Re: [Boost-users] [fusion] Using fusion::at_c on a sequence or an unused type

Subject: Re: [Boost-users] [fusion] Using fusion::at_c on a sequence or an unused type
From: Joel de Guzman (joel_at_[hidden])
Date: 2010-11-09 19:08:06

On 11/10/2010 6:01 AM, teajay wrote:
> Hi,
>
> I'm working on finalizing an extension to spirit/qi in which I need to extract an
> attribute from a fusion sequence in a parser context.
> The problem I'm facing is that this parser attribute can be an unused type or a valid
> fusion sequence.
>
> Just to illustrate my problem:
>
> template <typename Elements,typename Attribute>
> struct parser_dispachter
> {
> template <typename Index>
> void parse(Index &idx, Attribute &attr)
> {
> fusion::at_c<Index::value>(Elements).parse(fusion::at_c<Index::value>(attr));
>
> }
> };
>
> I actually have two cases:
> 1. Attribute = unused_type -> at_c replacement should return unused
> 2. Attribute = fusion sequence -> at_c replacement should return the corresponding
> attribute reference.
>
> What would be the best solution to extract the give attribute ?

Thomas, Spirit::support already takes care of that. See attribute_of,
attribute_not_unused, filter_unused_attributes and friends in
in support/attributes.hpp. See how they are used.

HTH

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