Parsing a boost::variant with User Defined Datatype containing std::string using Spirit Qi

I'm trying to create a GPIB parser using Spirit.Qi. Sometimes the response can be either an error or a normal response. This seemed like a good use case for the alternative parser which yields a boost::variant; however, if one of the variant types contains a string the code fails to compile. Here is a simplified version that reproduces the error.

struct C1 {
   std::string h{""};
   int i{0};
};
BOOST_FUSION_ADAPT_STRUCT(C1, (std::string, h)(int, i))

struct C2 {
   std::string h{""};
   std::string c{};
};
BOOST_FUSION_ADAPT_STRUCT(C2, (std::string, h)(std::string, c))

using VariantType = boost::variant<C1, C2>;

int main() {
   std::string s2{"C2:Zoo3"};
   VariantType v1;
   if(qi::parse(s1.begin(), s1.end(), 
                (qi::string("C1") >> ":" >> qi::int_) | 
                (qi::string("C2") >> ":" >> *qi::char_), 
                v1)) {
      if(boost::get<C1>(&v1)) {
         auto a1 = boost::get<C1>(v1);
         std::cout << "Parsing Succeeded, a1 = " << a1.h << ":" 
                   << a1.i << std::endl;
      }
      else {
         auto a2 = boost::get<C2>(v1);
         std::cout << "Parsing Succeeded, a2 = " << a2.h << ":" 
                   << a2.c << std::endl;
      }
   }
   else {
      std::cout << "Parsing Failed" << std::endl;
   }
   return 0;
}

I've experimented with various parsers other than *qi::char_ (eg. qi::string) to no avail. If I change C2::c to a char it works ok for 1 char. The best workaround I have found so far is to change C2::c to a std::vector<char>, this works fine but isn't ideal. I also tried telling Qi that std::string is a container like here. But qi knows what a std::string is so I'm pretty sure it ignores my customization. I think this is all because std::string isn't a POD and wasn't supported in unions until the rules were relaxed, but it works with boost::variant when it isn't in a struct, and std::vector works. Any ideas/workarounds would be appreciated.

*Note: I didn't post the compiler errors as they are long and mangled and I figured this was a known issue of std::string and variants. If they would be helpful let me know and I'll post them.

Solution

There's nothing wrong with the code, as far as I can tell.

I've tested on c++1{1,4,y} and boost 1.{57,58}.0, using gcc {4.9,5.x} and clang++ 3.5.

I suspect you may have an awkward Boost verion. Try using qi::as_string[*qi::char_] there. ¹

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#include <iostream>
#include <boost/fusion/adapted/struct.hpp>
#include <boost/spirit/include/qi.hpp>

struct C1 {
   std::string h{""};
   int i{0};

   friend std::ostream& operator<<(std::ostream& os, C1 const& c1) {
       return os << "C1 {h:'" << c1.h << "', i:'" << c1.i << "'}";
   }
};

struct C2 {
   std::string h{""};
   std::string c{};

   friend std::ostream& operator<<(std::ostream& os, C2 const& c2) {
       return os << "C2 {h:'" << c2.h << "', c:'" << c2.c << "'}";
   }
};

BOOST_FUSION_ADAPT_STRUCT(C1, (std::string, h)(int, i))
BOOST_FUSION_ADAPT_STRUCT(C2, (std::string, h)(std::string, c))

using VariantType = boost::variant<C1, C2>;
namespace qi = boost::spirit::qi;

int main() {
    VariantType value;
    for(std::string s1 : {
            "C2:Zoo3",
            "C1:1234"
            })
    {
        if(qi::parse(s1.begin(), s1.end(), 
                    (qi::string("C1") >> ":" >> qi::int_) | 
                    (qi::string("C2") >> ":" >> *qi::char_), 
                    value)) 
            std::cout << "Parsing Succeeded: " << value << "\n";
        else             
            std::cout << "Parsing Failed" << std::endl;
    }
}

Prints

Parsing Succeeded: C2 {h:'C2', c:'Zoo3'}
Parsing Succeeded: C1 {h:'C1', i:'1234'}

¹ I don't recommend qi::attr_cast<> as I remember this having had an awkward bug in awkwardly old boost versions.