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

问题描述

我只是读一些code，发现该人已被使用改编[-2] 之前访问第二个元素的 ARR ，就像这样：

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]

是允许的？

我知道改编[X] 相同 *（ARR + X）。因此，改编[-2] 是 *（ARR - 2），这似乎确定。你觉得呢？

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。从C99§6.5.2.1/ 2：

That is correct. From C99 §6.5.2.1/2:

标的德音响nition
  运算符[]是E1 [E2]是
  相同（*（（E1）+（E2）））。

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

有什么神奇的。这是一个1-1的等价。由于取消引用指针（*）总是时，你需要确保它指向一个有效的地址。

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.

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