How do I “expand” a compile-time std::array into a parameter pack?

Question

I\'d like to use partial template specialization in order to \'break down\' an array (which is created at compile time) into a parameter pack composed of its values (to inte

Answer 1

Inspired by @dfri 's answer, I transformed her / his solution to a version which can omit functions, but instead uses only one struct using partial template specialization for the std::integer_sequence which might also be interesting to others:

template <auto arr, template <typename X, X...> typename Consumer,
          typename IS = decltype(std::make_index_sequence<arr.size()>())> struct Generator;

template <auto arr, template <typename X, X...> typename Consumer, std::size_t... I>
struct Generator<arr, Consumer, std::index_sequence<I...>> {
  using type = Consumer<typename decltype(arr)::value_type, arr[I]...>;
};

Full example with usage:

#include <array>

/// Structure which wants to consume the array via a parameter pack.
template <typename StructuralType, StructuralType... s> struct ConsumerStruct {
  constexpr auto operator()() const { return std::array{s...}; }
};

/// Solution
template <auto arr, template <typename X, X...> typename Consumer,
          typename IS = decltype(std::make_index_sequence<arr.size()>())> struct Generator;

template <auto arr, template <typename X, X...> typename Consumer, std::size_t... I>
struct Generator<arr, Consumer, std::index_sequence<I...>> {
  using type = Consumer<typename decltype(arr)::value_type, arr[I]...>;
};

/// Helper typename
template <auto arr, template <typename T, T...> typename Consumer>
using Generator_t = typename Generator<arr, Consumer>::type;

// Usage
int main() {
  constexpr auto tup = std::array<int, 3>{{1, 5, 42}};
  constexpr Generator_t<tup, ConsumerStruct> tt;
  static_assert(tt() == tup);
  return 0;
}

Answer 2

A C++20 approach

See OP's own answer or, for possibly instructive but more verbose (and less useful) approach, revision 2 of this answer.

A C++17 approach

(This answer originally contained an approach using a minor C++20 feature (that a lambda without any captures may be default constructed), but inspired by the original answer the OP provided a much neater C++20 approach making use of the fact that a constexpr std::array falls under the kind of literal class that may be passed as a non-type template parameter in C++20 (given restraints on its ::value_type), combined with using partial specialization over the index sequence used to unpack the array into a parameter pack. This original answer, however, made use of a technique of wrapping std::array into a constexpr lambda (>=C++17) which acted as a constexpr (specific) std::array creator instead of an actual constexpr std::array. For details regarding this approach, see revision 2 of this answer)

Following OP's neat approach, below follows an adaption of it for C++17, using a non-type lvalue reference template parameter to provide, at compile time, a reference to the array to the array to struct target.

#include <array>
#include <cstdlib>
#include <tuple>
#include <type_traits>
#include <utility>

// Parameter pack structure (concrete target for generator below).
template <typename StructuralType, StructuralType... s>
struct ConsumerStruct
{
    // Use tuple equality testing for testing correctness.
    constexpr auto operator()() const { return std::tuple{s...}; }
};

// Generator: FROM std::array TO Consumer.
template <const auto& arr,
          template <typename T, T...> typename Consumer,
          typename Indices = std::make_index_sequence<arr.size()> >
struct Generator;

template <const auto& arr,
          template <typename T, T...> typename Consumer,
          std::size_t... I>
struct Generator<arr, Consumer, std::index_sequence<I...> >
{
    using type =
        Consumer<typename std::remove_cv<typename std::remove_reference<
                     decltype(arr)>::type>::type::value_type,
                 arr[I]...>;
};

// Helper.
template <const auto& arr, template <typename T, T...> typename Consumer>
using Generator_t = typename Generator<arr, Consumer>::type;

// Example usage.
int main()
{
    // As we want to use the address of the constexpr std::array at compile
    // time, it needs to have static storage duration.
    static constexpr std::array<int, 3> arr{{1, 5, 42}};
    constexpr Generator_t<arr, ConsumerStruct> cs;
    static_assert(cs() == std::tuple{1, 5, 42});
    return 0;
}

Note that this approach places a restriction on the std::array instance in that it needs to have static storage duration. If one wants to avoid this, using a constexpr lambda which generates the array may be used as an alternative.