C / C ++结构失调 [英] C/C++ Structure offset

问题描述

我找了一块code，可以告诉我一个字段的偏移结构中不分配结构的一个实例。

I'm looking for a piece of code that can tell me the offset of a field within a structure without allocating an instance of the structure.

IE浏览器：给定

struct mstct {
    int myfield;
    int myfield2;
};

我可以写：

mstct thing;
printf("offset %lu\n", (unsigned long)(&thing.myfield2 - &thing));

和得到弥补4的输出。我怎么能做到这一点不说，mstct事报关/分配呢？

And get "offset 4" for the output. How can I do it without that "mstct thing" declaration/allocating one?

我知道，和放大器;＆LT;＆结构GT;并不总是在结构的第一场的第一个字节指出，我可以占到稍后

I know that &<struct> does not always point at the first byte of the first field of the structure, I can account for that later.

推荐答案

如何offsetof（）宏（在STDDEF.H）标准？

How about the standard offsetof() macro (in stddef.h)?

编辑：人谁可能没有提供某种原因offsetof（）宏，您使用类似可以得到的效果：

for people who might not have the offsetof() macro available for some reason, you can get the effect using something like:

#define OFFSETOF(type, field)    ((unsigned long) &(((type *) 0)->field))

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