How to construct an std::array with index sequence?

Question

How can I construct an std::array with an index sequence, or a lambda which depends on a sequential index?

std::iota and std::generat

Answer 1

For ar, here's an approach:

namespace detail {
  template
  constexpr auto make_iota_array(T const offset, std::integer_sequence) noexcept
   -> std::array {
    return {{(Ns + offset)...}};
  }
}

template
constexpr auto make_iota_array(T const offset = {}) noexcept {
  static_assert(N >= T{}, "no negative sizes");
  return detail::make_iota_array(offset, std::make_integer_sequence{});
}

// ...

auto ar = make_iota_array(99);

Online Demo

For ar2, here's an approach:

namespace detail {
  template
  constexpr auto generate_array(F& f, std::index_sequence)
   -> std::array {
    return {{f(std::integral_constant{})...}};
  }
}

template
constexpr auto generate_array(F f) {
  return detail::generate_array(f, std::make_index_sequence{});
}

// ...

auto ar2 = generate_array([](auto i) -> C { return {i, i * 1.12f}; });

Online Demo

(noexcept is more-or-less optional here IMO, and omitted here for brevity, but is present in the demos.)

N.b. both are fully constexpr, but since generate_array is likely to be used with lambdas it won't be constexpr in practice until C++17 (demo). Also n.b. generate_array will work with non-copyable/non-movable types in C++17 due to guaranteed copy elision (demo).