零长度C-阵列结合指针类型 [英] Zero-length C-array binds to pointer type

问题描述

我最近写了一个函数模板这需要一个C数组的引用：

I recently wrote a function template which takes a reference to a C-array:

template <class T, size_t N>
void foo(T(&c_array)[N]);

假设T是一个字符的C字符串的长度 N - 1 由于空终止子。我意识到，我也许应该处理的边缘情况下ñ== 0 ，因为那时 N - 1 是的std :: numeric_limits＆LT;的std ::为size_t＆GT; :: MAX（）。

Assuming T is a char, the length of the C-string is N - 1 due to the null-terminator. I realized I should probably handle the edge-case where N == 0, because then N - 1 would be std::numeric_limits<std::size_t>::max().

因此​​，为了避免可能在极少数情况下随之而来，有人传递一个零长度数组此功能的混乱，我把一张支票ñ== 0 。

So in order to avoid the chaos that might ensue in the rare case that someone passes a zero-length array to this function, I placed a check for N == 0.

不过，出乎我的意料，似乎一个长度为零的数组实际上是连数组类型 - 或者至少，这就是GCC似乎相信。事实上，一个零长度数组不连的绑定的上述功能的签名，如果有一个指针类型签名功能可作为候选。

However, to my surprise, it seems that a zero-length array is actually not even an array type - or at least, that's what GCC seems to believe. In fact, a zero-length array doesn't even bind to the above function signature, if a function with a pointer-type signature is available as a candidate.

考虑以下code：

template <class T, size_t N>
void foo(T(&array)[N])
{
    std::cout << "Array" << std::endl;
}

void foo(const void* p)
{
    std::cout << "Pointer" << std::endl;
}

int main(int argc, char** argv)
{
    char array1[10] = { };
    const char* pointer = 0;
    char array2[0] = { };

    foo(array1);
    foo(pointer);
    foo(array2);
}

使用GCC 4.3.2，这个输出：

With GCC 4.3.2, this outputs:

Array
Pointer
Pointer

奇怪的是，零长度数组prefers绑定到需要的指针类型的功能。所以，这是海湾合作委员会中的错误，或者是有由C无授权的一些​​模糊的原因++标准，为什么这种行为是必要的吗？

Oddly, the zero-length array prefers to bind to the function that takes a pointer type. So, is this a bug in GCC, or is there some obscure reason mandated by the C++ standard why this behavior is necessary?

推荐答案

由于数组必须具有大于零的长度，如果你的编译器错误地接受零大小的数组的定义，那么你很安全的范围之外的语言标准。有没有必要为你处理的边缘情况ñ== 0 。

As arrays must have greater than zero length, if your compiler erroneously accepts a definition of a zero-sized array then you're "safely" outside of the scope of the language standard. There's no need for you to handle the edge case of N == 0.

这是用C ++真：8.3.5 [dcl.array]：如果恒恩pression（5.19）是present，它应是一个整型常量前pression及其价值应大于零。

This is true in C++: 8.3.5 [dcl.array]: If the constant-expression (5.19) is present, it shall be an integral constant expression and its value shall be greater than zero.

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