boost::spirit compile issue with BOOST_FUSION_ADAPT_STRUCT

Question

I'm very new to boost::spirit/fusion. Could someone please explain to me why the following is not compiling? It's compiling and working fine if I put the m_name and m_settings variables directly in the config struct, but fails to compile when I separate them into two different structs. What am I missing?

Btw, the code line that makes the compiler spew the gazillion boost::spirit errors is: cfg = section >> node;

#include <boost/config/warning_disable.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/spirit/include/phoenix_fusion.hpp>
#include <boost/spirit/include/phoenix_stl.hpp>
#include <boost/fusion/include/adapt_struct.hpp>

#include <iostream>
#include <string>
#include <vector>

namespace qi = boost::spirit::qi;
namespace fusion = boost::fusion;
namespace phoenix = boost::phoenix;
namespace ascii = boost::spirit::ascii;

struct config_section
{
  std::string m_name;
  std::string m_settings;
};

struct config
{
  config_section m_sections;
};

BOOST_FUSION_ADAPT_STRUCT(
    config_section,
    (std::string, m_name)
    (std::string, m_settings)
)

BOOST_FUSION_ADAPT_STRUCT(
    config,
    (config_section, m_sections)
)

template <typename Iterator>
struct config_grammar : qi::grammar<Iterator, config(), ascii::space_type>
{
  config_grammar() : config_grammar::base_type(cfg)
  {
    using qi::lexeme;
    using qi::lit;
    using ascii::string;
    using ascii::char_;
    using namespace qi::labels;

    section %= '[' >> lexeme[+(char_ - ']')] >> ']';

    node %= !lit('[') >> lexeme[+(char_ - '\n')];

    cfg %= section >> node;
  }

  qi::rule<Iterator, config(), ascii::space_type> cfg;
  qi::rule<Iterator, std::string(), ascii::space_type> node;
  qi::rule<Iterator, std::string(), ascii::space_type> section;
};

template <typename Iterator>
bool parse_config(Iterator first, Iterator last)
{
  using qi::double_;
  using qi::phrase_parse;
  using ascii::space;
  using boost::phoenix::ref;

  config result;
  config_grammar<Iterator> config_parser;

  bool r = phrase_parse(first, last, config_parser, space, result);
  if (first != last) // fail if we did not get a full match
    return false;

  return r;
}

int main()
{
  std::string input = "[section]\nstuff";

  bool b = parse_config(input.begin(), input.end());

  if (b)
    std::cout << "Success" << std::endl;
  else
    std::cout << "Failure" << std::endl;

  return 0;
}

Thanks!

Answer 1

The problem seems to be that you don’t specify how to convert from a config_section to a config. You told spirit that a config_section is composed of 2 strings but try to parse a config without a rule to link the conversion between a config and a config_section. This seems to compile.

template <typename Iterator>
struct config_grammar : qi::grammar<Iterator, config(), ascii::space_type>
{
  config_grammar() : config_grammar::base_type(cfg)
  {
    using qi::lexeme;
    using qi::lit;
    using ascii::string;
    using ascii::char_;
    using namespace qi::labels;

    section %= '[' >> lexeme[+(char_ - ']')] >> ']';

    node %= !lit('[') >> lexeme[+(char_ - '\n')];

    //create a rule to specify conversion of a config_section to a config
    cfg %= cfg_sec;
    //this a now a cfg_sec (which is what you declared to be composed of 2
    //  strings)
    cfg_sec %= section >> node;
  }

  qi::rule<Iterator, config(), ascii::space_type> cfg;
  //add new declaration here for cfg_sec
  qi::rule<Iterator, config_section(), ascii::space_type> cfg_sec;
  qi::rule<Iterator, std::string(), ascii::space_type> node;
  qi::rule<Iterator, std::string(), ascii::space_type> section;
};

Answer 2

The problem is that the attribute for your cfg rule and for config_grammar grammar is not config, but (as far as I understand -- because it is two consequent strings) config_section. After this replacements:

struct config_grammar : qi::grammar<Iterator, config_section(), ascii::space_type>
....
qi::rule<Iterator, config_section(), ascii::space_type> cfg;

It compiles.

I'd suggest to you to learn more about rules' attributes and how they work -- otherwise you'll get lost all the time.