spirit-general

[Spirit-general] error in "alternative" parser?

[Andrzej K. <akr...@gm...>]

Hi,
Ii this a bug in Spirit? (I am using boost 1.42 on Visual Studio 2005).

namespace qi = boost::spirit::qi;
typedef std::string::const_iterator Iter;

int main()
{
    qi::rule<Iter, std::string()> myParser
        = (qi::alnum >> qi::alnum >> qi::alpha)
        | (qi::alnum >> qi::alnum);

    std::string text = "MM";
    std::string result;
    Iter iter = text.begin(), end = text.end();

    bool ans = qi::parse( iter, end, myParser, result );
    bool full = iter == end;
}

once I run it, flags 'ans' and 'full' indicate a full match, but the
variable 'result' has value "MMMM". It should be "MM".
Regards,
&rzej

Re: [Spirit-general] error in "alternative" parser?

[Hartmut K. <har...@gm...>]

> Ii this a bug in Spirit? (I am using boost 1.42 on Visual Studio 2005).
> 
> namespace qi = boost::spirit::qi;
> typedef std::string::const_iterator Iter;
> 
> int main()
> {
>     qi::rule<Iter, std::string()> myParser
>         = (qi::alnum >> qi::alnum >> qi::alpha)
>         | (qi::alnum >> qi::alnum);
> 
>     std::string text = "MM";
>     std::string result;
>     Iter iter = text.begin(), end = text.end();
> 
>     bool ans = qi::parse( iter, end, myParser, result );
>     bool full = iter == end;
> }
> 
> once I run it, flags 'ans' and 'full' indicate a full match, but the
> variable 'result' has value "MMMM". It should be "MM".
> Regards,
> &rzej

Attributes are not automatically reset when alternatives require
backtracking. You need to employ (the undocumented) directive hold[] which
will 'commit' an attribute only after the embedded parser succeeded:

    qi::rule<Iter, std::string()> myParser
        = hold[(qi::alnum >> qi::alnum >> qi::alpha)]
        | (qi::alnum >> qi::alnum);

Regards Hartmut

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