使用 boost::spirit::x3 解析成向量<boost::string_view>

Posted 2023-02-22

【中文标题】使用 boost::spirit::x3 解析成向量<boost::string_view>

【英文标题】：parse into vector<boost::string_view> using boost::spirit::x3

【发布时间】：2016-08-30 14:25:17

【问题描述】：

这是我的previous one 关于boost::spirit::x3 和boost::string_view 的后续问题。

虽然我可以解析为 std::vector<std::string> (live example)，但解析为 std::vector<boost::string_view> 失败并出现以下编译错误：

#include <iostream>
#include <string>

#include <boost/utility/string_view.hpp>

namespace boost 
namespace spirit  namespace x3  namespace traits 

template <typename It>
void move_to(It b, It e, boost::string_view& v)

    v = boost::string_view(b, std::size_t(std::distance(b,e)));


  

 // namespace boost


#include <boost/spirit/home/x3.hpp>

namespace parser

    namespace x3 = boost::spirit::x3;
    using x3::char_;
    using x3::raw;

    const auto str_vec  = *(raw[ +~char_('_')] >> '_');


int main()

    std::string input = "hello_world_";

    std::vector<boost::string_view> strVec; 
    parse(input.data(), input.data()+input.size(), parser::str_vec, strVec);

    for(auto& x : strVec)  std::cout << x << std::endl; 

---

In file included from /usr/local/include/c++/6.1.0/bits/stl_tempbuf.h:60:0,

                 from /usr/local/include/c++/6.1.0/bits/stl_algo.h:62,

                 from /usr/local/include/c++/6.1.0/algorithm:62,

                 from /usr/local/include/boost/utility/string_view.hpp:27,

                 from main.cpp:4:

/usr/local/include/c++/6.1.0/bits/stl_construct.h: In instantiation of 'void std::_Construct(_T1*, _Args&& ...) [with _T1 = boost::basic_string_view<char, std::char_traits<char> >; _Args = const char&]':

/usr/local/include/c++/6.1.0/bits/stl_uninitialized.h:75:18:   required from 'static _ForwardIterator std::__uninitialized_copy<_TrivialValueTypes>::__uninit_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const char*; _ForwardIterator = boost::basic_string_view<char, std::char_traits<char> >*; bool _TrivialValueTypes = false]'

/usr/local/include/c++/6.1.0/bits/stl_uninitialized.h:126:15:   required from '_ForwardIterator std::uninitialized_copy(_InputIterator, _InputIterator, _ForwardIterator) [with _InputIterator = const char*; _ForwardIterator = boost::basic_string_view<char, std::char_traits<char> >*]'

/usr/local/include/c++/6.1.0/bits/stl_uninitialized.h:281:37:   required from '_ForwardIterator std::__uninitialized_copy_a(_InputIterator, _InputIterator, _ForwardIterator, std::allocator<_Tp>&) [with _InputIterator = const char*; _ForwardIterator = boost::basic_string_view<char, std::char_traits<char> >*; _Tp = boost::basic_string_view<char, std::char_traits<char> >]'

/usr/local/include/c++/6.1.0/bits/stl_vector.h:1288:33:   required from 'void std::vector<_Tp, _Alloc>::_M_range_initialize(_ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const char*; _Tp = boost::basic_string_view<char, std::char_traits<char> >; _Alloc = std::allocator<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/c++/6.1.0/bits/stl_vector.h:1261:4:   required from 'void std::vector<_Tp, _Alloc>::_M_initialize_dispatch(_InputIterator, _InputIterator, std::__false_type) [with _InputIterator = const char*; _Tp = boost::basic_string_view<char, std::char_traits<char> >; _Alloc = std::allocator<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/c++/6.1.0/bits/stl_vector.h:406:11:   [ skipping 9 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]

/usr/local/include/boost/spirit/home/x3/operator/detail/sequence.hpp:496:24:   required from 'static bool boost::spirit::x3::detail::parse_into_container_impl<boost::spirit::x3::sequence<L, R>, Context, RContext>::call(const parser_type&, Iterator&, const Iterator&, const Context&, RContext&, Attribute&) [with Iterator = const char*; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >; Left = boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >; Right = boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; boost::spirit::x3::detail::parse_into_container_impl<boost::spirit::x3::sequence<L, R>, Context, RContext>::parser_type = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >]'

/usr/local/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:287:74:   required from 'bool boost::spirit::x3::detail::parse_into_container(const Parser&, Iterator&, const Iterator&, const Context&, RContext&, Attribute&) [with Parser = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >; Iterator = const char*; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/boost/spirit/home/x3/operator/kleene.hpp:32:48:   required from 'bool boost::spirit::x3::kleene<Subject>::parse(Iterator&, const Iterator&, const Context&, RContext&, Attribute&) const [with Iterator = const char*; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >; Subject = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >]'

