Re: [Boost-users] [Proto] Building a mpl::vector from an expression

19 Aug 2009


      christophe henry wrote:
...
Hi Eric,
...
You could use reverse_fold instead of reversing the whole list after the
transform. Or you could use reverse_fold_tree to get the vector and then
pop the attribute_ off the front. That's probably fastest and simplest.
I must be doing something wrong (I suppose it has something to do with 
my second "when") because parsing:
attributes_ << int() << bool() << string()
With the above given grammar gives me a mpl::vector, 
which is normal, but if I simply and naively replace fold by 
reverse_fold, all I get is a mpl::vector<int>.
Am I missing something obvious?
Please see below:

   #include <string>
   #include 
   #include 
   #include 
   #include 
   #include 
   namespace proto = boost::proto;
   namespace mpl = boost::mpl;
   using proto::_;

   namespace My
   {
     struct my_tag {};

     template<class X = proto::is_proto_expr>
     struct attribute_
     {
       BOOST_PROTO_BASIC_EXTENDS(
           proto::terminal::type
         , attribute_
         , proto::default_domain
       )
     };

     attribute_<> const attribute = {{}};

     struct BuildAttributes
       : proto::make<
           mpl::pop_front<
             proto::fold_tree<
                 _
               , mpl::vector0<>()
               , mpl::push_back()
             >
           >
         >
     {};
   }

   int main()
   {
     mpl::vector3 x =
       My::BuildAttributes()(
         My::attribute << int() << std::string() << bool()
       );
   }

HTH,

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