软件设计（II）教学课件：Chapter9 Pointers, Virtual Functions and Polymorphism

1、Chapter 9 Pointers, Virtual Functions and Polymorphism9.1 Introduction9.2 Pointers to Objects9.3 Virtual Functions9.4 Dynamic Casting9.1 IntroductionPolymorphism: “one name, multiple forms”lFunctions, data typesFunction bindinglInvocation statement function bodyCompile time polymorphismlStatic (earl

2、y) binding, static linkinglFunction overloadinglOperator overloadingRun time polymorphismlDynamic (late) binding, dynamic linkinglVirtual function + pointer of base class2Techniques for Polymorphism3PolymorphismRun timepolymorphismCompile timepolymorphismFunctionOverloadingOperatoroverloadingVirtual

3、 functionExample Situation4class HM public: void show() coutHumann; ;class CN: public HM public: void show() cout“Chinesen; ;class CT: public CNpublic: void show() coutshow();(*it_ptr).show();Array of objectsitem * ptr = new item10;56The “this” PointerA special built-in pointer that points to the cu

4、rrent objectUsed to cope with hidden data fields/ Construct a circle object Circle:Circle(double radius) this-radius = radius; (*this).radius = radius; / Set a new radius void Circle:setRadius(double radius) this-radius = (radius = 0) ? radius : 0; The “this” PointerPerson &Person:greater(Person

5、& x) if (x.ageage) return x; else return *this;max = a.greater(b);7the invoking object!the argument object!Pointers to Derived ClassesPointers of a base class type is compatible with pointers of a derived classStatements for a base class is workable for derived classes8B *cptr;/pointer to class

6、B type variableB b;/base objectD d;/derived objectcptr = &b;/cptr points to object bcptr-show();cptr = &d; /cptr points to object dcptr-show();class Bpublic: int b; void show() coutb=bendl;class D : public Bpublic: int d; void show() coutb=bendl; coutd=db = 100; coutshow(); D derived; /deriv

7、ed object bptr = &derived; bptr-b=200; coutshow(); D *dptr; dptr= &derived; dptr-d=300; coutshow(); coutusing (D*)bptr)d=400; (D*)bptr)-show(); return 0;9.3 Virtual Functions10The function declared with the keyword “virtual”OverridinglTo redefine a virtual function in the derived classclass

8、B public: virtual string toString() return class B; ; class D: public B string toString() return class D; ; class B public: void display() cout“Display base; virtual void show() cout“n show base“;class D: public B public : void display() coutDisplay derived; void show() cout“show derived;11int main(

9、) B b; D d; B *bptr; coutdisplay(); bptr-show(); coutdisplay(); bptr-show(); return 0;Example of Virtual FunctionsRealize dynamic bindinglMust be accessed by using object pointers/referencesMust be defined in base classShould be redefined (overriding) in the derived classlIf not, base version will b

10、e invokedNo virtual constructorslVirtual destructors are allowedCan be a friend of another class12Notes on Virtual FunctionsVirtual Destructors13class B public: B() cout“Dest Bn”; ; class D: public B public: D() cout“Dest Dn”; ; int main() B *bp = new D(); delete bp; Dest BDest DDest Bvirtual B()Pur

11、e Virtual FunctionsA function without bodyldoes nothing, just a placeholderAlso called abstract functionClasses has pure virtual function cant be used to create objectsCompiler requires each derived class:ldefine the function, orlcontinue to declare it as a abstract function14virtual void display()

12、= 0;Abstract Classes15Class is the abstraction of instances/objectsA base class is more abstract/general than derived classesAbstract class (抽象类)lIn logic: A class so abstract that it cannot have any specific instancesIt can only be used as base classl In syntax:A class with abstract functions (抽象函数

13、)16Abstract FunctionI.e. Pure Virtual Function (纯虚函数)lCant be implemented in abstract classesFor example GeometricObject -color: string -filled: bool #GeometricObject() #GeometricObject(color: string, filled: bool) +getColor(): string +setColor(color: string): void +isFilled(): bool +setFilled(fille

14、d: bool): void +tostring(): string +getArea(): double +getPerimeter(): double “ protected” class GeometricObjectprotected: GeometricObject(); GeometricObject(string color, bool filled);public: string getColor(); void setColor(string color); bool isFilled(); void setFilled(bool filled); string toStri

15、ng(); virtual double getArea() = 0; virtual double getPerimeter() = 0;private: string color; bool filled; 17Abstract Class Example GeometricObject -color: string -filled: bool #GeometricObject() #GeometricObject(color: string, filled: bool) +getColor(): string +setColor(color: string): void +isFille

16、d(): bool +setFilled(filled: bool): void +tostring(): string +getArea(): double +getPerimeter(): double Circle -radius: double +Circle() +Circle(radius: double) +Circle(radius: double, color: string, filled: bool) +getRadius(): double +setRadius(radius: double): void +getDiameter(): double Rectangle

17、 -width: double -height: double +Rectangle() +Rectangle(width: double, height: double) +Rectangle(width: double, height: double, color: string, filled: bool) +getWidth(): double +setWidth(width: double): void +getHeight(): double +setHeight(height: double): void AbstractGeometricObject.hRunAbstractG

18、eometricObject.cppDerivedCircle2.hDerivedCircle2.cppRectangle2.hRectangle2.cppTestGeometricObject2 .cpp189.4 Dynamic CastingHow to display radius, diameter, area, and perimeter if the object is a circle?/ A function for displaying a geometric objectvoid displayGeometricObject(GeometricObject &ob

19、ject) cout The area is object.getArea() endl; cout The perimeter is object.getPerimeter() endl;19Dynamic CastingThe dynamic_cast operator checks if p points to a Circle objectlIf yes, p1 is assigned the address of the objectlIf no, p1 is assigned to NULL (the constant 0)GeometricObject *p = &object;Circle *p1 = dynamic_cast(p);if (p1 != NULL) cout The radius is getRadius() endl; cout The diameter is getDiameter() endl;RunDynamicCastingDemo20Upcasting and DowncastingUpcastinglAssigning a pointer of a derived class type to a p