How to Elegantly Initialize a `std::array` with Non-Default-Constructible Types?

Elegant Initialization of std::array with Non-Default-Constructible Type

Initializing a std::array with a non-default-constructible element type can be a cumbersome task. Manually repeating the value n times is inefficient and error-prone for large n.

To address this issue, a more elegant approach involves utilizing a sequence-type and a generator. The key idea is to create a sequence of indices from 0 to n-1 and then use a function to repeatedly apply a value to each index.

Here's an implementation:

<code class="cpp">template<typename T, int...N>
auto repeat(T value, seq<N...>) -> std::array<T, sizeof...(N)>
{
  // Unpack N, repeating `value` sizeof...(N) times
  // Note that (X, value) evaluates to value
  return {(N, value)...};
}</code>

To initialize an std::array using this approach:

<code class="cpp">template<typename T, int N>
void f(T value)
{
  // genseq_t<N> is seq<0,1,...N-1>
  std::array<T, N> items = repeat(value, genseq_t<N>{});
}</code>

Additionally, the following definitions are used:

<code class="cpp">template<int ... N>
struct seq
{
  using type = seq<N...>;

  static const std::size_t size = sizeof ... (N);

  template<int I>
  struct push_back : seq<N..., I> {};
};

template<int N>
struct genseq : genseq<N-1>::type::template push_back<N-1> {};

template<>
struct genseq<0> : seq<> {};

template<int N>
using genseq_t = typename genseq<N>::type;</code>

This solution provides an efficient and elegant way to initialize std::array with non-default-constructible types, regardless of the value of n.

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