Re: [Boost-users] [Proto] Using custom function tag in the simplified calalble context

Subject: Re: [Boost-users] [Proto] Using custom function tag in the simplified calalble context
From: Eric Niebler (eric_at_[hidden])
Date: 2008-10-04 11:14:51

Joel Falcou wrote:
> I've defined some custom tag for user specific function like :
> struct conj_tag {};
> 
> I have made the following expression builder using make_expr :
> 
> template<class X>
> typename proto::result_of::make_expr< conj_tag, X const &>::type
> conj( X const& x )
> {
>  return proto::make_expr<conj_tag>(boost::cref(x));
> }
> 
> And I match this function using the following grammar rule :
> struct my_grammar : or_< // some terminal
>                                           , bp::function< 
> bp::terminal<conj_tag>, my_grammar>
>                                          > {};

That's not right. Your conj() function is returning a unary expression 
with a conj_tag tag type. Your grammar is trying to match a binary 
expression with a proto::tag::function tag type. Your grammar should be:

   proto::unary_expr<conj_tag, my_grammar>

> Now, how should I overload operator() of a callable_context to match 
> this special tag ?

In your context, you should have an overloaded operator() like this:

   template<typename Expr>
   some-return-type operator()(conj_tag, Expr const &expr) const { ... }

HTH,

-- 
Eric Niebler
BoostPro Computing
http://www.boostpro.com