boost::spirit arithmetic formulas parser fails to compile

Question

I am trying to write a spirit parser for arithmetic expression which fills an abstract syntax tree. The parser compiles if I am not trying to fill the AST, but fails (with one 24K error) in the current version. I am using clang++ 3.5.0 with -std=c++11, and running on Ubuntu 14.4.

#include <string>
#include <vector>
#include <utility>

#include <boost/spirit/include/qi.hpp>

#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/adapted.hpp>
#include <boost/fusion/include/adapted.hpp>

#include <boost/variant/variant.hpp>
#include <boost/variant/recursive_wrapper.hpp>

using std::string;
using std::vector;
using std::pair;

using boost::spirit::qi::grammar;
using boost::spirit::qi::space_type;
using boost::spirit::qi::rule;

struct Term; // forward dec
typedef boost::recursive_wrapper<Term> RWTerm;
typedef pair<char, RWTerm> OpAndRWTerm;
typedef pair<RWTerm, vector<OpAndRWTerm> > Expr;
typedef boost::variant<Expr, double> Factor;
typedef pair<char, Factor> OpAndFactor;
struct Term : public pair<Factor, vector<OpAndFactor> >{};

template<typename It>
struct formula_parser : grammar<It, Expr(), space_type> {
  formula_parser() : formula_parser::base_type(expr_rule) {
    using boost::spirit::qi::double_;
    using boost::spirit::ascii::char_;

    factor_rule %= double_ | parenthesis_rule;
    parenthesis_rule %= '(' >> expr_rule >> ')';
    op_and_factor_rule %= char_("/*") >> factor_rule;
    term_rule %= factor_rule >> *op_and_factor_rule;
    op_and_term_rule %= char_("+-") >> term_rule;
    expr_rule %= term_rule >> *op_and_term_rule;
  }
  rule<It, OpAndRWTerm(), space_type> op_and_term_rule;
  rule<It, Expr(), space_type> expr_rule;
  rule<It, OpAndFactor(), space_type> op_and_factor_rule;
  rule<It, RWTerm(), space_type> term_rule;
  rule<It, Expr(), space_type> parenthesis_rule;
  rule<It, Factor(), space_type> factor_rule;
};

int main() {
  formula_parser<string::const_iterator> grammar;
}

What I understand from the error message is that fusion mixes up the Types Factor and RWTerm in the rule term_rule.

what am I doing wrong ?

Answer 1

It compiles for me if I change two things:

1. Since Term inherits from std::pair, Term is a new type. For this reason you need to apply BOOST_FUSION_ADAPT_STRUCT to Term, regardless whether this has been already done for std::pair in <boost/fusion/adapted/std_pair.hpp>:

   BOOST_FUSION_ADAPT_STRUCT(
       Term,
       (Factor, first)
       (std::vector<OpAndFactor>, second)
   )
   

   Alternatively, you could make Term a standalone struct with two members and then apply BOOST_FUSION_ADAPT_STRUCT on that:

   struct Term { Factor f; std::vector<OpAndFactor>  o;};
   
   BOOST_FUSION_ADAPT_STRUCT(
       Term,
       (Factor, f)
       (std::vector<OpAndFactor>, o)
   )
   

   By the way: You have to fully qualify std::vector here, because the following will not compile:

   using std::vector;
   BOOST_FUSION_ADAPT_STRUCT(
       Term,
       (Factor, f)
       (vector<OpAndFactor>, o)
   )
   

2. Use Term instead of RWTerm when declaring term_rule:

   rule<It, Term(), space_type> term_rule;
   

full code:

#include <string>
#include <vector>
#include <utility>

#include <boost/spirit/include/qi.hpp>

#include <boost/fusion/include/adapt_struct.hpp>
#include <boost/fusion/adapted.hpp>
#include <boost/fusion/include/adapted.hpp>

#include <boost/variant/variant.hpp>
#include <boost/variant/recursive_wrapper.hpp>

using boost::spirit::qi::grammar;
using boost::spirit::qi::space_type;
using boost::spirit::qi::rule;

struct Term; // forward dec
typedef boost::recursive_wrapper<Term> RWTerm;
typedef std::pair<char, RWTerm> OpAndRWTerm;
typedef std::pair<RWTerm, std::vector<OpAndRWTerm> > Expr;
typedef boost::variant<Expr, double> Factor;
typedef std::pair<char, Factor> OpAndFactor;
struct Term : public std::pair<Factor, std::vector<OpAndFactor> >{};

BOOST_FUSION_ADAPT_STRUCT(
    Term,
    (Factor, first)
    (std::vector<OpAndFactor>, second)
)


template<typename It>
struct formula_parser : grammar<It, Expr(), space_type> {
  formula_parser() : formula_parser::base_type(expr_rule) {
    using boost::spirit::qi::double_;
    using boost::spirit::ascii::char_;

    factor_rule %= double_ | parenthesis_rule;
    parenthesis_rule %= '(' >> expr_rule >> ')';
    op_and_factor_rule %= char_("/*") >> factor_rule;
    term_rule %= factor_rule >> *op_and_factor_rule;
    op_and_term_rule %= char_("+-") >> term_rule;
    expr_rule %= term_rule >> *op_and_term_rule;
  }
  rule<It, OpAndRWTerm(), space_type> op_and_term_rule;
  rule<It, Expr(), space_type> expr_rule;
  rule<It, OpAndFactor(), space_type> op_and_factor_rule;
  rule<It, Term(), space_type> term_rule;
  rule<It, Expr(), space_type> parenthesis_rule;
  rule<It, Factor(), space_type> factor_rule;
};

int main() {
  formula_parser<std::string::const_iterator> grammar;
}

live example