C++面向对象程序设计双语教程（第3版）课件 class 13-Polymorphism

ObjectivesTounderstandthemechanismsofstaticanddynamicpolymorphismTobeabletodeclarevirtualfunctionsandknowhowtousethemTobeabletodefineanabstractbaseclass01WhatisPolymorphism?04AbstractBaseClasses02DynamicPolymorphismandVirtualFunctions03UnderstandingofVirtualFunctions05InheritingInterfaceandImplementation01WhatisPolymorphism?Polymorphismforanaddition(+)operator,a+

bi+

jcomplex1+complex2foraprint

function, voidprint(int);voidprint(int,char*);Forexample,Whatispolymorphism?Wordmeaning--manyformsPolymorphismmeansthatsomecodes(functions),operatorsorobjectsbehavedifferentlyindifferentcontextsusingauniformform.PolymorphismisoneofOOPkeyfeatures.Polymorphism-BindingBindingisanassociation,suchasbetweenidentifiers(variablesorfunctionnames)andoperations(atypeorthespecificfunctionbody).Allfunctionshaveauniqueaddress.So,whenthecompilerencountersafunctioncall,itreplacesthefunctioncallwithamachinelanguageinstructionthattellstheCPUtojumptotheaddressofthefunction.Polymorphism-BindingTherearetwotypesofbindingaccordingtothetimeatwhichabindingtakesplace:(1)Compile-timebinding(calledstatic

binding,earlybinding)

–staticpolymorphism(2)Run-timebinding(calleddynamicbinding,latebinding)–dynamicpolymorphismStaticBinding(Compile-Time)–StaticPolymorphismclass

baseClass{public:voidf(){cout<<"CallingbaseClass\n";}};class

derivedClass:public

baseClass{public:voidf(){cout<<"CallingderivedClass\n";}};intmain(){baseClassone;one.f();derivedClasstwo;two.f();return0;}Staticbindingmeansthecompilerisabletodirectlyassociatetheidentifiername(suchasafunctionorvariable)withamachineaddress.Result:CallingbaseClassCallingderivedClassStaticBinding–StaticPolymorphismResult:CallingbaseClassCallingbaseClassclass

baseClass{public:voidf(){cout<<"CallingbaseClass\n";}};class

derivedClass:public

baseClass{public:voidf(){cout<<"CallingderivedClass\n";}};voidfn(baseClass&bc){bc.f();}intmain(){baseClassone;fn(one);derivedClasstwo;fn(two);return0;}A

top-level

functionwithaparameterofbaseclassreferenceorpointeri.e.referredtoasthederived-to-baseconversion02DynamicPolymorphismandVirtualFunctionsDynamicBinding(Run-Time)–DynamicPolymorphismResult:CallingbaseClassCallingderivedClassclass

baseClass{public:virtual

voidf(){cout<<"CallingbaseClass\n";}};class

derivedClass:public

baseClass{public:voidf(){cout<<"CallingderivedClass\n";}};voidfn(baseClass&bc){bc.f();}intmain(){baseClassone;fn(one);derivedClasstwo;fn(two);return0;}DynamicBinding

referstolinkingafunctioncalltothecodethatwillbeexecutedonlyatruntime.Thecodeassociatedwiththeproceduredosenotknowuntiltheprogramisexecuted.virtualfunctionVirtualFunctionsShape+volume():int+print():voidCube-edge:int+volume():int+print():voidCuboidlength:intwidth:intheight:int+volume():int+print():voidCaseStudyDefineclassescubeandcuboidaccordingtothegivenUMLdiagram.Cube-edge:int+volume():int+print():voidCuboidlength:intwidth:intheight:int+volume():int+print():voidVirtualFunctionsclass

Shape{public:Shape(){}virtual

intvolume(){cout<<"Shape'svolume.\n";return0;}virtual

voidprint(){cout<<"Shape’sprint.\n";}};class

Cube:public

Shape{public:Cube(int

e):edge(e){}virtual

intvolume(){cout<<"Cube’svolume\n";returnedge*edge*edge;}voidprint(){cout<<edge<<endl;}private:int

edge;};class

Cuboid:public

Shape{public:Cuboid(int

l,int

w,int

h):length(l),width(w),height(h){}virtual

intvolume(){cout<<"Cubiod’svolume\n";returnlength*width*height;}voidprint(){cout<<length<<width<<height<<endl;}private:intlength,width,height;};voidfn(Shape&

s){

s.print();cout<<s.volume();}intmain(){Cubec1(10);Cuboidc2(10,20,30);fn(&c1);fn(&c2);return0;}intmain(){Cubec1(10);Cuboidc2(10,20,30);fn(c1);fn(c2);return0;}voidfn(Shape*s){

s->print();cout<<s->volume();}ExtensibilityShape+volume():int+print():voidCube-edge:int+volume():int+print():voidCuboidlength:intwidth:intheight:int+volume():int+print():voidSphere-radius:int+volume():int+print():voidBenefitofusingvirtualfunctionsExtensibilityclass

