虚函数--手工实现简单虚表

/*******************************Person.h*****************************/

#if !defined(AFX_PERSON_H__7862BC0E_A397_4133_AEEF_0F6FEF6B9F43__INCLUDED_)
#define AFX_PERSON_H__7862BC0E_A397_4133_AEEF_0F6FEF6B9F43__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000

class CPerson;

//定义一个成员函数指针类型
typedef void (CPerson::*TYPE_pfn_Say)(void);
//new时编译器分析不出成员函数指针多大,所以在再定义一个全局函数指针
typedef void (*TYPE_pfn_g_Say)(void);


class CPerson
{
public:
    TYPE_pfn_Say *arypfnaSayPerson;
public:
    CPerson();
    ~CPerson();
public:
    void SayHello(void);
    void SayGoodbye(void);
};

#endif // !defined(AFX_PERSON_H__7862BC0E_A397_4133_AEEF_0F6FEF6B9F43__INCLUDED_)

 

/*******************************Person.cpp*****************************/

#include "stdafx.h"
#include "Person.h"
#include <iostream.h>

CPerson::CPerson()
{
    //基类申请用来放虚表的空间,用成员函数指针数组,编译器分不清成员函数指针的大小
    //所以用全局函数指针数组来确定大小
    TYPE_pfn_g_Say *arypfnSayTemp = new TYPE_pfn_g_Say[4];
    arypfnaSayPerson = (TYPE_pfn_Say *)arypfnSayTemp;

    //初始化虚表
    arypfnaSayPerson[0] = SayHello;
    arypfnaSayPerson[1] = SayGoodbye;
}

CPerson::~CPerson()
{
    if (arypfnaSayPerson)
    {
        delete [] arypfnaSayPerson;
        arypfnaSayPerson = NULL;
    }
}


void CPerson::SayHello(void)
{
    cout << "CPerson say Hello" << endl;
}

void CPerson::SayGoodbye(void)
{
    cout << "CPerson say goodbye" << endl;
}

 

/*******************************Student.h*******************************/

#if !defined(AFX_STUDENT_H__8CA11886_59C1_4C51_B79B_BE9B243CF034__INCLUDED_)
#define AFX_STUDENT_H__8CA11886_59C1_4C51_B79B_BE9B243CF034__INCLUDED_

#if _MSC_VER > 1000
#pragma once
#endif // _MSC_VER > 1000

#include "Person.h"

class CStudent;

//定义一个成员函数指针类型
typedef void (CStudent::*TYPE_pfn_SayStudent)(void);



class CStudent : public CPerson 
{
public:
    TYPE_pfn_SayStudent *arypfnaSayStudent;
public:
    CStudent();
    ~CStudent();
public:
    void SayHello(void);   
    void SayGoodbye(void);
};

#endif // !defined(AFX_STUDENT_H__8CA11886_59C1_4C51_B79B_BE9B243CF034__INCLUDED_)

 

/*******************************Student.cpp*******************************/

#include "stdafx.h"
#include "Student.h"
#include <iostream.h>

CStudent::CStudent()
{
    //子类构造会高用基类构造
    //所以会先在基类申请虚表空间,子类直接给值就行了
    arypfnaSayStudent = (TYPE_pfn_SayStudent *)arypfnaSayPerson;
    //把子类的成员函数覆盖掉基类的成员函数
    arypfnaSayStudent[0] = SayHello;
    arypfnaSayStudent[1] = SayGoodbye;
}

CStudent::~CStudent()
{
    arypfnaSayStudent = NULL;
}

void CStudent::SayHello(void)
{
    cout << "CStudent say Hello" << endl;
}

void CStudent::SayGoodbye(void)
{
    cout << "CStudent say goodbye" << endl;
}

 

/************************************************************************
手工简单模拟虚函数来表现出多态性：
写一基类Person
有sayHello，sayGoodbye函数
有一子类student
它也有自己的sayHello, sayGoodbye函数
请在这两个类里加入函数vsayHello, vsayGoodbye函数
来表现出对象的多态性(分别调用自己的对应的sayHello和sayGoodbye)

成员函数指针完成
************************************************************************/
#include "stdafx.h"
#include "Student.h"

void AllSay(CPerson *pthePerson, int nInedx)
{
    //成员函数指针数组调用,pthePerson->*这一段可以理解为this指针
    (pthePerson->*pthePerson->arypfnaSayPerson[nInedx])();
}

int main()
{
    CPerson thePerson;
    CStudent theStudent;

    AllSay(&thePerson, 0);
    AllSay(&thePerson, 1);
    AllSay(&theStudent, 0);
    AllSay(&theStudent, 1);

    return 0;
}