Re: [Boost-users] [Proto] Detecting if a tag is a proto type or not

17 Nov 2008


      Joel Falcou wrote:
...
Eric Niebler a écrit :
...
Well, they're defined in the boost::proto::tag namespace. You can play 
tricks with ADL and sizeof....
Care to elaborate a few ?
Something like this:

namespace boost { namespace proto { namespace tag {
   template<typename T>
   char check_is_proto_tag(T);
}}}

namespace not_proto
{
   char (&check_is_proto_tag(...))[2];
}

using not_proto::check_is_proto_tag;

template<typename T>
struct is_proto_tag
   : boost::mpl::bool_<sizeof(check_is_proto_tag(T()))==1>
{};

struct not_proto_tag {};

BOOST_MPL_ASSERT((is_proto_tag<boost::proto::tag::plus>));
BOOST_MPL_ASSERT_NOT((is_proto_tag<not_proto_tag>));


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