/usr/local/include/boost/spirit/home/x3/core/parse.hpp:35:68:   required from 'bool boost::spirit::x3::parse_main(Iterator&, Iterator, const Parser&, Attribute&) [with Iterator = const char*; Parser = boost::spirit::x3::kleene<boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> > >; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/boost/spirit/home/x3/core/parse.hpp:60:26:   required from 'bool boost::spirit::x3::parse(const Iterator&, Iterator, const Parser&, Attribute&) [with Iterator = const char*; Parser = boost::spirit::x3::kleene<boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> > >; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

main.cpp:36:75:   required from here

/usr/local/include/c++/6.1.0/bits/stl_construct.h:75:7: error: no matching function for call to 'boost::basic_string_view<char, std::char_traits<char> >::basic_string_view(const char&)'

      ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); 

       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In file included from main.cpp:4:0:

/usr/local/include/boost/utility/string_view.hpp:82:23: note: candidate: constexpr boost::basic_string_view<charT, traits>::basic_string_view(const charT*) [with charT = char; traits = std::char_traits<char>] <near match>

       BOOST_CONSTEXPR basic_string_view(const charT* str)

                       ^~~~~~~~~~~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:82:23: note:   conversion of argument 1 would be ill-formed:

In file included from /usr/local/include/c++/6.1.0/bits/stl_tempbuf.h:60:0,

                 from /usr/local/include/c++/6.1.0/bits/stl_algo.h:62,

                 from /usr/local/include/c++/6.1.0/algorithm:62,

                 from /usr/local/include/boost/utility/string_view.hpp:27,

                 from main.cpp:4:

/usr/local/include/c++/6.1.0/bits/stl_construct.h:75:7: error: invalid conversion from 'char' to 'const char*' [-fpermissive]

      ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); 

       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In file included from main.cpp:4:0:

/usr/local/include/boost/utility/string_view.hpp:85:23: note: candidate: constexpr boost::basic_string_view<charT, traits>::basic_string_view(const charT*, boost::basic_string_view<charT, traits>::size_type) [with charT = char; traits = std::char_traits<char>; boost::basic_string_view<charT, traits>::size_type = long unsigned int]

       BOOST_CONSTEXPR basic_string_view(const charT* str, size_type len)

                       ^~~~~~~~~~~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:85:23: note:   candidate expects 2 arguments, 1 provided

/usr/local/include/boost/utility/string_view.hpp:78:9: note: candidate: template<class Allocator> boost::basic_string_view<charT, traits>::basic_string_view(const std::__cxx11::basic_string<charT, traits, Allocator>&)

         basic_string_view(const std::basic_string<charT, traits,

         ^~~~~~~~~~~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:78:9: note:   template argument deduction/substitution failed:

In file included from /usr/local/include/c++/6.1.0/bits/stl_tempbuf.h:60:0,

                 from /usr/local/include/c++/6.1.0/bits/stl_algo.h:62,

                 from /usr/local/include/c++/6.1.0/algorithm:62,

                 from /usr/local/include/boost/utility/string_view.hpp:27,

                 from main.cpp:4:

/usr/local/include/c++/6.1.0/bits/stl_construct.h:75:7: note:   mismatched types 'const std::__cxx11::basic_string<char, std::char_traits<char>, Allocator>' and 'const char'

      ::new(static_cast<void*>(__p)) _T1(std::forward<_Args>(__args)...); 

       ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

In file included from main.cpp:4:0:

/usr/local/include/boost/utility/string_view.hpp:68:23: note: candidate: constexpr boost::basic_string_view<charT, traits>::basic_string_view(const boost::basic_string_view<charT, traits>&) [with charT = char; traits = std::char_traits<char>]

       BOOST_CONSTEXPR basic_string_view(const basic_string_view &rhs) BOOST_NOEXCEPT

                       ^~~~~~~~~~~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:68:23: note:   no known conversion for argument 1 from 'const char' to 'const boost::basic_string_view<char, std::char_traits<char> >&'

/usr/local/include/boost/utility/string_view.hpp:65:23: note: candidate: constexpr boost::basic_string_view<charT, traits>::basic_string_view() [with charT = char; traits = std::char_traits<char>]

       BOOST_CONSTEXPR basic_string_view() BOOST_NOEXCEPT

                       ^~~~~~~~~~~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:65:23: note:   candidate expects 0 arguments, 1 provided

In file included from /usr/local/include/c++/6.1.0/bits/char_traits.h:39:0,

                 from /usr/local/include/c++/6.1.0/ios:40,

                 from /usr/local/include/c++/6.1.0/ostream:38,

                 from /usr/local/include/c++/6.1.0/iostream:39,

                 from main.cpp:1:

