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

Subject: Re: [boost] [spirit][qi] strange attribute composition when using expectation points
From: Hartmut Kaiser (hartmut.kaiser_at_[hidden])
Date: 2010-09-11 08:54:40

> 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.

Generally, expectation points in terms of attribute handling do not mix well
with 'normal' sequences. The parser expression 

    int_ >> *(char_ - ( "," >> int_ ) ) >> int_ 

exposes as its attribute a tuple<int, vector<char>, int>, while the
expression

    int_ > *(char_ - ( "," >> int_ ) ) >> int_ 

will expose a tuple<tuple<int, vector<char>>, int>, which is the reason for
the problems you're seeing.

Regards Hartmut
---------------
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