700字范文 > C++对象模型——指向Data Members的指针(Pointer to Data Members)

C++对象模型——指向Data Members的指针(Pointer to Data Members)

时间：2022-11-06 21:09:02

本文所才用的编译环境是VC++6.0

作用

指向data members的指针,是一个有点神秘但颇有用处的语言特性,特别是如果需要详细调查 class members的底层布局的话,这样的调用可以决定vptr是放在 class 的起始位置或是尾端.另一个用途,可用来决定 class 中的access sections的次序

来看一个TEST类不带虚函数

class TEST{public:int nX;int nY;int nZ;double dJ;};

下面对该类进行测试

// Test.cpp : Defines the entry point for the console application.//#include "stdafx.h"#include <iostream>#include <stdio.h>using namespace std;int main(int argc, char* argv[]){cout<<sizeof(TEST)<<endl;int TEST::* p = &TEST::nX;printf("offest nX = %d\n", p);cout<<"offest nX = "<<p<<endl;p = &TEST::nY;printf("offest nY = %d\n", p);cout<<"offest nY = "<<p<<endl;p = &TEST::nZ;printf("offest nZ = %d\n", p);cout<<"offest nXZ = "<<p<<endl;p = NULL;printf("offest NULL = %d\n", p);cout<<"offest NULL = "<<p<<endl;double TEST::* pD = &TEST::dJ;printf("offest dJ = %d\n", pD);cout<<"offest dj = "<<pD<<endl;return 0;}

测试结果

①sizeof大小就是结构体的内存对齐24 没什么好讲的

②printf的结果可以看出 nX nY nZ dJ的偏移量就是按照他们在类声明的时候的顺序排列的

③一个指向NULL的Data Members的指针他的offset 会被编译器置-1

④为什么cout输出的只有0/1 因为输出的是一个bool值，代表有无偏移

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下面加上虚函数，来判断虚函数指针的摆放位置

class TEST{public:int nX;int nY;int nZ;virtual void Fun(void);};

#include "stdafx.h"#include <iostream>#include <stdio.h>using namespace std;int main(int argc, char* argv[]){cout<<sizeof(TEST)<<endl;int TEST::* p = &TEST::nX;printf("offest nX = %d\n", p);cout<<"offest nX = "<<p<<endl;p = &TEST::nY;printf("offest nY = %d\n", p);cout<<"offest nY = "<<p<<endl;p = &TEST::nZ;printf("offest nZ = %d\n", p);cout<<"offest nXZ = "<<p<<endl;p = NULL;printf("offest NULL = %d\n", p);cout<<"offest NULL = "<<p<<endl;return 0;}