C 中是否允许负数组索引? [英] Are negative array indexes allowed in C?

问题描述

我只是在阅读一些代码，发现此人正在使用 arr[-2] 访问 arr 之前的第二个元素，如下所示:

I was just reading some code and found that the person was using arr[-2] to access the 2nd element before the arr, like so:

|a|b|c|d|e|f|g|
       ^------------ arr[0]
         ^---------- arr[1]
   ^---------------- arr[-2]

允许吗?

我知道 arr[x] 和 *(arr + x) 是一样的.所以 arr[-2] 是 *(arr - 2)，这看起来没问题.你怎么看?

I know that arr[x] is the same as *(arr + x). So arr[-2] is *(arr - 2), which seems OK. What do you think?

推荐答案

这是正确的.来自 C99 §6.5.2.1/2:

That is correct. From C99 §6.5.2.1/2:

下标的定义运算符 [] 是 E1[E2] 是等同于 (*((E1)+(E2))).

The definition of the subscript operator [] is that E1[E2] is identical to (*((E1)+(E2))).

没有魔法.这是一个 1-1 的等价关系.在取消引用指针 (*) 时，您需要确保它指向有效地址.

There's no magic. It's a 1-1 equivalence. As always when dereferencing a pointer (*), you need to be sure it's pointing to a valid address.

这篇关于C 中是否允许负数组索引?的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！