[Boost-users] [spirit] need help with the parser

Subject: [Boost-users] [spirit] need help with the parser
From: Andrzej Krzemienski (akrzemi1_at_[hidden])
Date: 2010-05-05 16:08:43

Hi,
Could someone help me understand why my fairly simple spirit::qi
parser does not work as I expect?
I try to parse string "2 c" into the structure:

  struct Data {
    char val;
  };

  std::map<int, Data> map;

such that the map contains one element with an int 2 as key and char
"c" wrapped into structure Data as the value. I am using Boost 1.42.
The rules for the grammar w/o semantic actions are as follows.

  value = qi::char_;
  list = (qi::int_ >> value); // the start rule

I know the grammar could be expressed more simply; my real life
example is more complex and makes sense, but I tried to reduce it as
much as possible.
The full grammar definition reads:

  typedef std::string::const_iterator Iter;
  BOOST_FUSION_ADAPT_STRUCT(Data,(char, val));

  struct TestGrammar
  : qi::grammar<Iter, std::map<int,Data>(), ascii::space_type>
  {
    qi::rule<Iter, std::map<int,Data>(), ascii::space_type> list;
    qi::rule<Iter, Data(), ascii::space_type> value;

    TestGrammar() : TestGrammar::base_type(list) {
      value = qi::char_[ at_c<0>(_val) = _1 ];
      list = (qi::int_ >> value)[ _val[_1] = _2 ];
    }
  };

The code that does the parsing reads

  int main( ) {
    std::string input = "2 c";
    Iter iter = input.begin(), end = input.end();
    TestGrammar grammar;
    std::map<int,Data> map;
    bool ans = phrase_parse( iter, end, grammar, space, map );

    assert( ans );
    assert( iter == end );
    BOOST_AUTO( it, map.find(2) );
    assert( it != map.end() );
    assert( it->second.val == 'c' ); // FAILS!
  }

I marked my expectations with assertions. The parsing does succeed,
i.e. phrase_parse returns true and the entire input is consumed, the
key of the map's element is right, but its value is somehow lost. Can
anyone tell what I am missing? I enclose the entire program code in
the attachment.
Thanks in advance for help.

Regards,
&rzej