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:

template
auto repeat(T value, seq) -> std::array
{
  // Unpack N, repeating `value` sizeof...(N) times
  // Note that (X, value) evaluates to value
  return {(N, value)...};
}

To initialize an std::array using this approach:

template
void f(T value)
{
  // genseq_t is seq
  std::array items = repeat(value, genseq_t{});
}

Additionally, the following definitions are used:

template
struct seq
{
  using type = seq;

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

  template
  struct push_back : seq {};
};

template
struct genseq : genseq::type::template push_back {};

template
struct genseq : seq {};

template
using genseq_t = typename genseq::type;

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