Attributes from Boost.Spirit grammar: error from std:vector of boost::variant

Question

I got a working parser for reading position descriptions for a board game (international draughts, official grammar):

#include 

        

Answer 1

I've spent some time trying to understand the grammar.

I strongly suggest readable identifiers. It's very hard to understand what's going on, while I have the strong impression it's actually a really simple grammar

I suggest a simplification version shown below.

• Because your grammar doesn't use recursion there's no real need for the rule and tagged parser types.
• Also use a namespace for the parser artefacts.
• Consider encapsulation the use of a skipper instead of letting the caller decide (x3::skip[])

• Add a few helpers to be able to print the AST for verification:

  template <typename T> std::ostream& operator<<(std::ostream& os, std::vector<T> const& v) {
      os << "{"; for (auto& el : v) os << el << " "; return os << "}";
  }
  std::ostream& operator<<(std::ostream& os, num_pc const& p)  { if (p.k) os << p.k; return os << p.sq; }
  std::ostream& operator<<(std::ostream& os, num_rng const& r) { return os << r.pc << "-" << r.last; }
  std::ostream& operator<<(std::ostream& os, ccn const& o)     { return os << o.c << " " << o.seq;                      } 
  std::ostream& operator<<(std::ostream& os, num_not const& nn) { return os << nn.n; }
  

• I'd avoid wrapping the other vector unnecessarily too:

  using num_not = std::vector<ccn>;
  

• Use the modern ADAPT macros (as you're using C++14 by definition):

  BOOST_FUSION_ADAPT_STRUCT(ast::num_pc,  k, sq)
  BOOST_FUSION_ADAPT_STRUCT(ast::num_rng, pc, last)
  BOOST_FUSION_ADAPT_STRUCT(ast::ccn,     c, seq)
  BOOST_FUSION_ADAPT_STRUCT(ast::fen,     c, n)
  

-

Live Demo

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#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/fusion/include/io.hpp>
#include <boost/optional/optional_io.hpp>
#include <boost/optional.hpp>
#include <boost/spirit/home/x3.hpp>
#include <boost/variant.hpp>
#include <iostream>
#include <vector>

namespace ast {

    struct num_pc {
        boost::optional<char> k;
        int sq;
    };

    struct num_rng {
        num_pc pc;
        int last;
    };

    using rng_or_pc = boost::variant<num_rng, num_pc>;
    using num_seq = std::vector<rng_or_pc>;

    struct ccn {
        char c;
        boost::optional<num_seq> seq;
    };

    using num_not = std::vector<ccn>;

    struct fen {
        char c;
        num_not n;
    };

    template <typename T> std::ostream& operator<<(std::ostream& os, std::vector<T> const& v) {
        os << "{"; for (auto& el : v) os << el << " "; return os << "}";
    }
    std::ostream& operator<<(std::ostream& os, num_pc const& p)  { if (p.k) os << p.k; return os << p.sq; }
    std::ostream& operator<<(std::ostream& os, num_rng const& r) { return os << r.pc << "-" << r.last; }
    std::ostream& operator<<(std::ostream& os, ccn const& o)     { return os << o.c << " " << o.seq;                      } 
}

BOOST_FUSION_ADAPT_STRUCT(ast::num_pc,  k, sq)
BOOST_FUSION_ADAPT_STRUCT(ast::num_rng, pc, last)
BOOST_FUSION_ADAPT_STRUCT(ast::ccn,     c, seq)
BOOST_FUSION_ADAPT_STRUCT(ast::fen,     c, n)

namespace FEN {
    namespace x3 = boost::spirit::x3;

    namespace grammar
    {
        using namespace x3;

        template<typename T>
            auto as = [](auto p) { return rule<struct _, T>{} = as_parser(p); };

        uint_type const number {};
        auto const color   = char_("BW");
        auto const num_pc  = as<ast::num_pc>  ( -char_('K') >> number          ); 
        auto const num_rng = as<ast::num_rng> ( num_pc >> '-' >> number        ); 
        auto const num_seq = as<ast::num_seq> ( (num_rng | num_pc) % ','       ); 
        auto const ccn     = as<ast::ccn>     ( ':' >> color >> -num_seq       ); 
        auto const num_not = as<ast::num_not> ( repeat(2)[ccn]                 ); 
        auto const fen     = as<ast::fen>     ( color >> num_not >> -lit('.')  ); 
    }

    using grammar::fen;
}

int main() {
    for (std::string const t : {
        "B:W18,24,27,28,K10,K15:B12,16,20,K22,K25,K29",
        "B:W18,19,21,23,24,26,29,30,31,32:B1,2,3,4,6,7,9,10,11,12",
        "W:B1-20:W31-50",   // initial position
        "W:B:W",            // empty board
        "W:B1:W",           // only black pieces
        "W:B:W50"           // only white pieces
    }) {
        auto b = t.begin(), e = t.end();
        ast::fen data;
        bool ok = phrase_parse(b, e, FEN::fen, FEN::x3::space, data);

        std::cout << t << "\n";
        if (ok) {
            std::cout << "Parsed: " << boost::fusion::as_vector(data) << "\n";
        } else {
            std::cout << "Parse failed:\n";
            std::cout << "\t on input: " << t << "\n";
        }
        if (b != e)
            std::cout << "\t Remaining unparsed: '" << std::string(b, e) << '\n';
    }
}

Prints:

B:W18,24,27,28,K10,K15:B12,16,20,K22,K25,K29
Parsed: (B {W  {18 24 27 28  K10  K15 } B  {12 16 20  K22  K25  K29 } })
B:W18,19,21,23,24,26,29,30,31,32:B1,2,3,4,6,7,9,10,11,12
Parsed: (B {W  {18 19 21 23 24 26 29 30 31 32 } B  {1 2 3 4 6 7 9 10 11 12 } })
W:B1-20:W31-50
Parsed: (W {B  {1-20 } W  {31-50 } })
W:B:W
Parsed: (W {B -- W -- })
W:B1:W
Parsed: (W {B  {1 } W -- })
W:B:W50
Parsed: (W {B -- W  {50 } })