/usr/local/include/c++/6.1.0/bits/stl_algobase.h: In instantiation of 'static _OI std::__copy_move<false, false, std::random_access_iterator_tag>::__copy_m(_II, _II, _OI) [with _II = const char*; _OI = boost::basic_string_view<char, std::char_traits<char> >*]':

/usr/local/include/c++/6.1.0/bits/stl_algobase.h:386:44:   required from '_OI std::__copy_move_a(_II, _II, _OI) [with bool _IsMove = false; _II = const char*; _OI = boost::basic_string_view<char, std::char_traits<char> >*]'

/usr/local/include/c++/6.1.0/bits/stl_algobase.h:422:45:   required from '_OI std::__copy_move_a2(_II, _II, _OI) [with bool _IsMove = false; _II = const char*; _OI = __gnu_cxx::__normal_iterator<boost::basic_string_view<char, std::char_traits<char> >*, std::vector<boost::basic_string_view<char, std::char_traits<char> > > >]'

/usr/local/include/c++/6.1.0/bits/stl_algobase.h:455:8:   required from '_OI std::copy(_II, _II, _OI) [with _II = const char*; _OI = __gnu_cxx::__normal_iterator<boost::basic_string_view<char, std::char_traits<char> >*, std::vector<boost::basic_string_view<char, std::char_traits<char> > > >]'

/usr/local/include/c++/6.1.0/bits/vector.tcc:637:16:   required from 'void std::vector<_Tp, _Alloc>::_M_range_insert(std::vector<_Tp, _Alloc>::iterator, _ForwardIterator, _ForwardIterator, std::forward_iterator_tag) [with _ForwardIterator = const char*; _Tp = boost::basic_string_view<char, std::char_traits<char> >; _Alloc = std::allocator<boost::basic_string_view<char, std::char_traits<char> > >; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<boost::basic_string_view<char, std::char_traits<char> >*, std::vector<boost::basic_string_view<char, std::char_traits<char> > > >; typename std::_Vector_base<_Tp, _Alloc>::pointer = boost::basic_string_view<char, std::char_traits<char> >*]'

/usr/local/include/c++/6.1.0/bits/stl_vector.h:1375:4:   required from 'void std::vector<_Tp, _Alloc>::_M_insert_dispatch(std::vector<_Tp, _Alloc>::iterator, _InputIterator, _InputIterator, std::__false_type) [with _InputIterator = const char*; _Tp = boost::basic_string_view<char, std::char_traits<char> >; _Alloc = std::allocator<boost::basic_string_view<char, std::char_traits<char> > >; std::vector<_Tp, _Alloc>::iterator = __gnu_cxx::__normal_iterator<boost::basic_string_view<char, std::char_traits<char> >*, std::vector<boost::basic_string_view<char, std::char_traits<char> > > >; typename std::_Vector_base<_Tp, _Alloc>::pointer = boost::basic_string_view<char, std::char_traits<char> >*]'

/usr/local/include/c++/6.1.0/bits/stl_vector.h:1100:4:   [ skipping 12 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]

/usr/local/include/boost/spirit/home/x3/operator/detail/sequence.hpp:496:24:   required from 'static bool boost::spirit::x3::detail::parse_into_container_impl<boost::spirit::x3::sequence<L, R>, Context, RContext>::call(const parser_type&, Iterator&, const Iterator&, const Context&, RContext&, Attribute&) [with Iterator = const char*; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >; Left = boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >; Right = boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type>; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; boost::spirit::x3::detail::parse_into_container_impl<boost::spirit::x3::sequence<L, R>, Context, RContext>::parser_type = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >]'

/usr/local/include/boost/spirit/home/x3/core/detail/parse_into_container.hpp:287:74:   required from 'bool boost::spirit::x3::detail::parse_into_container(const Parser&, Iterator&, const Iterator&, const Context&, RContext&, Attribute&) [with Parser = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >; Iterator = const char*; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/boost/spirit/home/x3/operator/kleene.hpp:32:48:   required from 'bool boost::spirit::x3::kleene<Subject>::parse(Iterator&, const Iterator&, const Context&, RContext&, Attribute&) const [with Iterator = const char*; Context = boost::spirit::x3::unused_type; RContext = const boost::spirit::x3::unused_type; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >; Subject = boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> >]'

/usr/local/include/boost/spirit/home/x3/core/parse.hpp:35:68:   required from 'bool boost::spirit::x3::parse_main(Iterator&, Iterator, const Parser&, Attribute&) [with Iterator = const char*; Parser = boost::spirit::x3::kleene<boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> > >; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

