c++ - Spirit unable to assign attribute to single-element struct (or fusion sequence)

Question

My goal is to have my qi::grammar return an attribute. I'm having significant difficulty doing this with a spirit::lexer though.

I'd expect that with the given grammar below, if I called it with spirit::qi::parse(begin, end, grammar, output);, that the struct ident output would have the contents of the parsed lexeme.

The error seems to mostly flow out of this line: start %= lexer.identifier;

System Notes

• Boost 1.47.0
• Mac OS X 10.7.2
• clang++ or g++ (errors shown below are from clang++)

Compile Command

g++ -g -c -O0 -Wall -DBOOST_SPIRIT_DEBUG -DBOOST_SPIRIT_LEXERTL_DEBUG -DBOOST_SPIRIT_USE_PHOENIX_V3 -DBOOST_SPIRIT_ACTIONS_ALLOW_ATTR_COMPAT reduced.cpp

Source Code

#include <boost/fusion/include/adapt_struct.hpp>

#include <boost/spirit/home/lex.hpp>
#include <boost/spirit/home/lex/lexer/lexertl/lexer.hpp>
#include <boost/spirit/home/qi.hpp>

namespace spirit = boost::spirit;

struct ident {
    std::string value;
};

BOOST_FUSION_ADAPT_STRUCT(ident,
        (std::string, value)
        )

struct my_lexer : spirit::lex::lexer< spirit::lex::lexertl::actor_lexer<> > {
    spirit::lex::token_def<std::string> identifier;
};

struct my_grammar : spirit::qi::grammar<my_lexer::iterator_type, ident()> {
    my_grammar(const my_lexer & lexer) : my_grammar::base_type(start) {
        start %= lexer.identifier;
    }

    spirit::qi::rule<my_lexer::iterator_type, ident()> start;
};

