C語(yǔ)言程序設(shè)計(jì)第15章多態(tài)性

C語(yǔ)言程序設(shè)計(jì)第15章多態(tài)性講授內(nèi)容靜態(tài)綁定和動(dòng)態(tài)綁定的概念多態(tài)性的意義和作用虛函數(shù)的定義方法抽象基類的作用和定義方法虛析構(gòu)函數(shù)的作用和使用方法如何在程序中實(shí)現(xiàn)多態(tài)性15.1多態(tài)性的概念(1/5)多態(tài)性：具有繼承關(guān)系的類，其對(duì)象對(duì)同一個(gè)函數(shù)調(diào)用可以作出不同的響應(yīng)同一個(gè)函數(shù)調(diào)用——同一條函數(shù)調(diào)用語(yǔ)句不同的響應(yīng)——執(zhí)行不同的函數(shù)15.1多態(tài)性的概念(2/5)靜態(tài)綁定和動(dòng)態(tài)綁定靜態(tài)綁定——編譯時(shí)就能確定一條函數(shù)調(diào)用語(yǔ)句要調(diào)用的函數(shù)和面向?qū)ο蟮乃枷氩惶喾粭l指令對(duì)應(yīng)不同對(duì)象的不同操作——現(xiàn)實(shí)需要15.1多態(tài)性的概念(3/5)靜態(tài)綁定和動(dòng)態(tài)綁定動(dòng)態(tài)綁定：運(yùn)行時(shí)才能確定函數(shù)調(diào)用語(yǔ)句調(diào)用的函數(shù)調(diào)用對(duì)象的成員函數(shù)時(shí)（通過(guò)引用或指針），編譯器暫不確定要調(diào)用的函數(shù)編譯器檢查被調(diào)用函數(shù)是否存在（函數(shù)名、參數(shù)列表、返回值）程序運(yùn)行時(shí)，系統(tǒng)根據(jù)接收消息的對(duì)象類型來(lái)確定要調(diào)用的函數(shù)15.1多態(tài)性的概念(4/5)多態(tài)性的實(shí)現(xiàn)通過(guò)基類指針或引用調(diào)用虛函數(shù)動(dòng)態(tài)綁定多態(tài)性有助于更好地對(duì)程序進(jìn)行抽象控制模塊能專注于一般性問(wèn)題的處理具體的操作交給具體的對(duì)象去做15.1多態(tài)性的概念(5/5)多態(tài)性有助于提高程序的可擴(kuò)展性可以把控制模塊與被操作的對(duì)象分開(kāi)可以添加已定義類的新對(duì)象，并能管理該對(duì)象可以添加新類（已有類的派生類）的新對(duì)象，并能管理該對(duì)象常用的程序設(shè)計(jì)方法為具有繼承關(guān)系的類定義接口（虛函數(shù)）用鏈表或數(shù)組組織所有對(duì)象用基類指針操作這些對(duì)象15.2虛函數(shù)虛函數(shù)的定義在函數(shù)原型前加上關(guān)鍵字virtual如果一個(gè)函數(shù)在基類中被聲明為虛函數(shù)，則他在所有派生類中都是虛函數(shù)（包括重定義函數(shù)）

virtualvoidshow();只有通過(guò)基類指針或引用調(diào)用虛函數(shù)才能引發(fā)動(dòng)態(tài)綁定例子1：虛函數(shù)與多態(tài)性例子(1/3)//文件base.h//定義基類#if!defined__BASE__H__#define__BASE__H__#include<iostream.h>

classBase{public:virtualvoidshow(){cout<<"IamBase'sobject!\n";}};

#endif例子1：虛函數(shù)與多態(tài)性例子(2/3)//文件derived.h//定義基類Derived#if!defined__DERIVED__H__#define__DERIVED__H__#include"base.h"

classDerived:publicBase{public:voidshow(){cout<<"IamDerived'sobject!\n";}};

#endif例子1：虛函數(shù)與多態(tài)性例子(3/3)//文件ex15_2.cpp//測(cè)試多重繼承的類層次#include"derived.h"main(){Base*bPtr,bObj;DeriveddObj;bPtr=&bObj; //基類指針指向基類對(duì)象

bPtr->show(); //用基類指針調(diào)用成員函數(shù)，動(dòng)態(tài)綁定

bPtr=&dObj; //基類指針指向基類對(duì)象

bPtr->show(); //用基類指針調(diào)用成員函數(shù)，動(dòng)態(tài)綁定

return0;}程序運(yùn)行如果IamBase'sobject!IamDerived'sobject!15.3抽象基類和純虛函數(shù)(1/4)實(shí)現(xiàn)多態(tài)性的前提需要有共同的基類需要在基類中定義共同的接口接口要定義為虛函數(shù)如果基類的接口沒(méi)辦法實(shí)現(xiàn)怎么辦？如形狀類Shape15.3抽象基類和純虛函數(shù)(2/4)解決方法不實(shí)現(xiàn)這些接口：純虛函數(shù)包含純虛函數(shù)的類：抽象基類抽象基類不能實(shí)例化對(duì)象形狀類Shape的對(duì)象是什么？15.3抽象基類和純虛函數(shù)(3/4)純虛函數(shù)沒(méi)有函數(shù)體的“初始化為0”的函數(shù)