/usr/local/include/boost/spirit/home/x3/core/parse.hpp:60:26:   required from 'bool boost::spirit::x3::parse(const Iterator&, Iterator, const Parser&, Attribute&) [with Iterator = const char*; Parser = boost::spirit::x3::kleene<boost::spirit::x3::sequence<boost::spirit::x3::raw_directive<boost::spirit::x3::plus<boost::spirit::x3::negated_char_parser<boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, char> > > >, boost::spirit::x3::literal_char<boost::spirit::char_encoding::standard, boost::spirit::x3::unused_type> > >; Attribute = std::vector<boost::basic_string_view<char, std::char_traits<char> > >]'

main.cpp:36:75:   required from here

/usr/local/include/c++/6.1.0/bits/stl_algobase.h:324:18: error: no match for 'operator=' (operand types are 'boost::basic_string_view<char, std::char_traits<char> >' and 'const char')

        *__result = *__first;

        ~~~~~~~~~~^~~~~~~~~~

In file included from main.cpp:4:0:

/usr/local/include/boost/utility/string_view.hpp:71:26: note: candidate: boost::basic_string_view<charT, traits>& boost::basic_string_view<charT, traits>::operator=(const boost::basic_string_view<charT, traits>&) [with charT = char; traits = std::char_traits<char>]

       basic_string_view& operator=(const basic_string_view &rhs) BOOST_NOEXCEPT 

                          ^~~~~~~~

/usr/local/include/boost/utility/string_view.hpp:71:26: note:   no known conversion for argument 1 from 'const char' to 'const boost::basic_string_view<char, std::char_traits<char> >&'

live example

我在这里错过了什么？

【问题讨论】：

string_view 不拥有它引用的内存。那么......谁呢？

@NicolBolas 这里的内存归std::string input所有；但是为什么这对编译器错误很重要？

我不知道为什么会发生这个确切的错误，但我可以说强制 raw[...] 的属性为 boost::string_view makes it work。您可以手动使用额外的规则 (x3::rule<class my_rule,boost::string_ref> my_rule= x3::raw[....]; auto const str_vec=*my_rule;) 或使用我在示例中展示的 as 方法，我已经看到它多次使用（例如 here），最终是相同的。跨度>

AFAICT 问题似乎是 Spirit.X3 机器测试 raw 的合成属性（技术上是 boost::iterator_range<char*>，但显然当时仍然是 raw_attribute_type）是属性的 value_type（值类型为 boost::string_view）。该测试失败，X3 采用错误的代码路径并尝试操纵属性以使其适合，但未成功。使x3::traits::is_substitute<raw_attribute_type,boost::string_ref> 成为真实，使整个事情work again。

遗憾的是缺少 X3 的文档，但我认为即使在 Qi 中也没有。这是使 Spirit 的属性传播起作用的重要部分。 This thread 显示了类似的情况（最初通过额外的规则解决方法解决），this message from Joel de Guzman 谈到了 is_substitute。 PS：在我之前的评论中，当我说boost::string_ref时，我的意思是boost::string_view。

【参考方案1】：

通过重载/专门化move_to，您基本上是在告诉X3 如何从boost::iterator_range（实际上只是一对迭代器）转换为boost::string_view。不幸的是，它不知道两者都是“等价的”（一个是substitute，另一个似乎是 Spirit 中使用的命名法）。因此，您的原始示例有效，因为您明确请求从迭代器到 string_view 的转换，但是由于 X3 缺少推断适当属性类型所需的信息，所以这个失败了。

E特别是问题似乎出在here，这里测试解析器的属性（在本例中为raw_attribute_type）是否与预期属性的value_type（boost::string_view）兼容。此测试失败，然后选择了属性未“按原样通过”的代码路径，导致您得到错误。

考虑到所有这些，似乎有两种可能的方法：明确请求转换或向 Spirit 提供有关属性兼容性的信息。

缺少我们在 Qi 中的一些助手（attr_cast，as[]），似乎在 X3 中强制进行属性转换的方法是使用您想要的实际类型创建一个额外的规则：

x3::rule<class str,boost::string_view> str= x3::raw[ +~char_('_')] >> '_';
auto const str_vec = *str;

您也可以（特别是如果您需要在多个规则中执行此操作）使用一种我见过的方法（例如here），它创建一个可以像as<T>(parser) 一样使用的变量模板lambda隐藏该额外规则的创建并强制parser 的属性转换为T。

template<typename T>
auto as = [](auto p)  return rule<struct _, T> = as_parser(p); ;
auto const str_vec = *as<boost::string_view>(x3::raw[ +~char_('_')] >> '_');

向Spirit提供有关属性兼容性信息的方式似乎是专门化x3::traits::is_substitute：

namespace boost 
namespace spirit  namespace x3  namespace traits  
    template <>
    struct is_substitute<raw_attribute_type,boost::string_view> : boost::mpl::true_
    ;

但正如您在 cmets 中提到的，这似乎不是记录在案的自定义点，因此我不确定使用它是否是个好主意。理想情况下，这在 X3 中默认是专门的，但正如他在 previous question 中评论的那样，迭代器引用的存储必须是连续的这一事实使情况复杂化。