Re: [Boost-users] result_of protocol

Subject: Re: [Boost-users] result_of protocol
From: Hicham Mouline (hicham_at_[hidden])
Date: 2010-02-26 06:26:07

>----------------------------------------------------------------------------------
>struct extract_rule {
>
 > template <typename Sig> struct result;
>
 > template <typename Holder>
 >struct result< extract_rule(const Holder) > {
 > typedef const typename Holder::type& type;
>  };

>  template <typename Holder>
>  typename result< extract_rule(const Holder) >::type
>  operator()( const Holder& h ) const
>  {
>    return h.rule;
>  }
>};
>----------------------------------------------------------------------------------

>1>        with
>1>        [
>1>            Sig=extract_rule (const onevariantrule<parameters_t,0,boost::spirit::istream_iterator> )
>1>        ]

>The template onevariantrule<> is defined _before_ extract_rule.

>-------------------------------------------------------------------------------------------
>template <typename parameters_t, unsigned n, typename Iterator> 
>struct onevariantrule {
>  typedef .... type;
>};
>-------------------------------------------------------------------------------------------

>Holder matches onevariantrule<parameters_t,0,boost::spirit::istream_iterator>.
>That template instantiation is indeed a class.

Both specializations 

template <typename Holder>
struct result< extract_rule(const Holder) > {

};

and 

template <typename Holder>
struct result< extract_rule(const Holder& ) > { ///// the reference was missing here, doh!
typedef const typename Holder::type& type;
};

were needed.

Sorry for the spam,