//純虛函數(shù)virtualdoublearea()const=0;純虛函數(shù)不需要實(shí)現(xiàn)15.3抽象基類和純虛函數(shù)(4/4)抽象類和具體類包含純虛函數(shù)的類不能實(shí)例化對(duì)象，是抽象類如果抽象類的派生類實(shí)現(xiàn)了所有基類中的純虛函數(shù)，則不再是抽象類抽象類存在的意義是作為其它類的基類，也叫抽象基類例子2：抽象基類的例子(1/6)//文件shape.h//定義抽象基類Shape#if!defined__SHAPE__H__#define__SHAPE__H__#include<iostream.h>

classShape{public:virtualdoublearea()const=0;virtualvoidshow()const=0;};

#endif例子2：抽象基類的例子(2/6)//文件circle.h，定義派生類Circle#if!defined__CIRCLE__H__#define__CIRCLE__H__#include"shape.h"#definePI3.1416classCircle:publicShape{public:

Circle(double=0.0,double=0.0,double=1.0);doublearea()const;voidshow()const;private:doublex,y;doubler;};#endif例子2：抽象基類的例子(3/6)//文件circle.cpp，實(shí)現(xiàn)類Circle#include"circle.h"Circle::Circle(doublea,doubleb,doublec){x=a;y=b;r=c;}doubleCircle::area()const{returnPI*r*r;}

voidCircle::show()const{cout<<"IamaCircle:";}例子2：抽象基類的例子(4/6)//文件rectangle.h//定義派生類Rectangle#if!defined__RECTANGLE__H__#define__RECTANGLE__H__#include"shape.h"classRectangle:publicShape{public:Rectangle(double=1.0,double=1.0);doublearea()const;voidshow()const;private:doublelength;doublewidth;};#endif例子2：抽象基類的例子(5/6)//文件rectangle.cpp，實(shí)現(xiàn)類Rectangle#include"rectangle.h"Rectangle::Rectangle(doublea,doubleb){length=a;width=b;}doubleRectangle::area()const{returnlength*width;}voidRectangle::show()const{cout<<"IamaRectangle:";}例子2：抽象基類的例子(6/6)//文件ex15_3.cpp，測(cè)試類Shape的層次結(jié)構(gòu)#include"circle.h"#include"rectangle.h"voidcallArea(Shape&);main(){Circlecir(0.0,0.0,2.5);Rectanglerec(2.4,5.3);callArea(cir);callArea(rec);return0;}voidcallArea(Shape&obj){obj.show();cout<<"area="<<obj.area()<<endl;}程序運(yùn)行結(jié)果IamaCircle:area=19.635IamaRectangle:area=12.7215.4虛析構(gòu)函數(shù)構(gòu)造函數(shù)的執(zhí)行順序：從上到下析構(gòu)函數(shù)的執(zhí)行順序：從下到上創(chuàng)建對(duì)象時(shí)要執(zhí)行正確的構(gòu)造函數(shù)撤銷對(duì)象時(shí)要執(zhí)行正確的析構(gòu)函數(shù)問(wèn)題：動(dòng)態(tài)對(duì)象的創(chuàng)建和撤銷15.4虛析構(gòu)函數(shù)動(dòng)態(tài)對(duì)象的創(chuàng)建動(dòng)態(tài)創(chuàng)建的對(duì)象沒(méi)有問(wèn)題

newClassName(…);ClassName指明了要調(diào)用的構(gòu)造函數(shù)動(dòng)態(tài)對(duì)象的撤銷

delete基類指針;如果基類指針指向的是派生類的對(duì)象呢？15.4虛析構(gòu)函數(shù)析構(gòu)函數(shù)可以聲明為虛函數(shù)delete基類指針;程序會(huì)根據(jù)基類指針指向的對(duì)象的類型確定要調(diào)用的析構(gòu)函數(shù)基類的析構(gòu)函數(shù)為虛函數(shù)，所有派生類的析構(gòu)函數(shù)都是虛函數(shù)如果要操作具有繼承關(guān)系的類的動(dòng)態(tài)對(duì)象，最好使用虛析構(gòu)函數(shù)例子3：虛析構(gòu)函數(shù)的例子//文件employee.h，定義基類Employee#include<iostream.h>#if!defined__EMPLOYEE__H__#define__EMPLOYEE__H__classEmployee{public:Employee(){ cout<<"Employeebegin!"<<endl; }virtual~Employee(){cout<<"Employeeend!"<<endl;}};#endif例子3：虛析構(gòu)函數(shù)的例子//文件programmer.h，定義派生類Programmer#include<iostream.h>#include<string.h>#include"employee.h"classProgrammer:publicEmployee{public:Programmer(char*str){cout<<"Programmerbegin!"<<endl;name=newchar[strlen(str)+1];strcpy(name,str);}~Programmer(){deletename;cout<<"Programmerend!"<<endl;}private:char*name;};例子3：虛析構(gòu)函數(shù)的例子//文件accountant.h，定義派生類Accountant#include<iostream.h>#include"employee.h"classAccountant:publicEmployee{public:Accountant(intn){cout<<"Accountantbegin!"<<endl;age=n;}~Accountant(){cout<<"Accountantend!"<<endl;}private:intage;};例子3：虛析構(gòu)函數(shù)的例子//文件ex15_4.cpp#include"accountant.h"#include"programmer.h"constintMAX=100;main(){intno;//聲明儲(chǔ)存雇員信息的數(shù)組

Employee*ptr[MAX],*tptr;intENum=0;charname[100];intage;for(inti=0;i<MAX;i++){ptr[i]=NULL;}例子3：虛析構(gòu)函數(shù)的例子//輸入雇員信息

cout<<"Inputemployees'info:"<<endl;cout<<"1---Programmer"<<endl<<"2---Accountant"<<endl<<"0---exit"<<endl;cin>>no;while(no){switch(no){case1: //輸入程序員信息