This code is not my actual code, but just illustrates the issue. I have a rule that matches 0 or more digits, and an action that is supposed to count them and return that count as the synthesized attribute.
The attribute of *qi::ascii::digit should be a std::vector of the matched digits. So, I wrote the action so that it simply calls .size() and assigns the result to the synthesized attribute. To call .size(), I use boost.phoenix to bind a lambda that calls .size().
#include <boost/phoenix/bind/bind_function_object.hpp>
#include <boost/phoenix/core.hpp>
#include <boost/phoenix/operator.hpp>
#include <boost/spirit/include/qi.hpp>
#include <cstdint>
#include <iostream>
#include <string>
namespace phx = boost::phoenix;
namespace qi = boost::spirit::qi;
using It = std::string::iterator;
struct Parser : qi::grammar<std::string::iterator, size_t()> {
Parser() : Parser::base_type(r) {
auto count = [](auto&& v) { return v.size(); };
r %= (*qi::ascii::digit)[qi::_val = phx::bind(count, qi::_1)];
}
qi::rule<It, size_t()> r;
};
int main() {
using namespace std;
std::string s = "123";
int c;
Parser p;
qi::parse(s.begin(), s.end(), p, c);
std::cout << c << '\n';
}
But it doesn't work. I expect this code to compile and print 3. Instead, I get a very long compile error. The error is roughly what I would expect if the action were simply qi::_val = qi::_1, which perplexes me.
In file included from /usr/local/include/boost/type_traits/is_convertible.hpp:20,
from /usr/local/include/boost/type_traits/is_empty.hpp:12,
from /usr/local/include/boost/mpl/empty_base.hpp:23,
from /usr/local/include/boost/fusion/sequence/intrinsic/at.hpp:15,
from /usr/local/include/boost/phoenix/core/expression.hpp:11,
from /usr/local/include/boost/phoenix/bind/bind_function_object.hpp:17,
from spirit2.cpp:1:
/usr/local/include/boost/spirit/home/qi/parse.hpp: In instantiation of ‘bool boost::spirit::qi::parse(Iterator&, Iterator, const Expr&, Attr&) [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Expr = int; Attr = Parser]’:
/usr/local/include/boost/spirit/home/qi/parse.hpp:100:25: required from ‘bool boost::spirit::qi::parse(const Iterator&, Iterator, const Expr&, Attr&) [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Expr = int; Attr = Parser]’
spirit2.cpp:27:39: required from here
/usr/local/include/boost/spirit/home/qi/parse.hpp:85:9: error: static assertion failed: error_invalid_expression
BOOST_SPIRIT_ASSERT_MATCH(qi::domain, Expr);
^~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/local/include/boost/spirit/home/qi/detail/parse_auto.hpp:14,
from /usr/local/include/boost/spirit/home/qi/auto.hpp:16,
from /usr/local/include/boost/spirit/home/qi.hpp:15,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from spirit2.cpp:4:
/usr/local/include/boost/spirit/home/qi/parse.hpp:88:42: error: request for member ‘parse’ in ‘boost::spirit::compile<boost::spirit::qi::domain, int>((* & expr))’, which is of non-class type ‘boost::spirit::result_of::compile<boost::spirit::qi::domain, boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<int>, 0>, boost::spirit::unused_type, void>::type’ {aka ‘int’}
return compile<qi::domain>(expr).parse(first, last, context, unused, attr);
~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~
In file included from /usr/local/include/boost/spirit/home/qi/auxiliary/attr.hpp:18,
from /usr/local/include/boost/spirit/home/qi/auxiliary.hpp:19,
from /usr/local/include/boost/spirit/home/qi.hpp:16,
from /usr/local/include/boost/spirit/include/qi.hpp:16,
from spirit2.cpp:4:
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp: In instantiation of ‘static void boost::spirit::traits::assign_to_attribute_from_value<Attribute, T, Enable>::call(const T_&, Attribute&, mpl_::false_) [with T_ = std::vector<char, std::allocator<char> >; Attribute = long unsigned int; T = std::vector<char, std::allocator<char> >; Enable = void; mpl_::false_ = mpl_::bool_<false>]’:
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:171:17: required from ‘static void boost::spirit::traits::assign_to_attribute_from_value<Attribute, T, Enable>::call(const T&, Attribute&) [with Attribute = long unsigned int; T = std::vector<char, std::allocator<char> >; Enable = void]’
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:370:63: required from ‘void boost::spirit::traits::detail::assign_to(const T&, Attribute&, mpl_::false_) [with T = std::vector<char, std::allocator<char> >; Attribute = long unsigned int; mpl_::false_ = mpl_::bool_<false>]’
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:393:26: required from ‘void boost::spirit::traits::assign_to(const T&, Attribute&) [with T = std::vector<char, std::allocator<char> >; Attribute = long unsigned int]’
/usr/local/include/boost/spirit/home/qi/detail/attributes.hpp:27:30: required from ‘static void boost::spirit::qi::default_transform_attribute<Exposed, Transformed>::post(Exposed&, const Transformed&) [with Exposed = long unsigned int; Transformed = std::vector<char, std::allocator<char> >]’
