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: teajay (teajay_at_[hidden])
Date: 2010-11-10 16:44:58

Le 10/11/2010 01:08, Joel de Guzman a écrit :
> 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,
Hi Joel,

I'm aware of the templates you mention. I already use them in the 
solution I got working but I hoped there would be some better solution 
than what I have right now.

I guess I found my answer in the usages of those templates: there's 
nothing better than what I have right now. So I'll leave it is.

Thanks for the help.

Thomas Bernard