boost karma semantic action call map

Question

I need to map values to a std::string ( with the following map, and BOOST_FUSION_ADAPT_STRUCT )

    std::map< TYPEX, std::string> author2name;
    struct Emp
   {
     std::string name;
     TYPEX author;
   };

With the following code i want to generate my output:

karma::rule< it, std::string()> quote = '"' >> karma::string >> '"';
karma::rule< it, Emp> emp = karma::delimit('\t')[ quite << quite[ author2name[ karma::_1] ]];

Emp x;
karma::generate( std::ostream_iterator<char>(std::cout), emp, x);

But it doesn't compile.

And is there a way that i could write a header like this:

karma::rule< it, std::vector<std::string>()> header = karma::delimit('\t')[ % quote];
karma::rule< it, Emp> emp = header >> karma::eol >> karma::delimit('\t')[ quite << quite[ author2name[ karma::_1] ]];

karma::generate( std::ostream_iterator<char>(std::cout), {"A", "B", "C"},emp, x);

Answer 1

There is a number of small paper-cuts that killed you there :)

Working example:

Live On Coliru

#include <boost/fusion/adapted.hpp>
#include <boost/spirit/include/karma.hpp>
#include <boost/spirit/include/phoenix.hpp>
#include <map>

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

enum TYPEX { AUTHOR1, AUTHOR2, AUTHOR3, AUTHOR4 };

std::map<TYPEX, std::string> author2name;
struct Emp {
    std::string name;
    TYPEX author;
};

BOOST_FUSION_ADAPT_STRUCT(Emp, name, author) // boost 1_59
//BOOST_FUSION_ADAPT_STRUCT(Emp, (std::string, name)(std::string, author)) // older boost

int main() {
    using it = boost::spirit::ostream_iterator;

    karma::rule<it, std::string()> quote;
    karma::rule<it, TYPEX()> author;
    karma::rule<it, Emp()> emp;

    {
        using namespace karma;
        quote  %= '"' << string << '"';
        author  = quote [ _1 = phx::ref(author2name)[ _val ] ];
        emp    %= delimit('\t')[ quote << author ];
    }

    Emp x { "one", AUTHOR2 };
    author2name[AUTHOR2] = "TWO!";
    std::cout << karma::format(emp, x);
}

Prints:

"one"   "TWO!"  

The things that caused trouble:

• suggest to use boost::spirit::ostream_iterator and karma::format for more user-friendly API

• Add the missing parentheses on emp:

  karma::rule<it, Emp()> emp;
  

  NOTE: very recent boost (1_59 IIRC) doesn't not require these anymore. Which is why I found out only on Coliru

• Here:

  quote[ author2name[ karma::_1] ]
  

  you index [] into a std::map using ... qi::_1. That can't compile. What you wanted was to invoke the Phoenix lazy expression template of operator[]. You have to include the Phoenix header and force author2name to be a Phoenix reference actor:

  quote [ _1 = phx::ref(author2name)[_1] ]
  

  Note also, assigning back to _1 is important!

• Also, to have an auto-rule in the presence of Semantic Actions, you need to assign the rule using %= (otherwise Karma will suppress all automatic attribute propagation)