Re: [boost] [spirit][qi] strange attribute composition when using expectation points

Subject: Re: [boost] [spirit][qi] strange attribute composition when using expectation points
From: Joel de Guzman (joel_at_[hidden])
Date: 2010-09-10 22:21:35

On 9/10/2010 3:52 PM, Löw, Tobias (Evonik Energy Services GmbH) wrote:
> Hi,
>
> I get a different attribute composition depending on the use of expectation points. Here is a small example:
>
> ////////////////////////////////////////////////////////////////////////////////////////////////
> //
>
> BOOST_FUSION_DEFINE_STRUCT(
>      (client), input,
>      (int,    index1)
>      (std::string, text)
>      (int,    index2)
> )
>
>
>
> namespace client {
>
>
>      namespace fusion = boost::fusion;
>      namespace phoenix = boost::phoenix;
>      namespace qi = boost::spirit::qi;
>      namespace char_enc = boost::spirit::iso8859_1;
>
>      template<typename Iterator, typename Skipper>
>      struct line_grammar
>          : qi::grammar<Iterator, input(), Skipper>
>      {
>          line_grammar()
>            : line_grammar::base_type(line)
>          {
>              using qi::int_;
>              using qi::omit;
>              using char_enc::char_;
>              using qi::lit;
>
>
>              line =
>                      int_
>                      >>  lit(",")
> #ifdef USE_EXPECTATION_POINT
>                      >   *(char_ - ( ",">>  int_  ) )
> #else
>                      >>  *(char_ - ( ",">>  int_  ) )
> #endif
>                      >>  lit(",")
>                      >>  int_
>
>                  ;
>
>          }
>
>          qi::rule<Iterator, input(),  Skipper>  line;
>
>      };
>
>
>      bool read_line(  )
>      {
>          typedef line_grammar<std::string::const_iterator, char_enc::space_type>  grammar;
>
>          grammar gr; // Our grammar
>
>          input line_read;
>
>          std::string source = "1,x,2";
>          std::string::const_iterator iter = source.begin();
>          std::string::const_iterator end = source.end();
>          bool r = phrase_parse(iter, end, gr,
>              char_enc::space,
>              line_read);
>
>          return r&&  iter == end;
>      }
>
> }
>
> //
> /////////////////////////////////////////////////////////////////////////////////////////////////////////
>
> If USE_EXPECTATION_POINT is not defined then parsing succeeds and line_read == {1, "x", 2 }.
> But if USE_EXPECTATION_POINT  is defined then parsing also succeeds but line_read == {1, "x\0x2", 0 }.
> So, it seems that expectation points somehow break the semantics of attribute composition.

Yes. That is a known problem. Mixing >> with > is indeed a problem with
attribute composition. For now, there is no quick solution yet. You'll
just have to tweak the composed attribute type or use semantic actions to
"compile" your attribute.

Regards,
-- 
Joel de Guzman
http://www.boostpro.com
http://spirit.sf.net