#define offsetof(s,m) (size_t)&reinterpret_cast<const volatile char&>((((s *)0)->m))
该宏用于求结构体中一个成员在该结构体中的偏移量。第一个参数是结构体的名字，第二个参数是结构体成员的名字。该宏返回结构体structName s中成员memberName(m)的偏移量。偏移量是size_t类型的。
offsetof returns the offset in bytes of the specified member from the beginning of its parent data structure. It is undefined for bit fields.
示例
#include <stdio.h>
#include <stddef.h>
typedef struct
{
int iVal;
int iVal2;
}Test;
typedef struct
{
char ch;
int iNum;
}Test2;
int main(void)
{
Test t = {1, 2};
Test2 t2 = {'t', 100};
printf("\naddress of t : %p\naddress of t.iVal : %p\naddress of t.iVal2: %p\n\n", &t, &(t.iVal), &(t.iVal2));
printf("offset of iVal in t: %p\n", offsetof(Test, iVal));
printf("offset of iVal2 in t: %p\n", offsetof(Test, iVal2));
printf("\naddress of t2 : %p\naddress of t2.ch : %p\naddress of t2.iNum: %p\n\n", &t, &(t2.ch), &(t2.iNum));
printf("offset of ch in t2: %p\n", offsetof(Test2, ch));
printf("offset of iNum in t2: %p\n", offsetof(Test2, iNum));
return 0;
}
注意内存对齐。
原文
http://www.cppblog.com/lovedday/archive/2007/09/24/32801.html
http://baike.baidu.com/view/5513779.htm