEINVOCATIONLISTIFTHEDELEGATEINVOCATIONINCLUDESOUTPUTPARAMETERSORARETURNVALUE,THEIRFINALVALUEWILLCOMEFROMTHEINVOCATIONOFTHELASTDELEGATEINTHELISTIFANEXCEPTIONOCCURSDURINGPROCESSINGOFTHEINVOCATIONOFSUCHADELEGATE,ANDTHATEXCEPTIONISNOTCAUGHTWITHINTHEMETHODTHATWASINVOKED,THESEARCHFORANEXCEPTIONCATCHCLAUSECONTINUESINTHEMETHODTHATCALLEDTHEDELEGATE,ANDANYMETHODSFURTHERDOWNTHEINVOCATIONLISTARENOTINVOKEDATTEMPTINGTOINVOKEADELEGATEINSTANCEWHOSEVALUEISNULLRESULTSINANEXCEPTIONOFTYPESYSTEMNULLREFERENCEEXCEPTIONTHEFOLLOWINGEXAMPLESHOWSHOWTOINSTANTIATE,COMBINE,REMOVE,ANDINVOKEDELEGATESUSINGSYSTEMDELEGATEVOIDDINTXCLASSCPUBLICSTATICVOIDM1INTICONSOLEWRITELINE“CM1“IPUBLICSTATICVOIDM2INTICONSOLEWRITELINE“CM2“IPUBLICVOIDM3INTICONSOLEWRITELINE“CM3“ICLASSTESTSTATICVOIDMAINDCD1NEWDCM1CD11/CALLM1DCD2NEWDCM2CD22/CALLM2DCD3CD1CD2CD310/CALLM1THENM2CD3CD1CD320/CALLM1,M2,THENM1CCNEWCDCD4NEWDCM3CD3CD4CD330/CALLM1,M2,M1,THENM3CD3CD1/REMOVELASTM1CD340/CALLM1,M2,THENM3CD3CD4CD350/CALLM1THENM2CD3CD2CD360/CALLM1CD3CD2/IMPOSSIBLEREMOVALISBENIGNCD360/CALLM1CD3CD1/INVOCATIONLISTISEMPTYSOCD3ISNULL/CD370/SYSTEMNULLREFERENCEEXCEPTIONTHROWNCD3CD1/IMPOSSIBLEREMOVALISBENIGNASSHOWNINTHESTATEMENTCD3CD1,ADELEGATECANBEPRESENTINANINVOCATIONLISTMULTIPLETIMESINTHISCASE,ITISSIMPLYINVOKEDONCEPEROCCURRENCEINANINVOCATIONLISTSUCHASTHIS,WHENTHATDELEGATEISREMOVED,THELASTOCCURRENCEINTHEINVOCATIONLISTISTHEONEACTUALLYREMOVEDIMMEDIATELYPRIORTOTHEEXECUTIONOFTHEFINALSTATEMENT,CD3CD1,THEDELEGATECD3REFERSTOANEMPTYINVOCATIONLISTATTEMPTINGTOREMOVEADELEGATEFROMANEMPTYLISTORTOREMOVEANONEXISTENTDELEGATEFROMANONEMPTYLISTISNOTANERRORTHEOUTPUTPRODUCEDISCM11CM22CM110CM210CM120CM220CM120CM130CM230CM130CM330CM140CM240CM340CM150CM250CM160CM160译文委托委托是用来处理其他语言（如C、PASCAL和MODULA）需用函数指针来处理的情况的。不过与C函数指针不同，委托是完全面向对象的；另外，C指针仅指向成员函数，而委托同时封装了对象实例和方法。委托声明定义一个从SYSTEMDELEGATE类派生的类。委托实例封装了一个调用列表，该列表列出了一个或多个方法，每个方法称为一个可调用实体。对于实例方法，可调用实体由该方法和一个相关联的实例组成。对于静态方法，可调用实体仅由一个方法组成。用一个适当的参数集来调用一个委托实例，就是用此给定的参数集来调用该委托实例的每个可调用实体。委托实例的一个有趣且有用的属性是它不知道也不关心它所封装的方法所属的类；它所关心的仅限于这些方法必须与委托的类型兼容。这使委托非常适合于“匿名”调用。委托声明DELEGATEDECLARATION是一种TYPEDECLARATION，它声明一个新的委托类型。DELEGATEDECLARATIONATTRIBUTESOPTDELEGATEMODIFIERSOPTDELEGATERETURNTYPEIDENTIFIERVARIANTTYPEPARAMETERLISTOPTFORMALPARAMETERLISTOPTTYPEPARAMETERCONSTRAINTSCLAUSESOPTDELEGATEMODIFIERSDELEGATEMODIFIERDELEGATEMODIFIERSDELEGATEMODIFIERDELEGATEMODIFIERNEWPUBLICPROTECTEDINTERNALPRIVATE同一修饰符在一个委托声明中多次出现属于编译时错误。NEW修饰符仅允许在其他类型中声明的委托上使用，在这种情况下该修饰符表示所声明的委托会隐藏具有相同名称的继承成员。