Folding over arbitrarily many variadic packs

问题

I'm reading through Eric Niebler's post on his tiny metaprogramming library. In trying to implement the pieces that he omits / lists as challenges, I am left with the following implementation of transform:

template <typename F, typename... As>
using meta_apply = typename F::template apply<As...>;

template <typename... >
struct typelist_transform;

// unary
template <typename... T, typename F>
struct typelist_transform<typelist<T...>, F> 
{
    using type = typelist<meta_apply<F,T>...>;
};

// binary
template <typename... T, typename... U, typename F>
struct typelist_transform<typelist<T...>, typelist<U...>, F>
{
    using type = typelist<meta_apply<F,T,U>...>;
};

This works, but seems very unsatisfactory to me - I need a partial specialization of typelist_transform for every number of "arguments" into F. Is there a better way to implement this metafunction?

回答1:

With a slightly modified interface (taking the metafunction first, rather than last, and adding a couple members to typelist to get its size and the N-th member rather than using standalone metafunctions):

template <typename... Ts>
struct typelist {
    template<size_t N>
    using get = std::tuple_element_t<N, std::tuple<Ts...>>;
    static constexpr size_t size = sizeof...(Ts);
};

template<class, class, class...>
struct typelist_transform_helper;

template<size_t... Ints, class F, class...Lists>
struct typelist_transform_helper<std::index_sequence<Ints...>, F, Lists...>{
    template <class MF, size_t N, typename... Ls>
    using apply_to_Nth = meta_apply<MF, typename Ls::template get<N>...>;

    using type = typelist<apply_to_Nth<F, Ints, Lists...>...>;
};

template<class F, class L1, class... Lists>
struct typelist_transform : typelist_transform_helper<std::make_index_sequence<L1::size>, F, L1, Lists...> {
    // static assert on size of all lists being equal if wanted
};

Demo.

One problem with the above is that tuple_element usually requires N recursive template instantiations, making the total number of template instantiations quadratic in the length of the list. Given our use case (we access every single index), we can do better:

template<class L>
struct typelist_indexer {
    template<size_t N, class T> struct helper_base { using type = T; };

    template<class S, class> struct helper;
    template<size_t... Ns, class... Ts> 
    struct helper<std::index_sequence<Ns...>, typelist<Ts...>> : helper_base<Ns, Ts>... {};

    template<size_t N, class T>
    static helper_base<N, T> do_get(helper_base<N, T>);

    using helper_type = helper<std::make_index_sequence<L::size>, L>;

    template<size_t N>
    using get = typename decltype(do_get<N>(helper_type()))::type;
};

and then

template<size_t... Ints, class F, class...Lists>
struct typelist_transform_helper<std::index_sequence<Ints...>, F, Lists...>{
    template <class MF, size_t N, typename... Ls>
    using apply_to_Nth = meta_apply<MF, typename typelist_indexer<Ls>::template get<N>...>;

    using type = typelist<apply_to_Nth<F, Ints, Lists...>...>;
};

Now the number of template instantiations is linear in the size of the list, and in my tests this result in a significant compile time improvement. Demo.

来源：https://stackoverflow.com/questions/30492808/folding-over-arbitrarily-many-variadic-packs

标签

c++

c++11

metaprogramming

variadic-templates