Hi, I think that a reasonable implementation of at_c for the new phoenix3 could be: #pragma once // boost headers #include <boost/fusion/include/at.hpp> #include <boost/phoenix/function.hpp> #include <boost/type_traits/remove_reference.hpp> #include <boost/type_traits/remove_const.hpp> // std headers #include <algorithm> #include <iostream> namespace boost { namespace phoenix { template <int N> struct at_impl { template <class Signature> struct result; template <class This, class Arg> struct result<This(Arg)> { typedef typename boost::fusion::result_of::at_c< typename boost::remove_reference<Arg>::type , N >::type type; }; template <class Arg> typename boost::result_of<at_impl<N>(Arg &)>::type operator()(Arg &arg) const { return fusion::at_c<N>(arg); } }; } // namespace phoenix } // namespace boost If that piece of code is valid (I have tested but I'm not sure if that's the right way to do it) how would boost::phoenix::at_c be implemented? In phoenix2 at_impl was a eval so at_c was a template inline function. Best regards, Denis