ARRAYSIZE C ++宏：它是如何工作的？ [英] ARRAYSIZE C++ macro: how does it work?

问题描述

好的，我不是完全新手，但我不能说我理解下面的宏。最令人困惑的部分是将价值除以size_t：什么是完成的？特别是，因为我看到一个否定运算符，据我所知，可能会导致一个零值。这不是意味着它可以导致一个除以0的错误？ （顺便说一下，宏是正确的，并且工作得很漂亮。）

OK, I'm not entirely a newbie, but I cannot say I understand the following macro. The most confusing part is the division with value cast to size_t: what on earth does that accomplish? Especially, since I see a negation operator, which, as far as I know, might result in a zero value. Does not this mean that it can lead to a division-by-zero error? (By the way, the macro is correct and works beautifully.)

#define ARRAYSIZE(a) \
  ((sizeof(a) / sizeof(*(a))) / \
  static_cast<size_t>(!(sizeof(a) % sizeof(*(a)))))

推荐答案

第一部分 sizeof（a）/ sizeof（*（a）））是相当简单;它将整个数组的大小（假设你传递一个数组类型的对象，而不是一个指针）的大小除以第一个元素的大小。这给出了数组中元素的数量。

The first part (sizeof(a) / sizeof(*(a))) is fairly straightforward; it's dividing the size of the entire array (assuming you pass the macro an object of array type, and not a pointer), by the size of the first element. This gives the number of elements in the array.

第二部分不是那么简单。我认为潜在的除零是故意的;如果由于某种原因，数组的大小不是其中一个元素的整数倍，它将导致编译时错误。换句话说，它是某种编译时的健全检查。

The second part is not so straightforward. I think the potential division-by-zero is intentional; it will lead to a compile-time error if, for whatever reason, the size of the array is not an integer multiple of one of its elements. In other words, it's some kind of compile-time sanity check.

但是，我看不出在什么情况下可能发生...正如人们建议的下面，它会捕获一些误用（如在指针上使用 ARRAYSIZE（））。但它不会捕获所有的错误，

However, I can't see under what circumstances this could occur... As people have suggested in comments below, it will catch some misuse (like using ARRAYSIZE() on a pointer). It won't catch all errors like this, though.

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