fusion visit & assign

fva.cpp

namespace detail{
template <size_t I>
struct fusion_visit_impl
{
    template <typename Seq, typename F>
    static void visit(Seq& s, size_t idx, F&& fun)
    {
        static_assert(boost::fusion::result_of::size<Seq>::value >= I,"fusion index out of bounds");
        if (idx == I - 1) fun(boost::fusion::get<I - 1>(s));
        else fusion_visit_impl<I - 1>::visit(s, idx, std::forward<F>(fun));
    }
};

template <>
struct fusion_visit_impl<0>
{
    template <typename Seq, typename F>
    static void visit(Seq& , size_t , F&& ) { }
};
}

template <typename F, typename Seq>
void visit_fusion_sequence_at(Seq const& s, size_t idx, F&& fun)
{
    detail::fusion_visit_impl<boost::fusion::result_of::size<Seq>::value>::visit(s, idx, std::forward<F>(fun));
}
template <typename F, typename Seq>
void visit_fusion_sequence_at(Seq & s, size_t idx, F&& fun)
{
    detail::fusion_visit_impl<boost::fusion::result_of::size<Seq>::value>::visit(s, idx, std::forward<F>(fun));
}

//The F [&s](auto& x){assign(s,x);}

template<class T, class V>
typename std::enable_if< std::is_convertible< T,V >::value ,void>::type assign(T& to, const V& from)
{
    to = from;
}
// linker error when type conversion is not possible
template< class T, class V>
typename std::enable_if< !std::is_convertible< T,V >::value ,void>::type assign(T& , const V& );//{}