I noticed that in some contexts I have both the object class and the corresponding phoenix function class impl; ... impl f_impl; boost::phoenix::function<impl> f(f_impl); now to avoid the copying I could use: boost::phoenix::function<impl&> f(f_impl); which works. Although boost::phoenix::function<impl const&> f(f_impl); //doesn't work. (error below) Is there any problem with using references at all, in the first place? -- Thanks, Alfredo /home/correaa/usr/include/boost/spirit/home/phoenix/function/ function.hpp:31: error: conversion from ‘boost::phoenix::actor<boost::phoenix::composite<boost::phoenix::detail::function_eval<1>, boost::fusion::vector<boost::phoenix::value<gsl::interpolation::spline>, boost::phoenix::argument<0>, > > >’ to non-scalar type ‘boost::phoenix::actor<boost::phoenix::composite<boost::phoenix::detail::function_eval<1>, boost::fusion::vector<boost::phoenix::value<const gsl::interpolation::spline>, boost::phoenix::argument<0> > > >’ requested