Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT

Subject: Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT
From: Jens Weller (JensWeller_at_[hidden])
Date: 2016-03-03 05:16:24

> Gesendet: Donnerstag, 03. März 2016 um 03:12 Uhr
> Von: "Joel de Guzman" <djowel_at_[hidden]>
> An: boost_at_[hidden]
> Betreff: Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT
>
> On 03/03/2016 9:43 AM, Jens Weller wrote:
> > Hello,
> >
> > Code below, first what I'm trying to do is...
> > assign a value to a adapted struct (BOOST_FUSION_ADAPT_ADT), which works fine if both are of the same type or convertible:
> >
> > template<int I, class Seq, class V>
> > typename std::enable_if< std::is_convertible< typename fusion::result_of::at_c<Seq, I>::type, V>::value ,void>::type assign(Seq& s,  V& v)
> > {
> >      fusion::get<I>(s) = v;
> > }
> >
> > Some of my input is string, and the struct member is bool, unsigned int etc. In this case I'd need to know the type to convert from string.
> >
> > boost::fusion::get returns the attribute_proxy for the adapted struct, so does at_c<>::type.
> >
> > template<int I, class Seq, class V>
> > typename std::enable_if< !std::is_convertible< typename fusion::result_of::at_c<Seq, I>::type, V>::value /*&& !std::is_same<typename std::decay<V>::type,std::string>::value*/ ,void>::type assign(Seq& s, V& v)
> > {
> >      std::stringstream ss;
> >      ss << v;
> >      ss >> fusion::get<I>(s);
> > }
> >
> > Obviously this does not compile. Is there any trick to get to the type of the proxy?
> >
> > One idea I have is decltype(fusion::get<I>(s).get()), but not sure if I can get this to work in an enable_if...
> 
> Hi Jens,
> 
> Try attribute_proxy::type. It's documented here:
> http://www.boost.org/doc/libs/1_50_0/libs/fusion/doc/html/fusion/notes.html#fusion.notes.adt_attribute_proxy

Hi Joel,

great!

Is there a generic way to detect which sort of Adapted struct it is?
typename fusion::result_of::at_c<Seq, I>::type::type t;
Will probably only work for proxies, which would be fine for my code.

thanks,

Jens Weller