数组大小的宏拒绝指针 [英] Array-size macro that rejects pointers

问题描述

这是经常教导标准数组大小宏

The standard array-size macro that is often taught is

#define ARRAYSIZE(arr) (sizeof(arr) / sizeof(arr[0]))

或某些等价形式。然而，这种事情时，指针传递在默默的成功，并且给出的结果，可以在运行时似乎是可信直到事情神秘地土崩瓦解。

or some equivalent formation. However, this kind of thing silently succeeds when a pointer is passed in, and gives results that can seem plausible at runtime until things mysteriously fall apart.

这一切都太容易犯这个错误：具有本地数组变量重构功能，动了一下数组处理成调用数组作为参数新功能

It's all-too-easy to make this mistake: a function that has a local array variable is refactored, moving a bit of array manipulation into a new function called with the array as a parameter.

所以，问题是：是否有卫生宏在编译时pferably检测 ARRAYSIZE 宏滥用在C，$ P $？在C ++中，我们只希望使用专门用于仅数组参数的模板;在C，看来我们需要一些方法来区分数组和指针。 （如果我想拒绝的阵列，比如，我只是做例如（ARR =改编，...）因为数组赋值是非法的）。

So, the question is: is there a "sanitary" macro to detect misuse of the ARRAYSIZE macro in C, preferably at compile-time? In C++ we'd just use a template specialized for array arguments only; in C, it seems we'll need some way to distinguish arrays and pointers. (If I wanted to reject arrays, for instance, I'd just do e.g. (arr=arr, ...) because array assignment is illegal).

推荐答案

Linux内核使用一个很好的实施 ARRAY_SIZE 来解决这个问题：

Linux kernel uses a nice implementation of ARRAY_SIZE to deal with this issue:

#define ARRAY_SIZE(arr) (sizeof(arr) / sizeof((arr)[0]) + __must_be_array(arr))

与

#define __must_be_array(a) BUILD_BUG_ON_ZERO(__same_type((a), &(a)[0]))

和

#define __same_type(a, b) __builtin_types_compatible_p(typeof(a), typeof(b))

当然，这仅仅是在GNU C便携，因为它利用了两个instrinsics的：
   的typeof 运营商和 __ builtin_types_compatible_p 功能。此外，它使用自己的名牌 BUILD_BUG_ON_ZERO 宏这是只有在GNU C。

Of course this is portable only in GNU C as it makes use of two instrinsics: typeof operator and __builtin_types_compatible_p function. Also it uses their "famous" BUILD_BUG_ON_ZERO macro which is only valid in GNU C.

假设一个编译时评估的要求（这是我们想要的），我不知道任何便携执行这个宏的。

Assuming a compile time evaluation requirement (which is what we want), I don't know any portable implementation of this macro.

一个半便携的实施（且不会涵盖所有情况下）是：

A "semi-portable" implementation (and which would not cover all cases) is:

#define ARRAY_SIZE(arr)  \
    (sizeof(arr) / sizeof((arr)[0]) + STATIC_EXP(IS_ARRAY(arr)))

与

#define IS_ARRAY(arr)  ((void*)&(arr) == &(arr)[0])
#define STATIC_EXP(e)  \
    (0 * sizeof (struct { int ARRAY_SIZE_FAILED:(2 * (e) - 1);}))

使用 GCC 这一点，如果参数是 -std = C99 -Wall ，但<$数组没有给出警告C $ C> -pedantic 将发出警告。原因是 IS_ARRAY 前pression是不是一个整数常量前pression（转换为指针类型和下标操作者不得在整型常量前pressions）和 STATIC_EXP 位字段宽度需要整型常量前pression。

With gcc this gives no warning if argument is an array in -std=c99 -Wall but -pedantic would gives a warning. The reason is IS_ARRAY expression is not an integer constant expression (cast to pointer types and subscript operator are not allowed in integer constant expressions) and the bit-field width in STATIC_EXP requires an integer constant expression.

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