Sphere:public

Shape{public:Sphere(int

r):radius(r){}virtual

intvolume(){

cout<<"Sphere’svolume\n";

return3.14*(double)4*(double)radius*radius*radius/3;}virtual

voidprint(){cout<<radius<<endl;}private:intradius;};voidfn(Shape*s){

s->print();cout<<s->volume();}intmain(){Cubec1(10);Cuboidc2(10,20,30);Spheres(10);fn(&c1);fn(&c2);fn(&s);return0;}03UnderstandingofVirtualFunctionsTheWorkingPrincipleofVirtualFunctionsintmain(){Cubec1(10);cout<<"sizeofc1"<<sizeof(c1)<<endl;Cuboidc2(10,20,30);cout<<"sizeofc2"<<sizeof(c2)<<endl;return0;}(32-bitcompiler)sizeofc18sizeofc216VirtualFunctionTable—VTABLEThecompilercreatesasinglevirtualfunctiontable(VTABLE)foreachclassthatcontainsvirtualfunctions.ThecompileplacestheaddressesofvirtualfunctionsforthoseclassesintheVTABLE.Apointer(vptr)isplacessecretlyineachclasswithvirtualfunctions.ItpointstotheVTABLEforobjects.addressaddressaddressConversionsandInheritancevoidfn(Shape&

s){

s.print();cout<<s.volume();}intmain(){

Cubec1(10);

Cuboidc2(10,20,30);fn(c1);fn(c2);

return0;}intmain(){

Cubec1(10);

Shape&

s=c1;

s.print();cout<<s.volume();

Cuboidc2(10,20,30);

s=c2;

s.print();cout<<s.volume();return0;}derived-to-baseUnderstandingconversionsbetweenbaseandderivedclassesisessentialtounderstandinghowobject-orientedprogrammingworks.ConversionsandInheritancevoidfn(Shape*s){

s->print();cout<<s->volume();}intmain(){

Cubec1(10);

Cuboidc2(10,20,30);fn(&c1);fn(&c2);

return0;}intmain(){

Cubec1(10);

Shape*

s=&c1;

s->print();cout<<s->volume();

Cuboidc2(10,20,30);

s=&c2;

s->print();cout<<s->volume();

return0;}derived-to-baseConversionsandInheritancevoidmain(){

Shape

s;Cube

&c1=s;//errorCube

*c2=s;//error}Thisresultsinthatwemightattempttousec1orc2tousemembersthatdonotexistinShape.Thefactthatwecanbindareference(orpointer)toabase-classtypetoaderivedobjecthasacruciallyimportantimplication:Whenweuseareference(orpointer)toabase-classtype,wedon’tknowtheactualtypeoftheobjecttowhichthepointerorreferenceisbound.Thatobjectcanbeanobjectofthebaseclassoritcanbeanobjectofaderivedclass.ConversionsandInheritanceintmain(){

Cubec1(10);

Shape

s=c1;}ShapeCube’sdataCubeMemoryBecausetheCubeisignored,wesaythattheCubeportionofShapeissliceddown.intmain(){

Cubec1(10);

Shape

s;s=c1;}Shape(constShape&)Shape&operator=(constShape&)Theautomaticderived-to-baseconversionappliesonlyforconversionstoareferenceorpointertype.VirtualDectructorResult:ConstructingbaseclassConstructingderivedclassDosthinBase!BaseDestroyedConstructorcannotbevirtualfunction,butdestructoralwaysbevirtualfunction.class

Base{public:Base(){cout<<"Constructingbaseclass\n";}~Base(){cout<<"BaseDestroyed\n";}voidDoSomething(){cout<<"DosthinBase!\n";}};class

Derived:public

Base{public:Derived(){cout<<"Constructingderivedclass\n";}~Derived(){cout<<"Deriveddestroyed\n";}voidDoSomething(){cout<<"DosthinDerived!\n";}};intmain(){//upcastBase*pBase=new

Derived;pBase->DoSomething();deletepBase;return0;}BaseclasspointertothederivedclassobjectVirtualDectructorclass

Base{public:Base(){cout<<"Constructingbaseclass\n";}~Base(){cout<<"BaseDestroyed\n";}virtual

voidDoSomething(){cout<<"DosthinBase!\n";}};class

Derived:public

Base{public:Derived(){cout<<"Constructingderivedclass\n";}~Derived(){cout<<"Deriveddestroyed\n";}voidDoSomething(){cout<<"DosthinDerived!\n";}};intmain(){Base*pBase=new

