I am looking for an idiomatic way to apply a type-level transform for each element in a type list. So far I came up with the following, which works as expected:
namespace impl_
{
template <template <typename> typename, typename>
struct MapVariant;
template <template <typename> typename F, typename... Ts>
struct MapVariant<F, std::variant<Ts...>>
{
using Result = std::variant<F<Ts>...>;
};
}
template <typename Variant, template <typename> typename Transform>
using MapVariant = typename impl_::MapVariant<Transform, Variant>::Result;
Test:
using TestVariant = std::variant<int, char, std::array<float, 0>>;
template <typename Arg>
using TypeTransform = std::array<Arg, 0>;
static_assert(
std::is_same_v<MapVariant<TestVariant, TypeTransform>, //
std::variant<std::array<int, 0>, std::array<char, 0>, std::array<std::array<float, 0>, 0>> //
>);
Now I would like to generalize MapVariant
to accept an arbitrary container parameterized with a type list rather than only std::variant
(e.g., std::tuple
). My naive solution does not work:
template <template <typename...> class C, template <typename> typename F, typename... Ts>
using MapVariant = C<F<Ts>...>;
Here is a more verbose approach closer to the original that is also dysfunctional:
namespace impl_
{
template <template <typename> typename, typename, template <typename...> class, typename>
struct MapVariant;
template <template <typename> typename F, typename Unused, template <typename...> class C, typename... Ts>
struct MapVariant<F, Unused, C, C<Ts...>>
{
using Result = C<F<Ts>...>;
};
}
template <template <typename...> class Variant, template <typename> typename Transform, typename... Ts>
using MapVariant = typename impl_::MapVariant<Transform, Variant<Ts...>, Variant, Ts...>::Result;
Clang says "Template argument for template template parameter must be a class template or type alias template". I understand that the compiler is unable to unpack TestVariant
because apparently some type information is lost at the time of its definition via using
, but I don't understand how to work around this.
It is possible to do this at all?
Thanks to Fureeish I managed to fix this with a trivial change:
template <template <typename> typename, typename>
struct MapVariant;
template <template <typename...> class C, template <typename> typename F, typename... Ts>
struct MapVariant<F, C<Ts...>>
{
using Result = C<F<Ts>...>;
};
The outer alias need not be changed. The critical difference is that the order of template parameters was incorrect -- I would have noticed that if I just paid more attention to it. Smh. Thanks again, Fureeish.