PUBLIC、PROTECTED、INTERNAL和PRIVATE修饰符控制委托类型的可访问性。根据委托声明所在的上下文，可能不允许使用其中某些修饰符。上述的语法产生式中，IDENTIFIER用于指定委托的类型名称。可选的FORMALPARAMETERLIST用于指定委托的参数，而RETURNTYPE则指定委托的返回类型。可选的VARIANTTYPEPARAMETERLIST指定委托本身的类型形参。委托类型的返回类型必须为VOID或输出安全。委托类型的所有形参类型都必须是输入安全的。此外，所有OUT或REF形参类型也必须是输出安全的。请注意，由于基础执行平台的限制，甚至OUT形参也必须是输入安全的。C中的委托类型是名称等效的，而不是结构等效的。具体地说，对于两个委托类型，即使它们具有相同的参数列表和返回类型，仍被认为是不同的两个委托类型。但是，两个不同但结构上等效的委托类型的实例可能比较为相等。例如DELEGATEINTD1INTI,DOUBLEDCLASSAPUBLICSTATICINTM1INTA,DOUBLEBCLASSBDELEGATEINTD2INTC,DOUBLEDPUBLICSTATICINTM1INTF,DOUBLEGPUBLICSTATICVOIDM2INTK,DOUBLELPUBLICSTATICINTM3INTGPUBLICSTATICVOIDM4INTG委托类型D1和D2都与方法AM1和BM1兼容，这是因为它们具有相同的返回类型和参数列表；但是，这些委托类型是两个不同的类型，所以它们是不可互换的。委托类型D1和D2与方法BM2、BM3和BM4不兼容，这是因为它们具有不同的返回类型或参数列表。与其他泛型类型声明一样，必须提供类型实参才能创建构造委托类型。构造委托类型的形参类型和返回类型是通过将委托声明中的每个类型形参替换为构造委托类型的对应类型实参来创建的。结果返回类型和形参类型用于确定哪些方法与构造委托类型兼容。例如DELEGATEBOOLPREDICATETVALUECLASSXSTATICBOOLFINTISTATICBOOLGSTRINGS委托类型PREDICATE与方法XF兼容，委托类型PREDICATE与方法XG兼容。声明一个委托类型的唯一方法是通过DELEGATEDECLARATION。委托类型是从SYSTEMDELEGATE派生的类类型。委托类型隐含为SEALED，所以不允许从一个委托类型派生任何类型。也不允许从SYSTEMDELEGATE派生非委托类类型。请注意SYSTEMDELEGATE本身不是委托类型；它是从中派生所有委托类型的类类型。C提供了专门的语法用于委托类型的实例化和调用。除实例化外，所有可以应用于类或类实例的操作也可以相应地应用于委托类或委托实例。具体而言，可以通过通常的成员访问语法访问SYSTEMDELEGATE类型的成员。委托实例所封装的方法集合称为调用列表。从某个方法创建一个委托实例时，该委托实例将封装此方法，此时，它的调用列表只包含一个“入口点”。但是，当组合两个非空委托实例时，它们的调用列表将连接在一起（按照左操作数在前、右操作数在后的顺序）以组成一个新的调用列表，其中包含两个或更多个“入口点”。委托是使用二元和运算符进行组合的。可以使用二元和运算符将一个委托从委托组合中移除。委托间还可以进行比较以确定它们是否相等。下面的示例演示多个委托的实例化及其相应的调用列表DELEGATEVOIDDINTXCLASSCPUBLICSTATICVOIDM1INTIPUBLICSTATICVOIDM2INTICLASSTESTSTATICVOIDMAINDCD1NEWDCM1/M1DCD2NEWDCM2/M2DCD3CD1CD2/M1M2DCD4CD3CD1/M1M2M1DCD5CD4CD3/M1M2M1M1M2实例化CD1和CD2时，它们分别封装一个方法。实例化CD3时，它的调用列表有两个方法M1和M2，而且顺序与此相同。CD4的调用列表中依次包含M1、M2和M1。最后，CD5的调用列表中依次包含M1、M2、M1、M1和M2。有关组合以及移除委托的更多示例。11委托兼容性如果以下所有条件都成立，则方法或委托M与委托类型D兼容COMPATIBLED和M具有相同数量的形参，并且D中的每个形参都具有与M中对应形参相同的REF或OUT修饰符。对于每个值形参（没有REF或OUT修饰符的形参），存在从D中形参类型到M中对应形参类型的标识转换或隐式引用转换。对于每个REF或OUT参数，D中的参数类型与M中的参数类型相同。存在从M的返回类型到D的返回类型的标识或隐式引用转换。12委托实例化委托的实例通过DELEGATECREATIONEXPRESSION或到委托类型的转换进行创建。因此，新创建的委托实例将引用以下各项之一DELEGATECREATIONEXPRESSION中引用的静态方法，或者DELEGATECREATIONEXPRESSION中引用的目标对象（此对象不能为NULL）和实例方法，或者另一个委托。例如DELEGATEVOIDDINTXCLASSCPUBLICSTATICVOIDM1INTIPUBLICVOIDM2INTICLASSTESTSTATICVOIDMAINDCD1NEWDCM1/STATICMETHODCTNEWCDCD2NEWDTM2/INSTANCEMETHODDCD3NEWDCD2/ANOTHERDELEGATE委托实例一旦被实例化，它将始终引用同一目标对象和方法。记住，当组合两个委托或者从一个委托移除另一个时，将产生一个新的委托，该委托具有它自己的调用列表；被组合或移除的委托的调用列表将保持不变。13委托调用C为调用委托提供了专门的语法。当调用非空的、调用列表仅包含一个入