Derived;//upcastpBase->DoSomething();deletepBase;}class

Base{public:Base(){cout<<"Constructingbaseclass\n";}virtual~Base(){cout<<"BaseDestroyed\n";}virtual

voidDoSomething(){cout<<"DosthinBase!\n";}};class

Derived:public

Base{public:Derived(){cout<<"Constructingderivedclass\n";}~Derived(){cout<<"Deriveddestroyed\n";}voidDoSomething(){cout<<"DosthinDerived!\n";}};Result:(withvirtual)ConstructingbaseclassConstructingderivedclassDosthinDerived!BaseDestroyedResult:ConstructingbaseclassConstructingderivedclassDosthinDerived!DeriveddestroyedBaseDestroyed04AbstractBaseClassesAbstractBaseClassesSomeclassesrepresentabstractconceptsforwhichobjectscannotexist,thatis,theycannotbeinstantiated,becauseatleastonefunctionintheseclassesdonotknowwhattobeimplemented.However,theyexistsextensivelyforinheritance,andmostlyusedasabaseclass.Theseclassesarecalledabstractbaseclasses.class

Shape{public:Shape(){}virtual

intvolume();virtual

voidprint();};AbstractBaseClassesAclasswhichcontainsoneormorepurevirtualfunctioniscalledabstractbaseclass.class

Shape{public:Shape(){}virtual

intvolume()=0;virtual

voidprint()=0;};PurevirtualfunctionAbstractbaseclassApurevirtualfunction

isavirtualfunctionwhichcontainsnodefinitioninthebaseclass.Youdeclareapurevirtualfunctionbyusingapurespecifier(=0)inthedeclarationofavirtualmemberfunctionintheclassdeclaration.AbstractBaseClassesclass

Shape{public:Shape(){}virtual

intvolume()=0;virtual

voidprint()=0;};intShape::volume(){return

0;}voidShape::print(){}wecanprovideadefinitionforapurevirtual.However,thefunctionbodymustbedefinedoutsidetheclass.Thatis,wecannotprovideafunctionbodyinsidetheclassforafunctionthatis=0.AbstractBaseClassesShapes;//error–cannotbeinstantiatedShape*s;//okShape&s;//okAnobjectpointerAnobjectreferenceYoucannotcreateanobjectofanabstractclasstype;However,youcanusepointers

orreferencestoabstractclasstypes.AbstractBaseClassesclass

Shape{public:Shape(){}virtual

intvolume()=0;};class

Cube:public

Shape{public:Cube(int

e):edge(e){}intvolume(){

cout<<"Cube’svolume\n";

returnedge*edge*edge;}private:

intedge;};class

Cuboid:public

Shape{public:

Cuboid(int

l,int

w,int

h):

length(l),width(w),height(h){}

intvolume(){

cout<<"Cubiod’svolume\n";

returnlength*width*height;}private:

intlength,width,height;};intmain(){Shape*s;Cubec1(10);Cuboidc2(10,20,30);fn(&c1);fn(&c2);return0;}voidfn(Shape*s){

cout<<s->volume();}05InheritingImplementationand

Inheriting

InterfaceMemberFunctionsandInheritancebase+f():int+g():voidderived+f():intDistinguishbetweeninheritinginterfaceandinheritingimplementation.Interface-Overriding-VirtualfunctionsImplementation-

MemberfunctionsoftheclassesThememberfunctionsofabaseclasscanbeinheritedbyitsderivedclassintwoways:(1)Whenaderivedclassneedstobeabletoprovideitsowndefinitionforoperations,thederivedclassneedstooverridethedefinitionofthoseinheritedfromthebaseclass-interface(2)itsderivedclassonlyinheritthosewithoutanychange-implementationOverloadingandOverridingclass

Base{public:virtual

intf()const{cout<<"Base::f()\n";return1;}virtual

voidf(string

s)const{cout<<s<<"isaparameterintheBaseclass\n";}virtual

voidg()const{cout<<"Base::g()\n";}};class

Derived1:public

Base{public:voidg()const{cout<<"Overridingg()inDerived1\n";}};intmain(){Derived1d1;intx=d1.f();d1.f("hello");d1.g();return0;}Result:Base::f()HelloisaparameterintheBaseclassOverridingg()inDerived1OverloadingandOverridingclass

Base{public:virtual

intf()const{cout<<"Base::f()\n";return1;}virtual

voidf(string

s)const{cout<<s<<"isaparameterintheBaseclass\n";}virtual

voidg()const{cout<<"Base::g()\n";}};class

Derived1:public

Base{public:voidg()const{cout<<"Overridingg()inDerived1\n";}virtual

intf()const{cout<<“Dervied1::f()\n";return1;}};intmain(){Derived1d1;intx=d1.f();d1.f("hello");//errord1.g();return0;}nomatchingfunctionforcallto'Derived1::f(constchar[6])

d1.Base::f("hello");//okAderivedclassmemberwiththesamenameasamemberofthebaseclasshidesthedirectuseofthebaseclassmember.class

Base{public:virtual

intf()const{cout<<"Base::f()\n";return1;}virtual

voidf(string

s)const{cout<<s<<"isaparameterintheBaseclass\n";}};OverloadingandOverridingclass

Base{public:virtual

voidg()const{cout<<"Base::g()\n";}};