//Prototype an implementation of mpl::vector using variadic templates.
#ifndef __VARIADIC_TEMPLATES
  #error "compiler must implement variadic templates"
#endif
#include <boost/mpl/assert.hpp>
#include <boost/type_traits/is_same.hpp>
namespace boost
{
namespace mpl
{
  template
  < typename... Args
  >
  struct
vector
{
};
  template
  < typename Sequence
  , long N
  >
  struct
at_c
;
  template
  < typename Head
  , typename... Tail
  , long N
  >
  struct
at_c
  < vector<Head, Tail...>
  , N
  >
  : at_c<vector<Tail...>, N-1>
{  
};
  template
  < typename Head
  , typename... Tail
  >
  struct
at_c
  < vector<Head, Tail...>
  , 0
  >
{
        typedef
      Head
    type
    ;
};
    
}//exit mpl namespace
}//exit boost namespace

using namespace boost;

  template
  < unsigned I
  >
  struct
A
{};

    typedef
  mpl::vector
  < A<0>
  , A<1>
  , A<2>
  >
vec3
;
    typedef
  mpl::at_c<vec3,0>::type
at_0
;
BOOST_MPL_ASSERT((is_same<at_0, A<0> >));
    typedef
  mpl::at_c<vec3,1>::type
at_1
;
BOOST_MPL_ASSERT((is_same<at_1, A<1> >));
    typedef
  mpl::at_c<vec3,2>::type
at_2
;
BOOST_MPL_ASSERT((is_same<at_2, A<2> >));