2 Errors Reported

In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:13:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer.hpp:14:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/token_def.hpp:20:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/detail/assign_to.hpp:38:24: error: no matching constructor for initialization of 'ident'
                attr = Attribute(first, last);
                       ^         ~~~~~~~~~~~
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/detail/assign_to.hpp:94:13: note: in instantiation of member function 'boost::spirit::traits::assign_to_attribute_from_iterators<ident, char const *,
      void>::call' requested here
            call(first, last, attr);
            ^
In file included from ../reduced-example/reduced.cpp:4:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/lexertl/lexer.hpp:20:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/lexertl/token.hpp:530:13: note: in instantiation of function template specialization 'boost::spirit::traits::assign_to<char const *, ident>'
      requested here
            spirit::traits::assign_to(t.value().begin(), t.value().end(), attr);
            ^
In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:13:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer.hpp:14:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/token_def.hpp:20:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/detail/assign_to.hpp:330:59: note: in instantiation of member function 'boost::spirit::traits::assign_to_attribute_from_value<ident,
      boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, void>::call' requested here
            assign_to_attribute_from_value<Attribute, T>::call(val, attr);
                                                          ^
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/detail/assign_to.hpp:353:9: note: in instantiation of function template specialization
      'boost::spirit::traits::detail::assign_to<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, ident>' requested here
        detail::assign_to(val, attr, is_not_wrapped_container());
        ^
In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:13:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer.hpp:14:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/token_def.hpp:106:21: note: in instantiation of function template specialization
      'boost::spirit::traits::assign_to<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, ident>' requested here
                    spirit::traits::assign_to(t, attr);
                    ^
In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:13:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer.hpp:14:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/lexer/token_def.hpp:21:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/reference.hpp:16:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/reference.hpp:43:20: note: (skipping 5 contexts in backtrace; use -ftemplate-backtrace-limit=0 to see all)
            return ref.get().parse(first, last, context, skipper, attr);
                   ^
In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:15:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/tokenize_and_parse.hpp:15:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/nonterminal/grammar.hpp:18:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/nonterminal/rule.hpp:16:
In file included from /home/wlynch/Boost/1.47.0/include/boost/function.hpp:64:
In file included from /home/wlynch/Boost/1.47.0/include/boost/preprocessor/iteration/detail/iter/forward1.hpp:67:
In file included from /home/wlynch/Boost/1.47.0/include/boost/function/detail/function_iterate.hpp:14:
In file included from /home/wlynch/Boost/1.47.0/include/boost/function/detail/maybe_include.hpp:33:
/home/wlynch/Boost/1.47.0/include/boost/function/function_template.hpp:1042:5: note: in instantiation of function template specialization 'boost::function4<bool,
      boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>,
      lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > &, boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *,
      boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > const &, boost::spirit::context<boost::fusion::cons<ident &,
      boost::fusion::nil>, boost::fusion::vector0<void> > &, boost::spirit::unused_type const
      &>::function4<boost::spirit::qi::detail::parser_binder<boost::spirit::lex::reference<boost::spirit::lex::token_def<std::basic_string<char>, char, unsigned long> const, unsigned long>, mpl_::bool_<1> > >'
      requested here
    base_type(f)
    ^
/home/wlynch/Boost/1.47.0/include/boost/function/function_template.hpp:1083:5: note: in instantiation of function template specialization 'boost::function<bool
      (boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>,
      lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > &, boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *,
      boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > const &, boost::spirit::context<boost::fusion::cons<ident &,
      boost::fusion::nil>, boost::fusion::vector0<void> > &, boost::spirit::unused_type const
      &)>::function<boost::spirit::qi::detail::parser_binder<boost::spirit::lex::reference<boost::spirit::lex::token_def<std::basic_string<char>, char, unsigned long> const, unsigned long>, mpl_::bool_<1> > >'
      requested here
    self_type(f).swap(*this);
    ^
In file included from ../reduced-example/reduced.cpp:3:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex.hpp:15:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/lex/tokenize_and_parse.hpp:15:
In file included from /home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/nonterminal/grammar.hpp:18:
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/nonterminal/rule.hpp:182:19: note: in instantiation of function template specialization 'boost::function<bool
      (boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>,
      lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > &, boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *,
      boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned long>, lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > > const &, boost::spirit::context<boost::fusion::cons<ident &,
      boost::fusion::nil>, boost::fusion::vector0<void> > &, boost::spirit::unused_type const
      &)>::operator=<boost::spirit::qi::detail::parser_binder<boost::spirit::lex::reference<boost::spirit::lex::token_def<std::basic_string<char>, char, unsigned long> const, unsigned long>, mpl_::bool_<1> > >'
      requested here
            lhs.f = detail::bind_parser<Auto>(
                  ^
/home/wlynch/Boost/1.47.0/include/boost/spirit/home/qi/nonterminal/rule.hpp:230:13: note: in instantiation of function template specialization
      'boost::spirit::qi::rule<boost::spirit::lex::lexertl::iterator<boost::spirit::lex::lexertl::functor<boost::spirit::lex::lexertl::token<char const *, boost::mpl::vector0<mpl_::na>, mpl_::bool_<1>, unsigned
      long>, lexertl::detail::data, char const *, mpl_::bool_<1>, mpl_::bool_<1> > >, ident (), boost::spirit::unused_type, boost::spirit::unused_type, boost::spirit::unused_type>::define<mpl_::bool_<1>,
      boost::spirit::lex::token_def<std::basic_string<char>, char, unsigned long> >' requested here
            define<mpl::true_>(r, expr, traits::matches<qi::domain, Expr>());
            ^
../reduced-example/reduced.cpp:23:9: note: in instantiation of function template specialization 'boost::spirit::qi::operator%=<boost::spirit::lex::token_def<std::basic_string<char>, char, unsigned long> >'
      requested here
           

Answer 1

I ended up realizing that the struct I defined is being used in spirit as a tuple. Because spirit will try to minimize the groups (like, an optional<int, int> is an optional<int>). Therefore, I guessed that a tuple<A> will be converted to an A. Which seems to be the case.

I was able to further reduce the test case of broken code to the following:

#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/spirit/home/qi.hpp>
#include <string>

struct ident {
    std::string a;
};

BOOST_FUSION_ADAPT_STRUCT(ident,
        (std::string, a)
        )

int main() {
    boost::spirit::qi::rule<const char*, ident()> r;
    r = boost::spirit::lexeme["abc"];
}

From the following mailing list postings (1, 2) that I found, I can work around this issue by doing:

r = boost::spirit::lexeme["abc"] >> boost::spirit::eps;

While not really elegant, it solves the problem at least. If someone else has a method to do the single element struct, I'd be greatly interested though.