面向对象程序设计Chapter08.ppt

1 Chapter8Scope Lifetime andMoreonFunctions 2 Chapter8Topics LocalScopeandGlobalScopeofanIdentifierDeterminewhichVariableinaprogramarelocalDeterminingtheLifetimeofaVariableWritingaValue ReturningFunctionforaTask 3 8 1ScopeofIdentifier Scope theregionofprogramcodewhereitislegaltousethatidentifier ClassscopeLocalscope blockscope Globalscope filescope 4 8 1ScopeofIdentifier Scope theregionofprogramcodewhereitislegaltousethatidentifier includevoidmain intx 256 cout x x endl AlocalvariabledeclaredinablockhasaLocalscope 5 if alpha 3 intn cin n beta beta n Thescopeoflocalvariablen Exampleofalocalvariabledeclaredinablock 6 8 1 1LocalScopeandGlobalScope foranidentifierthatisdeclaredinsideablock thisincludesfunctionparameters foranidentifierthatisdeclaredoutsideofallnamespaces functionsandclasses extendsfromthepointofdeclarationtotheendoftheblock extendsfrompointofdeclarationtotheendoftheentirefile LocalScope GlobalScope 7 constfloatTAX RATE 0 05 globalconstantfloattipRate globalvariablevoidhandle int float functionprototypeintmain intage ageandbilllocaltothisblockfloatbill handle age bill return0 voidhandle inta floatb floattax a b andtaxlocaltothisblock 8 8 1 2NamePrecedence orNameHiding whenafunctiondeclaresalocalidentifierwiththesamenameasaglobalidentifier thelocalidentifiertakesprecedencewithinthatfunction 9 includeusingnamespacestd voidsomefunc float constinta 17 intb c intmain b 8 c 6 somefunc 42 8 return0 voidsomefunc floatc floatb b c cout b 10 NamePrecedence Whenanexpressionreferstoanidentifier 1 firstchecksthelocaldeclarations2 ifitisn tlocal compilerworksoutwardthrougheachlevelofnestinguntilitfindsanidentifierwithsamename 3 ifcompilerreachesglobaldeclarationsandstillcan tfindtheidentifier anerrormessageresults 11 NamePrecedenceImplementedbyCompiler includeusingnamespacestd voidSomeFunc float constinta 17 intb c intmain b 8 c 6 SomeFunc 42 8 return0 voidSomeFunc floatc floatb b c 12 inthefunctionbodyaccessestheglobevariable includeintx voidmain intx 256 cout globalvariablex x endl cout localvariablex x endl 13 globevariable inthefunctionbodyaccessestheglobevariable includeintx voidmain intx 256 cout globalvariablex x endl cout localvariablex x endl 14 localvariable inthefunctionbodyaccessestheglobevariable includeintx voidmain intx 256 cout globalvariablex x endl cout localvariablex x endl 15 inthefunctionbodyaccessestheglobevariable includeintx voidmain intx 256 cout globalvariablex x endl cout localvariablex x endl 16 8 1 2ScopeRules Functionnamehasglobalscope Functionparameterscopeisidenticaltoscopeofalocalvariabledeclaredintheoutermostblockofthefunctionbody Globalvariable orconstant scopeextendsfromdeclarationtotheendofthefile exceptasnotedinrule5 17 Localvariable orconstant scopeextendsfromdeclarationtotheendoftheblockwheredeclared Thisscopeincludesanynestedblocks exceptasnotedinrule5 5 Anidentifier sscopedoesnotincludeanynestedblockthatcontainsalocallydeclaredidentifierwiththesamename localidentifiershavenameprecedence 8 1 2ScopeRules 18 includeinti voidprint voidmain for i 0 i 5 i print voidprint for i 0 i 5 i cout cout n 19 includeinti voidprint voidmain for i 0 i 5 i print voidprint for i 0 i 5 i cout cout n Output 20 8 1 3Variabledeclarationanddefinition intSquare intn intresult result n n returnresult intSquare intn Memoryallocated内存分配 Nomemoryreserved无内存保留 intsomeInt externintsomeInt 21 Externaldeclaration Itstatesaglobalvariablelocatedinanotherfile 它声明了一个在其它文件中定义的变量 Nomemoryisreservedforanexternaldeclaration 对外部变量不分配存储空间 Thepurposeofanexternaldeclarationisthatthecompilercandotypechecking 声明外部变量的目的是让编译软件作类型检查 22 8 1 4Namespace AmechanismthattheprogrammercancreateanamedscopeThescopeofanidentifierdeclaredinanamespaceextendsfromthepointofdeclarationtotheendofthenamespacebodyAnditsscopeincludesthescopeofausingdirectivespecifyingthatnamespace 23 8 1 4Namespace namespacestd intabs int Anamespacedefinition namespaceidentifier namespace body 24 3WaystoUseNamespaceIdentifiers useaqualifiednameitconsistingofthenamespace thescoperesolutionoperator andthedesiredtheidentifieralpha std abs beta writeausingdeclarationusingstd abs alpha abs beta writeausingdirectivelocallyorgloballyusingnamespacestd alpha abs beta 25 Scopecategories作用域类型 Classscope localscope globalscope Namespacescope 26 8 2LifetimeofVariables变量的生存期 Theperiodoftimeduringprogramexecutionwhenanidentifierhasmemoryallocatedtoit 