Filter the synthesized attribute through a std::map in a boost spirit semantic action

Question

I have a case where I\'d like to filter the value that comes up as a synthesized attribute inside of a rule through a std::map.

• The map is pre-generated and wi

Answer 1

_{Reposting from comment}

Try qi::_val = phx::ref(myMap)[qi::_1] rather than qi::_val = phx::at(myMap, qi::_1).

Answer 2

The problem is that the return type of phoenix::at is defined in this header as container::value_type. This is a pair in the case of a map. Simply specializing the result for map makes it work (using ref as noted by ildjarn).

#define BOOST_SPIRIT_USE_PHOENIX_V3
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <boost/foreach.hpp>
#include <string>
#include <iostream>
#include <vector>
#include <map>

namespace qi = boost::spirit::qi;
namespace phx = boost::phoenix;

namespace boost { namespace phoenix { namespace stl {
    template <typename This, typename Key, typename Value, typename Compare, typename Allocator, typename Index>
        struct at_impl::result<This(std::map<Key,Value,Compare,Allocator>&, Index)>
        {
            typedef Value & type;
        };
    template <typename This, typename Key, typename Value, typename Compare, typename Allocator, typename Index>
        struct at_impl::result<This(std::map<Key,Value,Compare,Allocator> const&, Index)>
        {
            typedef Value const& type;
        };
}}}

int main() {

    std::map<unsigned int, unsigned int> myMap;
    myMap[1] = 100; myMap[2] = 200; myMap[3] = 300;

    std::string test = "1 2 3";
    std::vector<unsigned int> results;

    qi::rule<std::string::iterator, unsigned int()> r
                      = qi::uint_ [qi::_val = phx::at(phx::cref(myMap), qi::_1)];

    qi::parse(test.begin(), test.end(), ( r % " " ), results);

    BOOST_FOREACH(unsigned int &x, results) {
        std::cout << x << "\n";
    }
}