0321537904_ppt73679-0321537904_ppt幻灯片Savitch_ch_17

Copyright2008PearsonAddison-Wesley.Allrightsreserved.,Chapter17,Templates,Slide17-3,Overview,17.1TemplatesforAlgorithmAbstraction17.2TemplatesforDataAbstraction,Copyright2008PearsonAddison-Wesley.Allrightsreserved.,17.1,TemplatesforAlgorithmAbstraction,Slide17-5,TemplatesforAlgorithmAbstraction,FunctiondefinitionsoftenuseapplicationspecificadaptationsofmoregeneralalgorithmsForexample:Thegeneralalgorithmusedinswap_valuescouldswapvariablesofanytype:voidswap_values(type_of_var,Slide17-6,swap_valuesforchar,Hereisaversionofswap_valuestoswapcharactervariables:voidswap_values(char,Slide17-7,AGeneralswap_values,Ageneralizedversionofswap_valuesisshownhere.voidswap_values(type_of_varThisfunction,iftype_of_varcouldacceptanytype,couldbeusedtoswapvaluesofanytype,Slide17-8,AC+functiontemplatewillallowswap_valuestoswapvaluesoftwovariablesofthesametypeExample:templatevoidswap_values(T,Templateprefix,TemplatesforFunctions,Slide17-9,TemplateDetails,templateisthetemplateprefixTellscompilerthatthedeclarationordefinitionthatfollowsisatemplateTellscompilerthatTisatypeparameterclassmeanstypeinthiscontext(typenamecouldreplaceclassbutclassisusuallyused)Tcanbereplacedbyanytypeargument(whetherthetypeisaclassornot)AtemplateoverloadsthefunctionnamebyreplacingTwiththetypeusedinafunctioncall,Slide17-10,CallingaTemplateFunction,CallingafunctiondefinedwithatemplateisidenticaltocallinganormalfunctionExample:Tocallthetemplateversionofswap_valueschars1,s2;inti1,i2;swap_values(s1,s2);swap_values(i1,i2);Thecompilercheckstheargumenttypesandgeneratesanappropriateversionofswap_values,Slide17-11,TemplatesandDeclarations,AfunctiontemplatemayalsohaveaseparatedeclarationThetemplateprefixandtypeparameterareusedDependingonyourcompilerYoumay,ormaynot,beabletoseparatedeclarationanddefinitionsoftemplatefunctionsjustasyoudowithregularfunctionsTobesafe,placetemplatefunctiondefinitionsinthesamefilewheretheyareusedwithnodeclarationAfileincludedwith#includeis,inmostcases,equivalenttobeinginthesamefile“Thismeansincludingthe.cppfileor.hfilewithimplementationcode,Slide17-12,TisthetraditionalnameforthetypeparameterAnyvalid,non-keyword,identifiercanbeusedVariableTypecouldbeusedtemplatevoidswap_values(VariableType,Display17.1,TheTypeParameterT,Slide17-13,TemplateswithMultipleParameters,FunctiontemplatesmayusemorethanoneparameterExample:templateAllparametersmustbeusedinthetemplatefunction,Slide17-14,AlgorithmAbstraction,UsingatemplatefunctionwecanexpressmoregeneralalgorithmsinC+AlgorithmabstractionmeansexpressingalgorithmsinaverygeneralwaysowecanignoreincidentaldetailThisallowsustoconcentrateonthesubstantivepartofthealgorithm,Slide17-15,ProgramExample:AGenericSortingFunction,Thesortfunctionbelowusesanalgorithmthatdoesnotdependonthebasetypeofthearrayvoidsort(inta,intnumber_used)intindex_of_next_smallest;for(intindex=0;indexnumber_used-1;index+)index_of_next_smallest=index_of_smallest(a,index,number_used);swap_values(aindex,aindex_of_next_smallest);Thesamealgorithmcouldbeusedtosortanarrayofanytype,Slide17-16,sortusestwohelperfunctionsindex_of_smallestalsousesageneralalgorithmandcouldbedefinedwithatemplateswap_valueshasalreadybeenadaptedasatemplateAllthreefunctions,definedwithtemplates,aredemonstratedin,Display17.2,Display17.3(1-2),GenericSorting:HelpingFunctions,Slide17-17,TemplatesandOperators,Thefunctionindex_of_smallestcomparesitemsinanarrayusingtheoperatorIfatemplatefunctionusesanoperator,suchas,thatoperatormustbedefinedforthetypesbeingcomparedIfaclasstypehastheoperatoroverloadedfortheclass,thenanarrayofobjectsoftheclasscouldbesortedwithfunctiontemplatesort,Slide17-18,DefiningTemplates,WhendefiningatemplateitisagoodideaTostartwithanordinaryfunctionthataccomplishesthetaskwithonetypeItisofteneasiertodealwithaconcretecaseratherthanthegeneralcaseThendebugtheordinaryfunctionNextconvertthefunctiontoatemplatebyreplacingtypenameswithatypeparameter,Slide17-19,InappropriateTypesforTemplates,Templatescanbeusedforanytypeforwhichthecodeinthefunctionmakessenseswap_valuesswapsindividualobjectsofatypeThiscodewouldnotwork,becausetheassignmentoperatorusedinswap_valuesdoesnotworkwitharrays:inta10,b10;swap_values(a,b);,Slide17-20,Section17.1Conclusion,CanyouIdentifyatemplateprefix?Identifyaparametertypeinatemplateprefix?Compareandcontrastfunctionoverloadingwiththeuseoftemplates?Whatadditionalcomplexitiesareinvolvedwhenclasstypesareinvolvedasparametertypes?,Copyright2008PearsonAddison-Wesley.Allrightsreserved.,17.2,TemplatesforDataAbstraction,Slide17-22,TemplatesforDataAbstraction,ClassdefinitionscanalsobemademoregeneralwithtemplatesThesyntaxforclasstemplatesisbasicallythesameasforfunctiontemplatestemplatecomesbeforethetemplatedefinitionTypeparameterTisusedintheclassdefinitionjustlikeanyothertypeTypeparameterTcanrepresentanytype,Slide17-23,AClassTemplate,ThefollowingisaclasstemplateAnobjectofthisclasscontainsapairofvaluesoftypeTtemplateclassPairpublic:Pair();Pair(Tfirst_value,Tsecond_value);continuedonnextslide,Slide17-24,TemplateClassPair(cont.),voidset_element(intposition,Tvalue);/Precondition:positionis1or2/Postcondition:positionindicatedissettovalueTget_element(intposition)const;/Precondition:positionis1or2/Returnsvalueinpositionindicatedprivate:Tfirst;Tsecond;,Slide17-25,DeclaringTemplateClassObjects,Oncetheclasstemplateisdefined,objectsmaybedeclaredDeclarationsmustindicatewhattypeistobeusedforTExample:Todeclareanobjectsoitcanholdapairofintegers:Pairscore;orforapairofcharacters:Pairseats;,Slide17-26,UsingtheObjects,Afterdeclaration,objectsbasedonatemplateclassareusedjustlikeanyotherobjectsContinuingthepreviousexample:score.set_element(1,3);score.set_element(2,0);seats.set_element(1,A);,Slide17-27,DefiningtheMemberFunctions,MemberfunctionsofatemplateclassaredefinedthesamewayasmemberfunctionsofordinaryclassesTheonlydifferenceisthatthememberfunctiondefinitionsarethemselvestemplates,Slide17-28,ThisisadefinitionoftheconstructorforclassPairthattakestwoargumentstemplatePair:Pair(Tfirst_value,Tsecond_value):first(first_value),second(second_value)/NobodyneededduetoinitializationaboveTheclassnameincludes,DefiningaPairConstructor,Slide17-29,Definingset_element,Hereisadefinitionforset_elementinthetemplateclassPairvoidPair:set_element(intposition,Tvalue)if(position=1)first=value;elseif(position=2)second=value;else,Slide17-30,TemplateClassNamesasParameters,ThenameofatemplateclassmaybeusedasthetypeofafunctionparameterExample:TocreateaparameteroftypePair:intadd_up(constPair/Returnsthesumoftwointegersinthe_pair,Slide17-31,TemplateFunctionswithTemplateClassParameters,Functionadd_upfromapreviousexamplecanbema