Question

我尝试使用Range-V3库（用于MSVC），但由于缺少文档，我不明白如何做一件事。

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
}

我从find_if获得了什么？这是迭代器吗？我希望得到一个基本元素的迭代器，即在ss map中得到值。

这是我得到的错误：

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

这看起来像范围V3的MSVC implementation中的错误¹（或者更确切地说是缺失的特征）。一个简单的解决方法是用以下代码替换您的代码：

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

这应该有效，因为basic_iterator operator->的重载是operator->而$posts = $this->Posts->find() ->select() ->join([ 'a' => [ 'table' => 'animals', 'type' => 'INNER', 'conditions' =>'a.id = Posts.animal_id' ], 'u' => [ 'table' => 'users', 'type' => 'INNER', 'conditions' =>'u.id = a.user_id' ] ]) ->where(['u.id' => '2', 'a.id'=> $jqueryanimalvalue //here is null ]);没有。{/ p>

¹如果您查看operator*的MSVC来源，您会注意到$posts = $this->Posts->find() ->select() ->join([ 'a' => [ 'table' => 'animals', 'type' => 'INNER', 'conditions' => [ 'a.id = Posts.animal_id', 'a.id IS ' . $jqueryanimalvalue // if it is null ] ], 'u' => [ 'table' => 'users', 'type' => 'INNER', 'conditions' => [ 'u.id = a.user_id', 'u.id' => '2' ] ] ]);没有重载，而原始范围中有一个-v3 { {3}}

Range-V3：获取基本迭代器

1 个答案: