英文:
Does std::ranges::views::enumerate use the wrong type for indexing on GCC?
问题
首先，使用Range-v3，我注意到以下代码：
std::vector<int> v;
auto w = v | ranges::views::enumerate;
auto [x, y] = *w.begin();
static_assert(std::is_same_v<std::size_t, decltype(x)>);
这对我来说是有道理的，因为根据我所理解的，std::size_t 是用于索引对象的正确类型。
然后，使用C++20的范围，静态断言失败。相反，以下内容通过了：
static_assert(std::is_same_v<long, decltype(x)>);
但是在我的系统上，long 与 std::size_t 不同，我验证了以下内容：
static_assert(std::is_same_v<unsigned long, std::size_t>)
在GCC 13.2.1上，这是示例。
英文:
First, using Range-v3, I noticed that
std::vector<int> v;
auto w = v | ranges::views::enumerate;
auto [x, y] = *w.begin();
static_assert(std::is_same_v<std::size_t, decltype(x)>);
which makes sense to me because std::size_t is the right type to use for indexing into objects, as far as I've understood.
Then, with C++20 ranges, the static assertion fails. Instead, the following passes,
static_assert(std::is_same_v<long, decltype(x)>);
but long is not the same as std::size_t on my system, where I verified that
static_assert(std::is_same_v<unsigned long, std::size_t>)
On, GCC 13.2.1, here's the example.
答案1
得分: 4
No.
std::views::enumerate 会使用 range_difference_t<Base> 作为其索引值，gcc 的实现符合规范。
然而，range-v3 的实现使用 make_unsigned<range_difference_t<Base>> 作为其索引值。在这种情况下，使用有符号类型似乎更合适，因为你可能希望减法运算是明智的。尽管在 P2164（或其任何修订版）中，我看不到关于这个选择的讨论。
但是，在我的系统上，long 不等同于 std::size_t。
在任何系统上，long 和 std::size_t 不相同。long 是有符号的，而 size_t 是无符号的。
英文:
> Does std::ranges::views::enumerate use the wrong type for indexing on GCC?
No.
std::views::enumerate is specified to use range_difference_t<Base> as its index value. gcc implementation is correct to the specification.
On the other hand, range-v3's implementation used make_unsigned<range_difference_t<Base>> as its index value.
Signed seems like a much better choice here, given that you probably want subtraction to be sensible. Although there's no discussion of this choice as far as I can see in P2164 (or any revision thereof).
> but long is not the same as std::size_t on my system
long is not the same as std::size_t on any system. long is signed and size_t is unsigned.
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