C++20 范围和 std::views::take 出错

Question

我正在尝试使用 C++20 范围，并且在使用 GCC 11.1.0 和 CMake 3.20.3 进行编译时出现以下奇怪行为。 具体来说，以下代码无法编译：

auto Foo() {
    std::vector<long int> x{1, 2, 3, 4, 5, 6};
    return std::views::all(x) | std::views::take(x.size());
    // return std::views::all(x) | std::views::take(static_cast<int>(x.size()));
}

导致非常长的错误消息，例如

[...] error: no match for ‘operator|’ (operand types are ‘std::ranges::ref_view<std::vector<long int> >’ and ‘std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>’)
[build]   230 |     return std::views::all(x) | std::views::take(x.size());
[build]       |            ~~~~~~~~~~~~~~~~~~ ^ ~~~~~~~~~~~~~~~~~~~~~~~~~~
[build]       |                           |                     |
[build]       |                           |                     std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>
[build]       |                           std::ranges::ref_view<std::vector<long int> >

和

[build] /usr/include/c++/11/ranges:739:13:   required for the satisfaction of ‘__adaptor_invocable<_Self, _Range>’ [with _Self = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>; _Range = std::ranges::ref_view<std::vector<long int, std::allocator<long int> > >]
[build] /usr/include/c++/11/ranges:740:9:   in requirements  [with _Args = {std::ranges::ref_view<std::vector<long int, std::allocator<long int> > >}; _Adaptor = std::ranges::views::__adaptor::_Partial<std::ranges::views::_Take, long unsigned int>]
[build] /usr/include/c++/11/ranges:740:44: note: the required expression ‘declval<_Adaptor>()((declval<_Args>)()...)’ is invalid, because
[build]   740 |       = requires { std::declval<_Adaptor>()(declval<_Args>()...); };
[build]       |                    ~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~
[build] /usr/include/c++/11/ranges:740:44: error: use of deleted function ‘constexpr auto std::ranges::views::__adaptor::_Partial<_Adaptor, _Arg>::operator()(_Range&&) const && [with _Range = std::ranges::ref_view<std::vector<long int> >; _Adaptor = std::ranges::views::_Take; _Arg = long unsigned int]’
[build] /usr/include/c++/11/ranges:862:9: note: declared here
[build]   862 |         operator()(_Range&& __r) const && = delete;
[build]       |         ^~~~~~~~
[build] In file included from /usr/include/c++/11/streambuf:41,
[build]                  from /usr/include/c++/11/bits/streambuf_iterator.h:35,
[build]                  from /usr/include/c++/11/iterator:66,
[build]                  from /usr/include/c++/11/ranges:43,
[build]                  from ../mmpc/test/parametric_controller.test.cpp:27:
[build] /usr/include/c++/11/bits/ios_base.h:87:3: note: candidate: ‘constexpr std::_Ios_Fmtflags std::operator|(std::_Ios_Fmtflags, std::_Ios_Fmtflags)’
[build]    87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:87:27: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Fmtflags’
[build]    87 |   operator|(_Ios_Fmtflags __a, _Ios_Fmtflags __b)
[build]       |             ~~~~~~~~~~~~~~^~~
[build] /usr/include/c++/11/bits/ios_base.h:129:3: note: candidate: ‘constexpr std::_Ios_Openmode std::operator|(std::_Ios_Openmode, std::_Ios_Openmode)’
[build]   129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:129:27: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Openmode’
[build]   129 |   operator|(_Ios_Openmode __a, _Ios_Openmode __b)
[build]       |             ~~~~~~~~~~~~~~^~~
[build] /usr/include/c++/11/bits/ios_base.h:169:3: note: candidate: ‘constexpr std::_Ios_Iostate std::operator|(std::_Ios_Iostate, std::_Ios_Iostate)’
[build]   169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
[build]       |   ^~~~~~~~
[build] /usr/include/c++/11/bits/ios_base.h:169:26: note:   no known conversion for argument 1 from ‘std::ranges::ref_view<std::vector<long int> >’ to ‘std::_Ios_Iostate’
[build]   169 |   operator|(_Ios_Iostate __a, _Ios_Iostate __b)
[build]       |             ~~~~~~~~~~~~~^~~

但是，当我在注释行中将x的大小转换为long int时，一切都编译得很好。 这种行为是预期的吗？ 还是我的编译器设置有问题？

最终，当我使用 Clang 12.0.1 时，根本没有编译，我收到以下错误：

[build] /usr/bin/../lib/gcc/x86_64-linux-gnu/11/../../../../include/c++/11/ranges:3392:19: error: missing 'typename' prior to dependent type name 'iterator_traits<iterator_t<_Base>>::iterator_category'
[build]             using _Cat = iterator_traits<iterator_t<_Base>>::iterator_category;
[build]                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

Clang 12.0.1 不应该支持包括范围在内的 C++20 的特性吗？ 为什么我会收到此错误？

Answer 1

这是一个库错误，将由P2367 修复，因为它被指定为执行take_view{E, F} （这将是一个缩小转换）而不是仅仅take_view(E, F) （这很好）。 与此同时，您必须相应地进行演员阵容。

---

还有这个：

std::views::all(x) | std::views::take(x.size());

只是很长的一段写法：

x | std::views::take(x.size());

您应该很少（如果有的话？）需要在用户代码中编写views::all 。 图书馆为你做这件事。

---

Clang 12.0.1 不应该支持包括范围在内的 C++20 的特性吗？ 为什么我会收到此错误？

这与范围无关，clang 只是不完全支持 libstdc++ 在其实现中使用的 C++20 语言功能（在这种情况下，使用类型名 Down！ ）。