这是有效的ANSI C ++代码吗?尝试在编译时生成结构成员的偏移量

问题描述

    #define _OFFS_OF_MEMBER(p_type, p_member) (size_t)&(((p_type *)NULL)->p_member)

    struct a
    {
             int a, b;
    };

    size_t l = _OFFS_OF_MEMBER(struct a, b);

我与一些其他用户进行了一些聊天/交谈，其中一个用户说这是取消引用并访问地址NULL附近的地址空间.我说过:获取成员的地址将不会访问，触摸或读取该成员的值.根据标准，它是完全安全的.

I had a little chat/conversation with some fellow users, and one of them said that this is dereferencing and accessing the address space near address NULL. I said: taking an address of a member will not access, touch, or read the value of that member. According to standard it is completely safe.

    struct a* p = NULL;
    size_t offset = &p->b; // this may NOT touch b, it is not dereferencing
    // p->b = 0; // now, we are dereferincing: acccess violation time!

这始终是计算偏移量的安全方法，还是编译器可以根据标准自由地取消引用并弄乱地址NULL附近的内存?

Is this always a safe way to calculate offset, or are compilers free to dereference and mess up the memory near address NULL according to standards?

我知道有一种安全的方法可以计算标准所提供的偏移量，但是我很好奇您对此有何评论.所有人都赞成我的夸奖:投票赞成这个问题:-)

I know there is a safe way to calculate offsets provided by the standard, but I am curious what you have to say about this. All in favor of my explenation: up-vote this question :-)

推荐答案

您不会在这里取消引用任何无效的内容.宏所做的全部工作就是告诉编译器，地址为NULL的内存中存在类型为p_type的结构.然后，它使用p_member的地址，该地址是此虚拟结构的成员.因此，在任何地方都不会取消引用.

You're not dereferencing anything invalid here. All that macro does is tell the compiler that a structure of type p_type exists in memory at the address NULL. It then takes the address of p_member, which is a member of this fictitious structure. So, no dereferencing anywhere.

实际上，这正是 offsetof 宏的定义在stddef.h中.

正如某些评论所言，这可能不适用于C ++和继承，我只对C中的POD结构使用offsetof.

As some of the comments say, this may not work well with C++ and inheritance, I've only used offsetof with POD structures in C.

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