我有一对迭代器,我想使用ranges::views::filter(some_predicate)
就可以了(使用管道操作符)。 AFAIU 我应该首先将我的迭代器对转换为视图。我尝试使用ranges::subrange(first, last)
这样做,但我收到了可怕的错误消息。
注1:我使用的是C++14和range-v3版本0.9.1(与gcc-5.5兼容的最后一个版本)。如果使用 C++17/20 和/或使用 C++20 std::ranges 时解决方案有所不同,我也有兴趣知道发生了什么变化。
注2:我发现文档 https://ericniebler.github.io/range-v3/range-v3 严重缺乏,所以我正在使用cppreference.com https://en.cppreference.com/w/cpp/ranges/filter_view。如果您知道更好的文档,我非常感兴趣。
EDIT:
在我的真实代码中,我包装了一个 java 风格的遗留迭代器(它有一个next()
方法而不是operator++
/operator*
。我将它们包装在 C++ 兼容的包装器中。然后我尝试将该包装器转换为视图,最后对其进行过滤。我重现了一个最小的例子godbolt https://godbolt.org/z/n7W5rKxra。这个用iterator_range
按照建议,但它仍然无法编译(请参阅下面的第二次编辑)。
#include "range/v3/all.hpp"
#include "range/v3/iterator_range.hpp"
class LegacyIteratorWrapper {
public:
using value_type = int;
using difference_type = std::ptrdiff_t;
using pointer = value_type*;
using reference = value_type&;
using iterator_category = std::input_iterator_tag;
// The type isn’t default-constructible, the error comes from here
LegacyIteratorWrapper() = delete;
static LegacyIteratorWrapper create();
reference operator*() const;
pointer operator->();
LegacyIteratorWrapper& operator++();
LegacyIteratorWrapper operator++(int);
friend bool operator==(const LegacyIteratorWrapper& a, const LegacyIteratorWrapper& b);
friend bool operator!=(const LegacyIteratorWrapper& a, const LegacyIteratorWrapper& b);
};
void foo()
{
LegacyIteratorWrapper begin { LegacyIteratorWrapper::create() };
LegacyIteratorWrapper end { LegacyIteratorWrapper::create() };
ranges::iterator_range<LegacyIteratorWrapper, LegacyIteratorWrapper> rng {begin, end};
auto _ = rng
| ranges::views::filter(
[&](auto _) { return true; }
)
;
}
In file included from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/reference_wrapper.hpp:24:0,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/detail/variant.hpp:33,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/iterator/common_iterator.hpp:26,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/interface.hpp:24,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/ref.hpp:25,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action/action.hpp:29,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action.hpp:17,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/all.hpp:17,
from <source>:1:
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/pipeable.hpp: In instantiation of 'constexpr auto ranges::operator|(Arg&&, Pipe) [with Arg = ranges::iterator_range<LegacyIteratorWrapper, LegacyIteratorWrapper>&; Pipe = ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_>; bool CPP_false_ = false; typename concepts::detail::identity<typename std::enable_if<(static_cast<bool>(((! is_pipeable_v<Arg>) && is_pipeable_v<Pipe>)) || CPP_false_), void>::type>::invoke<int> <anonymous> = 0]':
<source>:33:11: required from here
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/pipeable.hpp:63:53: error: no matching function for call to 'ranges::pipeable_access::impl<ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_> >::pipe(ranges::iterator_range<LegacyIteratorWrapper, LegacyIteratorWrapper>&, ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_>&)'
return pipeable_access::impl<Pipe>::pipe(static_cast<Arg &&>(arg), pipe);
^
In file included from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/range_fwd.hpp:22:0,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action/action.hpp:21,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action.hpp:17,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/all.hpp:17,
from <source>:1:
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/view.hpp:114:35: note: candidate: template<class Rng, class Vw> static constexpr auto ranges::views::view<View>::pipe(Rng&&, Vw&&, concepts::detail::enable_if_t<concepts::detail::Nil, (static_cast<bool>((viewable_range<Rng> && invocable<View&, Rng>)) || concepts::detail::CPP_false(concepts::detail::Nil{}))>) [with Rng = Rng; Vw = Vw; View = ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_]
static constexpr auto CPP_fun(pipe)(Rng && rng, Vw && v)( //
^
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/view.hpp:114:35: note: template argument deduction/substitution failed:
<source>: In function 'void foo()':
<source>:33:11: error: 'void _' has incomplete type
)
^
ASM generation compiler returned: 1
In file included from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/reference_wrapper.hpp:24:0,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/detail/variant.hpp:33,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/iterator/common_iterator.hpp:26,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/interface.hpp:24,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/ref.hpp:25,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action/action.hpp:29,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action.hpp:17,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/all.hpp:17,
from <source>:1:
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/pipeable.hpp: In instantiation of 'constexpr auto ranges::operator|(Arg&&, Pipe) [with Arg = ranges::iterator_range<LegacyIteratorWrapper, LegacyIteratorWrapper>&; Pipe = ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_>; bool CPP_false_ = false; typename concepts::detail::identity<typename std::enable_if<(static_cast<bool>(((! is_pipeable_v<Arg>) && is_pipeable_v<Pipe>)) || CPP_false_), void>::type>::invoke<int> <anonymous> = 0]':
<source>:33:11: required from here
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/functional/pipeable.hpp:63:53: error: no matching function for call to 'ranges::pipeable_access::impl<ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_> >::pipe(ranges::iterator_range<LegacyIteratorWrapper, LegacyIteratorWrapper>&, ranges::views::view<ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_>&)'
return pipeable_access::impl<Pipe>::pipe(static_cast<Arg &&>(arg), pipe);
^
In file included from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/range_fwd.hpp:22:0,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action/action.hpp:21,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/action.hpp:17,
from /opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/all.hpp:17,
from <source>:1:
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/view.hpp:114:35: note: candidate: template<class Rng, class Vw> static constexpr auto ranges::views::view<View>::pipe(Rng&&, Vw&&, concepts::detail::enable_if_t<concepts::detail::Nil, (static_cast<bool>((viewable_range<Rng> && invocable<View&, Rng>)) || concepts::detail::CPP_false(concepts::detail::Nil{}))>) [with Rng = Rng; Vw = Vw; View = ranges::make_pipeable_fn::operator()(Fun) const [with Fun = ranges::detail::bind_back_fn_<ranges::views::cpp20_filter_fn, foo()::<lambda(auto:15)> >]::_]
static constexpr auto CPP_fun(pipe)(Rng && rng, Vw && v)( //
^
/opt/compiler-explorer/libs/rangesv3/0.9.1/include/range/v3/view/view.hpp:114:35: note: template argument deduction/substitution failed:
<source>: In function 'void foo()':
<source>:33:11: error: 'void _' has incomplete type
)
^
Execution build compiler returned: 1
EDIT2(已解决):我收到错误,因为LegacyIteratorWrapper
不可默认构造。这是满足常规特征(由 range-v3 模拟)所必需的,这是 C++ 迭代器所要求的,并且包括默认可构造。