继承多态信息工程学院计算机技术教研室

1、概述 多态性（Polymorphism）是指一个名字，多种语义；或界面 相同，多种实现。即当发出同样的消息被不同类型的对象接 收时，导致完全不同的行为。 多态性是面向对象程序设计的重要特征之一。 多态的实现：函数重载运算符重载虚函数联编是指一个程序模块、代码之间互相关联的过程。 静态联编(Static Binding)：是指程序的匹配、连接在编译阶段，即程序运行之前完成，也称为早期匹配。重载函数使用静态联编。 动态联编(Dynamic Binding) ：是指程序联编推迟到运行时进行，所以又称为晚期联编。继承和虚函数使用的是动态联编。8.6 联编(Binding)普通成员函数重载可表达为两种形

2、式：1. 在一个类说明中重载例如：Show ( int , char ) ;Show ( char * , float ) ;8.7 静态联编普通成员函数重载可表达为两种形式：1. 在一个类说明中重载例如：Show ( int , char )； 与Show ( char * , float )； 不是同一函数，编译能够区分2. 基类的成员函数在派生类重载。有 3 种编译区分方法：（1）根据参数的特征加以区分8.7 静态联编普通成员函数重载可表达为两种形式：1. 在一个类说明中重载2. 基类的成员函数在派生类重载。有 3 种编译区分方法：（1）根据参数的特征加以区分例如：A : Show (

3、)；有别于B : Show ( )；（2）使用“ : ”加以区分8.7 静态联编普通成员函数重载可表达为两种形式：1. 在一个类说明中重载2. 基类的成员函数在派生类重载。有 3 种编译区分方法：（1）根据参数的特征加以区分（2）使用“ : ”加以区分例如：Aobj . Show ( )调用A : Show ( )Bobj . Show ( )调用B : Show ( )（3）根据类对象加以区分8.7 静态联编 冠以关键字 virtual 的成员函数称为虚函数 实现运行时多态的关键首先是要说明虚函数，另外，必须 用基类指针调用派生类的不同实现版本8.8 虚函数和动态联编 基类指针虽然获取派生类

4、对象地址，却只能访问派生类从基类继承的成员8.8.1 虚函数和基类指针#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d

5、 : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针#includeclass Base public : Base(

6、char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : F

7、irst_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_obj#includeclass Base public : Base(char xx) x = xx; void who() cout Base class:

8、 x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Sec

9、ond derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp定义基类指针#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class Firs

10、t_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ;

11、p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx,

12、 char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - w

13、ho() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout

14、First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) -

15、 who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: c

16、har y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_

17、objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d pub

18、lic : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : B

19、ase(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz )

20、 : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base c

21、lass: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cou

22、t Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class First

23、_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p

24、 = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() cout First derived class: x , y n ; #includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: char x; ;class F

25、irst_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who()

26、 ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() cout First derived class: x , y n ; 通过对象调用成员函数#includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected: ch

27、ar x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x

28、 , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() cout Second derived class: x , y , z n ; #includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; prot

29、ected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived

30、 class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() cout Second derived class: x , y , z n ; 基类指针做类型转换#includeclass Base public : Base(char xx) x = xx; void who() cout Base

31、 class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() c

32、out Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp通过基类指针只能访问从基类继承的成员 #includeclass Base public : Base(char xx) x = xx; void who() cout Base class: x n ; protected:

33、 char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class

34、: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp修改程序定义虚函数#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d :

35、public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F

36、_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针基类定义虚函数#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(x

37、x) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who

38、() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针派生类的重定义版本默认为虚函数#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived clas

39、s: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动

40、8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_obj#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;cla

41、ss Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_o

42、bjp#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public

43、 : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base

44、(char xx) x = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char

45、 zz ) : First_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; virtual void who(

46、) cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; v

47、oid who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() cout First derived class: x , y n ; #includeclass Base public : Base(char xx) x = xx; virtu

48、al void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy

49、 ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objvoid who() cout First derived class: x , y n ; p#includeclass Base public : Base(char xx) x

50、 = xx; virtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : Fir

51、st_d( xx, yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp#includeclass Base public : Base(char xx) x = xx; virtual void who() cout Base

52、 class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz; void who() c

53、out Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objpvoid who() coutSecond derived class: x, y, zn ; #includeclass Base public : Base(char xx) x = xx; virtual void wh

54、o() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx, yy ) z = zz;

55、 void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objvoid who() coutSecond derived class: x, y, zn ; p#includeclass Base public : Base(char xx) x = xx; vi

56、rtual void who() cout Base class: x n ; protected: char x; ;class First_d : public Base public : First_d(char xx, char yy):Base(xx) y = yy; void who() cout First derived class: x , y n ; protected: char y; ;class Second_d : public First_d public : Second_d( char xx, char yy, char zz ) : First_d( xx,

57、 yy ) z = zz; void who() cout Second derived class: x , y , z who() ; p = &F_obj ; p - who() ; p = &S_obj ; p - who() ; F_obj.who() ; ( ( Second_d * ) p ) - who() ;演示基类指针的移动 8.8.1 虚函数和基类指针ATOENDxyzB_objF_objS_objp由于who()的虚特性随着p指向不同对象，执行不同实现版本 注意： 一个虚函数，在派生类层界面相同的重载函数都保持虚特性 虚函数必须是类的成员函数，不能是全局函数和静态函数

58、不能将友元说明为虚函数，但虚函数可以是另一个类的友元 析构函数可以是虚函数，但构造函数不能是虚函数 动态联编只能通过指针或引用来操作虚函数，如果采用其他标识对象来操作虚函数，将采用静态联编方式调用虚函数。8.8.1 虚函数和基类指针 在派生类中重载基类的虚函数要求函数名、返回类型、参数 个数、参数类型和顺序完全相同 如果仅仅返回类型不同，C+认为是错误重载 如果函数原型不同，仅函数名相同，丢失虚特性 8.8.2 虚函数的重载特性void g ( ) derived d ; base * bp = & d ; / 基类指针指向派生类对象 bp - vf1 ( ) ; / 调用 derived :

59、 vf1 ( ) bp - vf2 ( ) ; / 调用 base : vf2 ( ) bp - f ( ) ; / 调用 base : f ( ) ; 例：class base public : virtual void vf1 ( ) ; virtual void vf2 ( ) ; virtual void vf3 ( ) ; void f ( ) ; ;class derived : public base public : void vf1 ( ) ;/ 虚函数 void vf2 ( int ) ;/ 重载，参数不同，虚特性丢失 char vf3 ( ) ;/ error，仅返回类型

60、不同 void f ( ) ;/ 非虚函数重载 ;8.8.2 虚函数的重载特性 构造函数不能是虚函数。建立一个派生类对象时，必须从类 层次的根开始，沿着继承路径逐个调用基类的构造函数 析构函数可以是虚的。虚析构函数用于指引 delete 运算符正 确析构动态对象 8.8.3 虚析构函数#includeclass A public: A() cout A:A() is called.n ; ;class B : public A public: B() cout B:B() is called.n ; ;void main() A *Ap = new B ; B *Bp2 = new B ; c