Range-V3：获取基本迭代器

Question

I try to use Range-V3 library (for MSVC), but due to lack of documentation I don't understand how to do one thing.

std::map<int, std::wstring> ss = { {1,L"1"}, {2,L"2"}, {3,L"3"} };
auto rng = ss | ranges::view::reverse | ranges::view::values;
auto it = ranges::find_if(rng, [](auto&&x) {return x == L"2"; });
if (it != rng.end()) {
    assert(it.base()->first == 2); // this does not compile
}

What do I get from find_if ? Is this iterator? I want to get an iterator to base element, ie to value in ss map.

Here is the error I get:

1>d:\sources\ranges_test\ranges_test.cpp(11): error C2819: type 'ranges::v3::basic_iterator<ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>,ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>>' does not have an overloaded member 'operator ->'
1>        with
1>        [
1>            _Kty=int,
1>            _Ty=std::wstring
1>        ]
1>d:\sources\fingrad\dev.fingrad\src\vc\lib\range\v3\utility\basic_iterator.hpp(656): note: see declaration of 'ranges::v3::basic_iterator<ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>,ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>>'
1>        with
1>        [
1>            _Kty=int,
1>            _Ty=std::wstring
1>        ]
1>d:\sources\ranges_test\ranges_test.cpp(11): note: did you intend to use '.' instead?
1>d:\sources\ranges_test\ranges_test.cpp(11): error C2039: 'first': is not a member of 'ranges::v3::basic_iterator<ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>,ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>>'
1>        with
1>        [
1>            _Kty=int,
1>            _Ty=std::wstring
1>        ]
1>d:\sources\fingrad\dev.fingrad\src\vc\lib\range\v3\utility\basic_iterator.hpp(656): note: see declaration of 'ranges::v3::basic_iterator<ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>,ranges::v3::adaptor_cursor<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,ranges::v3::reverse_view<ranges::v3::sized_range<std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>,std::_Tree_iterator<std::_Tree_val<std::_Tree_simple_types<std::pair<const _Kty,_Ty>>>>>>::adaptor<true>>>'
1>        with
1>        [
1>            _Kty=int,
1>            _Ty=std::wstring
1>        ]

Answer 1

This looks like a bug ¹ in MSVC implementation of range-V3 (or rather a missing feature). An easy workaround would be to replace your code with:

assert((*it.base()).first == 2);

This should work because operator* is overload for basic_iterator while operator-> was not.

---

¹ If you look at MSVC source for basic_iterator , you will notice that there are no overload for operator-> , while there is one in the original range-v3 basic_iterator .