named_params_test.cpp isn't working for me on g++ 3.3.3/3.4.1 on linux. I get the following error: /home/daniel/projects/boost/boost/mpl/eval_if.hpp: In instantiation of `boost::mpl::eval_if<boost::detail::is_const_reference_wrapper<const boost::reference_wrapper<int> >, const boost::reference_wrapper<int>, boost::mpl::identity<const boost::reference_wrapper<int> > >': /home/daniel/src/boost/boost/named_params.hpp:431: instantiated from `boost::detail::unwrap_cv_reference<const boost::reference_wrapper<int> >' named_params_test.cpp:159: instantiated from here /home/daniel/projects/boost/boost/mpl/eval_if.hpp:33: error: base `boost::reference_wrapper<int>' with only non-default constructor in class without a constructor The problem seems to be that eval_if inherits from boost::reference_wrapper<int> which doesn't have a default constructor. Is this a bug in g++? Since eval_if is never constructed I guess it might be? The following patch seems to work. Daniel diff -ru boost/boost/named_params.hpp boost-new/boost/named_params.hpp --- boost/boost/named_params.hpp 2004-11-08 01:20:28.000000000 +0000 +++ boost-new/boost/named_params.hpp 2004-11-13 22:47:07.000000000 +0000 @@ -424,11 +424,11 @@ template <class T> struct unwrap_cv_reference { - typedef typename mpl::eval_if< + typedef typename mpl::if_< is_const_reference_wrapper<T> , T , mpl::identity<T> - >::type type; + >::type::type type; }; } // namespace detail