chap15-polymorphism_et_virtual_func .pdf

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1、15:Polymorphism&VirtualFunctionsPolymorphism(implementedinC++withvirtualfunctions)isthethirdessentialfeatureofanobject-orientedprogramminglanguage,afterdataabstractionandinheritance.659Itprovidesanotherdimensionofseparationofinterfacefromimplementation,todecouplewhatfromhow.

2、Polymorphismallowsimprovedcodeorganizationandreadabilityaswellasthecreationofextensibleprogramsthatcanbe“grown”notonlyduringtheoriginalcreationoftheproject,butalsowhennewfeaturesaredesired.Encapsulationcreatesnewdatatypesbycombiningcharacteristicsandbehaviors.Accesscontrolse

3、paratestheinterfacefromtheimplementationbymakingthedetailsprivate.Thiskindofmechanicalorganizationmakesreadysensetosomeonewithaproceduralprogrammingbackground.Butvirtualfunctionsdealwithdecouplingintermsoftypes.InChapter14,yousawhowinheritanceallowsthetreatmentofanobjectasit

4、sowntypeoritsbasetype.Thisabilityiscriticalbecauseitallowsmanytypes(derivedfromthesamebasetype)tobetreatedasiftheywereonetype,andasinglepieceofcodetoworkonallthosedifferenttypesequally.Thevirtualfunctionallowsonetypetoexpressitsdistinctionfromanother,similartype,aslongasthey

5、’rebothderivedfromthesamebasetype.Thisdistinctionisexpressedthroughdifferencesinbehaviorofthefunctionsthatyoucancallthroughthebaseclass.Inthischapter,you’lllearnaboutvirtualfunctions,startingfromthebasicswithsimpleexamplesthatstripawayeverythingbutthe“virtualness”oftheprogra

6、m.EvolutionofC++programmersCprogrammersseemtoacquireC++inthreesteps.First,assimplya“betterC,”becauseC++forcesyoutodeclareallfunctionsbeforeusingthemandismuchpickierabouthowvariablesareused.YoucanoftenfindtheerrorsinaCprogramsimplybycompilingitwithaC++compiler.660ThinkinginC+

7、+www.BruceEckel.comThesecondstepis“object-based”C++.Thismeansthatyoueasilyseethecodeorganizationbenefitsofgroupingadatastructuretogetherwiththefunctionsthatactuponit,thevalueofconstructorsanddestructors,andperhapssomesimpleinheritance.MostprogrammerswhohavebeenworkingwithCfo

8、rawhilequicklyseetheusefulnessofthisbecause,whenevertheycreatealibrary,this