#include<初始化列表>需要在基于范围的 for 中使用初始值设定项列表吗？

发布于 2024-12-27 14:12:31 字数 1097 浏览 1 评论 0原文

最终的 C++11 标准包括基于范围的 for 的规定，以便“仅适用于”本机数组，而无需包含或任何其他标头。据我所知，这个问题首先在工作文件 n2900 作为 UK 78 和 79 评论的结果。

该提案还包含隐式 <code>#include 的规定每个翻译单元中的，因此即使

#include <iostream>

int main()
{
    for (auto i : { 1, 2, 3, 4, 5 })
        std::cout << i << "\n";
}

不包含，程序也将符合标准。

然而，当概念从 C++11 中删除时，基于范围的 for 被修改，如 n2930。虽然数组“正常工作”的规定仍然存在，但没有提及初始化列表也是如此；事实上，各种标准库容器标头将 #include 的规范和 8.5.4.2 的最终文本对我来说意味着相反的内容。

据我所知，这与该主题的最终措辞非常接近。那么，上面的程序是否符合最终标准，或者我是否需要 #include 甚至在基于范围的 for 中使用它？换句话说，在基于范围的 for 中使用初始化列表是否构成“std::initializer_list 的使用——甚至是类型未命名的隐式使用”（根据 8.5） .4.2 FDIS？

原文

The final C++11 standard includes provisions for range-based for to "just work" for native arrays without having to include <iterator> or any other header. This was addressed first, as far as I can tell, in working paper n2900 as a result of comments UK 78 and 79.

This proposal also included a provision to implicitly #include <initializer_list> in every translation unit, so that e.g. the program

#include <iostream>

int main()
{
    for (auto i : { 1, 2, 3, 4, 5 })
        std::cout << i << "\n";
}

would be standard-conforming even without including <initializer_list>.

However, when concepts were removed from C++11, range-based for was revised as seen in n2930. While the provision for arrays to "just work" remains, there is no mention that the same is true for initializer lists; indeed the specification that various standard library container headers will #include <initializer_list> and the final text of 8.5.4.2 implies the opposite to me.

As far as I can tell, this is pretty close to the final wording on the topic. So, is the program above well-formed with regard to the final standard, or do I need to #include <initializer_list> even to use it in a range-based for? Put another way, does the use of an initializer list in a range-based for constitute a "use of std::initializer_list---even an implicit use in which the type is not named" per 8.5.4.2 of the FDIS?

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想挽留 2025-01-03 14:12:31

我会说是的。根据 §6.5.4[stmt.ranged]/1，该语句

for (auto i : { 1, 2, 3, 4, 5 })
    ...

相当于

auto&& __range = { 1, 2, 3, 4, 5 };
...

和这意味着使用 initializer_list，并且需要包含标题。

I'd say yes. According to §6.5.4[stmt.ranged]/1, the statement

for (auto i : { 1, 2, 3, 4, 5 })
    ...

is just equivalent to

auto&& __range = { 1, 2, 3, 4, 5 };
...

and that means an initializer_list<int> is used, and the <initializer_list> header needs to be included.

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初熏 2025-01-03 14:12:31

如果不包含，GCC 7.1 会产生以下错误：

error: deducing from brace-enclosed initializer list requires #include <initializer_list>
     for (auto i : { 1, 2, 3, 4, 5 })
                                   ^

要查看此错误，应省略，因为包含 会产生以下错误： 还将包括。

GCC 7.1 produces the following error if <initializer_list> is not included:

error: deducing from brace-enclosed initializer list requires #include <initializer_list>
     for (auto i : { 1, 2, 3, 4, 5 })
                                   ^

To see this error one should omit <iostream>, because including <iostream> will also include <initializer_list>.

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