I have been encountering a strange problem with MPL under GCC 4.1.1. The following code snippet compiles fine under GCC 4.0.2 but fails under GCC 4.1.1. Any idea? Hailin -- #include <boost/mpl/vector.hpp> #include <boost/mpl/transform.hpp> struct int_t { typedef unsigned int type; }; template <typename T2> struct unsigned_t { struct unsigned_t_func { template <typename T> struct apply { typedef typename T::type type; }; }; typedef typename boost::mpl::transform<T2, unsigned_t_func>::type type; }; int main() { typedef unsigned_t<boost::mpl::vector<int_t> >::type const_t; return 0; }