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  typename Consumer,
          typename IS = decltype(std::make_index_sequence())> struct Generator;

template  typename Consumer, std::size_t... I>
struct Generator> {
  using type = Consumer;
};

Full example with usage:

#include 

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

/// Solution
template  typename Consumer,
          typename IS = decltype(std::make_index_sequence())> struct Generator;

template  typename Consumer, std::size_t... I>
struct Generator> {
  using type = Consumer;
};

/// Helper typename
template  typename Consumer>
using Generator_t = typename Generator::type;

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