m.s. m.s. - 3 months ago 26
C++ Question

parse into vector<boost::string_view> using boost::spirit::x3

This is a follow-up question to my previous one regarding

boost::spirit::x3
and
boost::string_view
.

While I can parse into a
std::vector<std::string>
(live example), parsing into a
std::vector<boost::string_view>
fails with the following compile errors:

#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

What am I missing here?

jv_ jv_
Answer

By overloading/specializing move_to you are basically telling X3 how to convert from a boost::iterator_range (actually just a pair of iterators) to boost::string_view. Unfortunately it doesn't know that both are "equivalent" (one is a substitute of the other seems to be the nomenclature used in Spirit). And so your original example works because you are explicitly requesting the transformation from iterators to string_view but this one fails since X3 lacks the required information to infer the appropriate attribute type.

Especifically the problem seems to be here, where it is tested whether the attribute of the parser (in this case raw_attribute_type) is compatible with the value_type of the expected attribute (boost::string_view). This test fails and then a code path where the attribute is not "passed as is" is chosen causing the error you get.

With all this in mind there seem to be two possible approaches: either explicitly request the transformation or provide Spirit with the information about the compatibility of the attributes.

Missing some of the helpers we had in Qi (attr_cast, as[]) it seems that the way to force an attribute transformation in X3 is by creating an extra rule with the actual type you want:

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

You could also (especially if you need to do this in several rules) use an approach I've seen sehe use several times (like here) that creates a variable template lambda that can be used like as<T>(parser) that hides the creation of that extra rule and forces the attribute of parser to transform into 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_('_')] >> '_');

The way to provide Spirit with the information about the compatibility of the attributes seems to be to specialize 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_
    {};
}}}
}

But as you mentioned in the comments this doesn't seem to be a documented customization point and so I'm not sure if it's a good idea to use it. Ideally this would be specialized by default in X3, but as sehe commented in the previous question, the fact that the storage referred by the iterators must be contiguous complicates the situation.