Question

I have a rule which is supposed to return a Boost.Fusion ASSOC_STRUCT. I am trying to assign to _val the results parsed by the rule’s parsers, but I cannot make it work. I will skip the talking and present you directly with the relevant code.

#include <boost/fusion/include/define_assoc_struct.hpp>
#include <boost/spirit/include/qi.hpp>
#include <boost/spirit/include/phoenix_core.hpp>
#include <boost/spirit/include/phoenix_operator.hpp>
#include <boost/fusion/include/as_vector.hpp>
#include <boost/spirit/include/qi_symbols.hpp>
#include <iostream>
#include <string>

namespace keys {
   struct actor_key;
   struct action_key;
   struct money_key;
 }

 BOOST_FUSION_DEFINE_ASSOC_STRUCT(
    (), ActionLine,
    (int, action, keys::action_key)
    (int, amount, keys::money_key)
    (int, actor, keys::actor_key)
  )

int main ()
{
  namespace qi = boost::spirit::qi;
  using qi::_1;
  using qi::_2;
  using qi::_a;
  using qi::int_;
  using qi::lit;
  using boost::spirit::_val;
  using boost::fusion::as_vector;

  qi::symbols<char, int> name_parser_;
  name_parser_.add("name4", 4);

  std::string input("string1 ($7) string2 name4");
  std::string::const_iterator f(input.begin()), l(input.end());  
  ActionLine expected{0, 7, 4}, result;

  //The following rule is supposed to parse input of the form
  //"string1 ($[integer]) string2 [name]"
  //and return a triple (ActionLine) with the values { 0, value of (int_), value of (name_parser_) }
  qi::rule<std::string::const_iterator, ActionLine()> action_line_ =
     lit("string1 ($") >> int_ >> lit(") string2 ") >> name_parser_
     // [ _val = ActionLine{0, _1, _2} ]; // this is what I am trying to achieve
    [ _val = ActionLine{0, 7, 4} ]; //this compiles, but of course is not what I need

   bool b =
     qi::parse(f, l, action_line_, result) &&
     as_vector(result) == as_vector(expected);

   std::cout << "test: " << std::boolalpha << b << std::endl;
   return 0;
 }

(Compile with g++ above_file.cpp -std=c++0x)
Compiler error is somewhat different in my real application than in this example, but it is something like (in the line of _val = ActionLine{0, _1, _2} ):
No matching function of call ::ActionLine::ActionLine(), and I guess it cannot convert _1 and _2 to ints.

I also tried to add local int variables and use them to copy the parsed values, but it did not work, not did using boost::phoenix::at(_1,0), boost::phoenix::at(_1,1) ( I found those ideas here boost spirit semantic action parameters)

Answer 1

You need to use phoenix::construct in your semantic action to do what you want.

#include <boost/fusion/include/define_assoc_struct.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_object.hpp> //CHANGE:you need to include this to use "phoenix::construct"
#include <boost/fusion/include/as_vector.hpp>
#include <boost/spirit/include/qi_symbols.hpp>
#include <iostream>
#include <string>

namespace keys {
   struct actor_key;
   struct action_key;
   struct money_key;
 }

 BOOST_FUSION_DEFINE_ASSOC_STRUCT(
    (), ActionLine,
    (int, action, keys::action_key)
    (int, amount, keys::money_key)
    (int, actor, keys::actor_key)
  )

int main ()
{
  namespace qi = boost::spirit::qi;
  namespace phx = boost::phoenix;
  using qi::_1;
  using qi::_2;
  using qi::_a;
  using qi::int_;
  using qi::lit;
  using boost::spirit::_val;
  using boost::fusion::as_vector;

  qi::symbols<char, int> name_parser_;
  name_parser_.add("name4", 4);

  std::string input("string1 ($7) string2 name4");
  std::string::const_iterator f(input.begin()), l(input.end());  
  ActionLine expected{0, 7, 4}, result;

  //The following rule is supposed to parse input of the form
  //"string1 ($[integer]) string2 [name]"
  //and return a triple (ActionLine) with the values { 0, value of (int_), value of (name_parser_) }
  qi::rule<std::string::const_iterator, ActionLine()> action_line_ =
     (lit("string1 ($") >> int_ >> lit(") string2 ") >> name_parser_) //CHANGE:the parentheses are important otherwise the semantic action would be attached to name_parser_
    [ _val = phx::construct<ActionLine>(0, _1, _2) ]; //CHANGE: you need to use phoenix to use the placeholders _1, _2, etc

   bool b =
     qi::parse(f, l, action_line_, result) &&
     as_vector(result) == as_vector(expected);

   std::cout << "test: " << std::boolalpha << b << std::endl;
   std::cout << at_key<keys::action_key>(result)<< ", " << at_key<keys::money_key>(result)<< ", " << at_key<keys::actor_key>(result) << std::endl;

   return 0;
 }