第51课 继承对象模型分析——多态的本质分析
多态的本质分析
用C写面向对象,用C实现多态
#ifndef _51_2_H_
#define _51_2_H_
typedef void Demo;
typedef void Derived;
Demo* Demo_Create(int i, int j);
int Demo_GetI(Demo* pThis);
int Demo_GetJ(Demo* pThis);
int Demo_Add(Demo* pThis, int value);
void Demo_Free(Demo* pThis);
Derived* Derived_Create(int i, int j, int k);
int Derived_GetK(Derived* pThis);
int Derived_Add(Derived* pThis, int value);
#endif
#include "51-2.h"
#include "malloc.h"
static int Demo_Virtual_Add(Demo* pThis, int value);
static int Derived_Virtual_Add(Demo* pThis, int value);
struct VTable // 2. 定义虚函数表数据结构
{
int (*pAdd)(void*, int); // 3. 虚函数表里面存储什么???
};
struct ClassDemo
{
struct VTable* vptr; // 1. 定义虚函数表指针 ==》 虚函数表指针类型???
int mi;
int mj;
};
struct ClassDerived
{
struct ClassDemo d;
int mk;
};
static struct VTable g_Demo_vtbl =
{
Demo_Virtual_Add
};
static struct VTable g_Derived_vtbl =
{
Derived_Virtual_Add
};
Demo* Demo_Create(int i, int j)
{
struct ClassDemo* ret = (struct ClassDemo*)malloc(sizeof(struct ClassDemo));
if( ret != NULL )
{
ret->vptr = &g_Demo_vtbl; // 4. 关联对象和虚函数表
ret->mi = i;
ret->mj = j;
}
return ret;
}
int Demo_GetI(Demo* pThis)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi;
}
int Demo_GetJ(Demo* pThis)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mj;
}
// 6. 定义虚函数表中指针所指向的具体函数
static int Demo_Virtual_Add(Demo* pThis, int value)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->mi + obj->mj + value;
}
// 5. 分析具体的虚函数!!!!
int Demo_Add(Demo* pThis, int value)
{
struct ClassDemo* obj = (struct ClassDemo*)pThis;
return obj->vptr->pAdd(pThis, value);
}
void Demo_Free(Demo* pThis)
{
free(pThis);
}
Derived* Derived_Create(int i, int j, int k)
{
struct ClassDerived* ret = (struct ClassDerived*)malloc(sizeof(struct ClassDerived));
if( ret != NULL )
{
ret->d.vptr = &g_Derived_vtbl;
ret->d.mi = i;
ret->d.mj = j;
ret->mk = k;
}
return ret;
}
int Derived_GetK(Derived* pThis)
{
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->mk;
}
static int Derived_Virtual_Add(Demo* pThis, int value)
{
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->mk + value;
}
int Derived_Add(Derived* pThis, int value)
{
struct ClassDerived* obj = (struct ClassDerived*)pThis;
return obj->d.vptr->pAdd(pThis, value);
}
#include "stdio.h"
#include "51-2.h"
void run(Demo* p, int v)
{
int r = Demo_Add(p, v);
printf("r = %d\n", r);
}
int main()
{
Demo* pb = Demo_Create(1, 2);
Derived* pd = Derived_Create(1, 22, 333);
printf("pb->add(3) = %d\n", Demo_Add(pb, 3));
printf("pd->add(3) = %d\n", Derived_Add(pd, 3));
run(pb, 3);
run(pd, 3);
Demo_Free(pb);
Demo_Free(pd);
return 0;
}
继承的本质就是父子间成员变量的叠加
C++中的多态是通过虚函数表实现的
虚函数表是由编译器自动生成与维护的
虚函数的调用效率低于普通成员函数