Implementation C++14 make_integer_sequence

Question

I tried to implement the C++14 alias template make_integer_sequence, which simplifies the creation of the class template integer_sequence.

templ         


        

Answer 1

Here is another implementation technique (for T=size_t), it uses C++17 fold expressions and bitwise generation (i.e. O(log(N)):

template <size_t... Is>
struct idx_seq {
  template <size_t N, size_t Offset>
  struct pow2_impl {
    using type = typename idx_seq<Is..., (Offset + Is)...>::template pow2_impl<N - 1, Offset + sizeof...(Is)>::type;
  };
  template <size_t _> struct pow2_impl<0, _> { using type = idx_seq; };
  template <size_t _> struct pow2_impl<(size_t)-1, _> { using type = idx_seq<>; };

  template <size_t Offset>
  using offset = idx_seq<(Offset + Is)...>;
};

template <size_t N>
using idx_seq_pow2 = typename idx_seq<0>::template pow2_impl<N, 1>::type;

template <size_t... Is, size_t... Js>
constexpr static auto operator,(idx_seq<Is...>, idx_seq<Js...>)
  -> idx_seq<Is..., Js...>
{ return {}; }

template <size_t N, size_t Mask, size_t... Bits>
struct make_idx_seq_impl {
  using type = typename make_idx_seq_impl<N, (N >= Mask ? Mask << 1 : 0), Bits..., (N & Mask)>::type;
};

template <size_t N, size_t... Bits>
struct make_idx_seq_impl<N, 0, Bits...> {
  using type = decltype((idx_seq<>{}, ..., typename idx_seq_pow2<Bits>::template offset<(N & (Bits - 1))>{}));
};

template <size_t N>
using make_idx_seq = typename make_idx_seq_impl<N, 1>::type;