/usr/local/include/boost/spirit/home/qi/action/action.hpp:71:36: required from ‘bool boost::spirit::qi::action<Subject, Action>::parse(Iterator&, const Iterator&, Context&, const Skipper&, Attribute&) const [with Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; Context = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >; Skipper = boost::spirit::unused_type; Attribute = long unsigned int; Subject = boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >; Action = boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >]’
/usr/local/include/boost/spirit/home/qi/nonterminal/detail/parser_binder.hpp:73:54: [ skipping 3 instantiation contexts, use -ftemplate-backtrace-limit=0 to disable ]
/usr/local/include/boost/function/function_template.hpp:720:7: required from ‘boost::function4<R, T1, T2, T3, T4>::function4(Functor, typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1068:16: required from ‘boost::function<R(T0, T1, T2, T3)>::function(Functor, typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), int>::type = int]’
/usr/local/include/boost/function/function_template.hpp:1121:5: required from ‘typename boost::enable_if_<(! boost::is_integral<Functor>::value), boost::function<R(T0, T1, T2, T3)>&>::type boost::function<R(T0, T1, T2, T3)>::operator=(Functor) [with Functor = boost::spirit::qi::detail::parser_binder<boost::spirit::qi::action<boost::spirit::qi::kleene<boost::spirit::qi::char_class<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> > >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> > >, mpl_::bool_<true> >; R = bool; T0 = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T1 = const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&; T2 = boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&; T3 = const boost::spirit::unused_type&; typename boost::enable_if_<(! boost::is_integral<Functor>::value), boost::function<R(T0, T1, T2, T3)>&>::type = boost::function<bool(__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&, const __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >&, boost::spirit::context<boost::fusion::cons<long unsigned int&, boost::fusion::nil_>, boost::fusion::vector<> >&, const boost::spirit::unused_type&)>&]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:185:19: required from ‘static void boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>::define(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, const Expr&, mpl_::true_) [with Auto = mpl_::bool_<true>; Expr = boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript, boost::proto::argsns_::list2<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::dereference, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> >, 0>&>, 1>&, const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >&>, 2>; Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; T1 = long unsigned int(); T2 = boost::spirit::unused_type; T3 = boost::spirit::unused_type; T4 = boost::spirit::unused_type; mpl_::true_ = mpl_::bool_<true>]’
/usr/local/include/boost/spirit/home/qi/nonterminal/rule.hpp:249:31: required from ‘boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>& boost::spirit::qi::operator%=(boost::spirit::qi::rule<Iterator, T1, T2, T3, T4>&, Expr&&) [with Expr = const boost::proto::exprns_::expr<boost::proto::tagns_::tag::subscript, boost::proto::argsns_::list2<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::dereference, boost::proto::argsns_::list1<const boost::proto::exprns_::expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<boost::spirit::tag::char_code<boost::spirit::tag::digit, boost::spirit::char_encoding::ascii> >, 0>&>, 1>&, const boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::assign, boost::proto::argsns_::list2<boost::phoenix::actor<boost::spirit::attribute<0> >, boost::phoenix::actor<boost::proto::exprns_::basic_expr<boost::phoenix::detail::tag::function_eval, boost::proto::argsns_::list2<boost::proto::exprns_::basic_expr<boost::proto::tagns_::tag::terminal, boost::proto::argsns_::term<Parser::Parser()::<lambda(auto:1&&)> >, 0>, boost::phoenix::actor<boost::spirit::argument<0> > >, 2> > >, 2> >&>, 2>; Iterator = __gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >; T1 = long unsigned int(); T2 = boost::spirit::unused_type; T3 = boost::spirit::unused_type; T4 = boost::spirit::unused_type]’
spirit2.cpp:16:69: required from here
/usr/local/include/boost/spirit/home/qi/detail/assign_to.hpp:153:20: error: invalid static_cast from type ‘const std::vector<char, std::allocator<char> >’ to type ‘long unsigned int’
attr = static_cast<Attribute>(val);
^~~~~~~~~~~~~~~~~~~~~~~~~~~
The solution is to drop %:
r = (*qi::ascii::digit)[qi::_val = phx::bind(count, qi::_1)];
The important error message is in the last line: error: invalid static_cast from type ‘const std::vector<char, std::allocator<char> >’ to type ‘long unsigned int’
For why it produces vector<char> see here.
There is use of semantic action and:
r %= p and r = p are equivalent if there are no semantic actions associated with p.
Read more about it here.