Distribute template wrapper across parameter pack

Question

I have a couple of templated types, Egg<T> and Chick<T>.

template<typename T>
struct Egg{};

template<typename T>
struct Chick{};

The chicks are contained in a class LoudNest<Chicks...> and the eggs in QuietNest<Eggs...>:

template <typename... Chicks>
struct LoudNest {
  std::tuple<Chicks...> chicks;
};

template <typename... Eggs>
struct QuietNest {
  std::tuple<Eggs...> eggs;

  // more here.
};

I want to have a hatch method on QuietNest<Eggs...> that produces a LoudNest. The LoudNest should have a Chick<T> for each Egg<T> in the QuietNest. I have a function QuietNest<Eggs...>::hatch_impl that can create a std::tuple<Chicks...> where the Chicks all have the correct type parameters. That is, QuietNest<Egg<double>, Egg<string>, Egg<char>>::hatch_impl will return std::tuple<Chick<double>, Chick<string>, Chick<char>>. I'm getting stuck trying to wrap that in a LoudNest constructor:

template <typename... Eggs>
struct QuietNest {
  std::tuple<Eggs...> eggs;

  auto hatch() const {
    // hatchlings is a std::tuple of chicks templated how I want.
    auto hatchlings = std::apply(
      [&](auto... args) { return hatch_impl(std::make_tuple(), args...); },
      eggs);
    // This line causes an error:
    return LoudNest{hatchlings};
    // error: cannot refer to class template 'LoudNest' without a template argument
  }

  // The rest of this all works, but is included in case you want to poke at it:

  // base case: only one parameter was passed—the tuple of hatched chicks.
  template<typename...Chicks>
  std::tuple<Chicks...> hatch_impl(std::tuple<Chicks...> chicks) {
    return chicks;
  }

  // recursive case: in addition to the tuple of hatched chicks,
  // at least one egg was passed (possibly more in the tail)
  template<typename...Chicks, typename T, typename...Unhatched>
  std::tuple<Chicks..., Chick<T>> hatch_impl(
    std::tuple<Chicks...> chicks,
    const Egg<T>& egg,
    Unhatched... tail
  ) const {
    Chick<T> babyBird = hatchOne(egg);
    return hatch_impl(
      std::tuple_cat(chicks, std::make_tuple(babyBird)),
      tail...);
  }

  template<T>
  Chick<T> hatchOne(Egg<T> egg) { return Chick<T>{}; }
};

I'm thinking I need to make a "converter" that accepts a parameter pack of eggs and produces a LoudNest with chicks of the corresponding types. Starting with converting a single Egg<T> to a Chick<T>, I have:

template<typename T>
struct AsChick {
  using type = T;
};

template< template <typename> class E, typename T>
struct AsChick<E<T>> {
  static_assert(std::is_same<E<T>, Egg<T>>::value, "Expected AsChick to be used with an Egg<T>");
  using type = Chick<T>;
};

Where I'm getting stuck is when I try to do the same for a parameter pack:

template<typename... Eggs>
struct AsLoudNest1 {
    using type = LoudNest<
        (AsChick<Eggs>::type)...
        // I want this to expand Eggs to produce
        // AsChick<Eggs0>::type, AsChick<Eggs1>::type, AsChick<Eggs2>::type, ...
        // but it doesn't looks like that's a supported type of expansion
    >;
};
static_assert(std::is_same<
  AsLoudNest1<Egg<int>, Egg<double>>::type,
  LoudNest<Chick<int>, Chick<double>>
>::value, "Expected AsLoudNest1 to convert my Egg<T>s to Chick<T>s");

And try number two:

template <
    class E, // does this need to be template<typename> class E?
    typename... Rest>
struct AsLoudNest2 {
    using type = LoudNest<
      // Pretty sure the beginning is right.
      AsChick<E>::type,

      // This line feels wrong, AsLoudNest2<...>::type is a concrete type, not a parameter pack
      AsLoudNest2<Rest...>::type...
    >;
};
// also, feels like I need a base case for AsLoudNest2?

My actual problem has to do with implementing an interpreter, and the classes are FormalParameter<T> (Egg<T>), ActualParameter<T> (Chick<T>), etc. However, I wanted to avoid using the word "parameter" for in the example code, as we're already talking about Parameter Packs in a different sense.

code from this post: https://godbolt.org/z/XBIEhm

Answer 1

I was able to fix your example with a few changes: https://godbolt.org/z/3VW68f

1. Add a deduction guide to LoudNest to alleviate the problem deducing the Chicks... types in LoudNest{hatchlings} (this probably isn't the only solution, but seemed clean in that it didn't require complicating the hatch() implementation):

   template<typename... Chicks>
   LoudNest(const std::tuple<Chicks...>& chicks) -> LoudNest<Chicks...>;
   

2. (Add hatchOne which was present in your question but not the godbolt link you shared)

3. Get rid of hatch_impl in favor of just calling hatchOne during pack expansion:

   auto hatchlings = std::apply(
     [&](auto... args) { return std::make_tuple(hatchOne(args)...); },
     eggs);
   

4. Use a specialization to deduce the inner T types of the Egg parameters to AsLoudNest1:

   template<typename... Eggs>
   struct AsLoudNest1 {};
   
   template<typename... Ts>
   struct AsLoudNest1<Egg<Ts>...> {
       using type = LoudNest<Chick<Ts>...>;
   };