问题描述
经常教的标准数组大小的宏是
The standard array-size macro that is often taught is
#define ARRAYSIZE(arr) (sizeof(arr) / sizeof(arr[0]))
或某种等效的形式.然而，当传入一个指针时，这种事情会悄悄地成功，并给出在运行时看似合理的结果，直到事情神秘地崩溃.
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.
犯这个错误太容易了:一个具有局部数组变量的函数被重构，将一些数组操作移到一个以数组作为参数调用的新函数中.
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.
那么，问题是:是否有一个卫生"宏来检测 C 中 ARRAYSIZE 宏的滥用，最好是在编译时?在 C++ 中，我们只使用专门用于数组参数的模板；在 C 中，似乎我们需要某种方法来区分数组和指针.(例如，如果我想拒绝数组，我只会做例如 (arr=arr, ...) 因为数组分配是非法的).
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 内核使用了一个很好的 ARRAY_SIZE 实现来处理这个问题:
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，这仅在 GNU C 中是可移植的，因为它使用了两个内在函数:typeof 运算符和 __builtin_types_compatible_p 函数.它还使用了他们著名的"BUILD_BUG_ON_ZERO 宏，该宏仅在 GNU C 中有效.
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.
假设有一个编译时评估要求(这是我们想要的)，我不知道这个宏的任何可移植实现.
Assuming a compile time evaluation requirement (which is what we want), I don't know any portable implementation of this macro.
半便携式"实现(并不会涵盖所有情况)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 如果参数是 -std=c99 -Wall 中的数组，则不会发出警告，但 -pedantic 会发出警告.原因是 IS_ARRAY 表达式不是整数常量表达式(整数常量表达式中不允许转换为指针类型和下标运算符)并且 STATIC_EXP 中的位域宽度需要一个整数常量表达式.
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.
这篇关于拒绝指针的数组大小的宏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！