Gesendet: Donnerstag, 03. März 2016 um 03:12 Uhr Von: "Joel de Guzman" An: boost@lists.boost.org Betreff: Re: [boost] [fusion] getting type of attribute_proxy for BOOST_FUSION_ADAPT_ADT

On 03/03/2016 9:43 AM, Jens Weller wrote:

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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 typename std::enable_if< std::is_convertible< typename fusion::result_of::at_c::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 typename std::enable_if< !std::is_convertible< typename fusion::result_of::at_c::type, V>::value /*&& !std::is_same::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#...

Hi Joel, great! Is there a generic way to detect which sort of Adapted struct it is? typename fusion::result_of::at_c::type::type t; Will probably only work for proxies, which would be fine for my code. thanks, Jens Weller

[snip]

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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#...

Hi Joel,

great!

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

Hmmm, well adt_attribute_proxy only happens, erm with proxies. So I guess the more relevant question is, how do you easily detect if at_c or any iterator function returns a proxy(?). Alas, there's no easy way at the moment. It's easy to add one and perhaps that might be a good idea. Please file a ticket and I'll see what I can do.

Ticket created under: https://svn.boost.org/trac/boost/ticket/12041 Which brings me to another fusion question/improvement: ADT structs don't know their member names, which can be fixed with such a construct: #define ADT_MEMBER_NAME(CLASS, INDEX, MEMBER) \ template <> struct struct_member_name { typedef char const *type; static type call() { return #MEMBER; } }; namespace boost { namespace fusion { namespace extension { ADT_MEMBER_NAME(ListEntry, 0, text) ... ADT_MEMBER_NAME(ListEntry, 6, published) } } } Could that be integrated into the BOOST_FUSION_ADAPT_STRUCT_ADT etc. macros? thanks, Jens Weller