变量的生存期是指在程序运行过程中 变量实际占用内存或寄存器的时间 27 8 2 1LifetimeofLocalVariablesinFunctions theirstorageiscreated allocated whencontrolentersthefunction 当执行函数的时候 为函数中声明的局部变量分配存储空间 localvariablesare alive whilefunctionisexecutingtheirstorageisdestroyed deallocated whenfunctionexits 28 8 2 2LifetimeofGlobalVariables theirlifetimeisthelifetimeoftheentireprogramtheirmemoryisallocatedwhenprogrambeginsexecutiontheirmemoryisdestroyedwhentheentireprogramterminates 29 8 2 3AutomaticVariableandStaticVariable storageforautomaticvariableisallocatedatblockentryanddestroyedatblockexit storageforstaticvariableremainsallocatedthroughoutexecutionoftheentireprogram AutomaticVariable StaticVariable 30 Bydefault localvariablesareautomatictoobtainastaticlocalvariable youmustusethereservedwordstaticinitsdeclaration 31 8 2 4Initializationindeclaration声明时初始化 Forautomaticvariables initializationoccurseachtimecontrolenterstheblockForstaticvariables initializationonlyoccursonce intsum sum 0 intsum 0 32 StaticandAutomaticLocalVariables intpopularSquare intn staticinttimesCalled 0 initializedonlyonceintresult n n initializedeachtimetimesCalled timesCalled 1 cout Call timesCalled endl returnresult 33 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 35 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 36 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 37 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 38 1 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 39 2 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 40 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 41 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 42 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 43 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 44 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 45 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 46 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 47 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 48 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 49 1 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 50 autoa 1staticb 23autoa 1staticb 34 3 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 51 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 52 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 53 例4 12静态变量与自动变量的测试 includeintfunc voidmain cout func endl cout func endl intfunc inta 0 自动变量staticintb 1 静态变量a b cout autoa a endl cout staticb b endl returna b autoa 1staticb 23autoa 1staticb 34 54 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 55 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 56 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 57 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 58 autoa 1staticb 23autoa 1staticb 34 exampleStaticandAutomaticLocalVariables includeintfunc voidmain cout func endl cout func endl intfunc inta 0 staticintb 1 a b cout autoa a endl cout staticb b endl returna b 59 8 3Interfacedesign DonotletfunctionscommunicatebydirectlychangeglobalvariablesSideeffect anyeffectofonefunctiononanotherthatisnotapartoftheexplicitlydefinedinterfacebetweenthemTheonlyexternaleffectthatafunctionshouldhaveistotransferinformationthroughthewell structuredinterfaceoftheparameterlist 60 8 3 1SideEffect NonlocalvariablesareaccessedonlythroughtheparameterlistIncoming onlyparametersaredefinedasvalueparameters 61 8 3 2GlobalConstants Therearetwoadvantages Easeofchange 易于修改 Consistency 能保持数据的一致性 62 8 4Value returningFunctions includeintSquare int prototypesintCube int usingnamespacestd intmain cout Thesquareof27is Square 27 endl functioncallcout Thecubeof27is Cube 27 endl functioncallreturn0 63 RestofProgram intSquare intn returnn n intCube intn returnn n n 64 Featuresofvalue returningfunction HeadingbeginswiththedatatypeThereturnstatementhasareturnvalue Functionvaluetype thedatatypeoftheresultvaluereturnedbyafunction 65 SyntaxTemplateforFunctionDefinition DataTypeFunctionName ParameterList Statement Ifdatatypeisomitted intisthedefaulttype 66 8 4 1ABooleanFunction boolIsTriangle in floatangle1 in floatangle2 in floatangle3 Functionchecksif3incomingvaluesaddupto180degrees formingavalidtriangle PRECONDITION angle1 angle2 angle3areassigned POSTCONDITION FCTNVAL true ifsumiswithin0 000001of 180 0degrees false otherwise return fabs angle1 angle2 angle3 180 0 0 000001 67 SomePrototypesinHeaderFile intisalpha charc