array_cat.cpp

#include <array>
#include <tuple>
#include <cstddef> // for size_t

// concatenate multiple arrays - requires C++20
//
// The usual approach is to default-construct `std::array<T, Na + Nb>`, then
// overwrite the elements, e.g. with `std::copy()` or loops.
//
// The problem with that is it requires that `T` is default-constructable;
// this implementation does not have that requirement.
template <class T, size_t Na, size_t Nb>
constexpr auto array_cat(
  const std::array<T, Na>& a,
  const std::array<T, Nb>& b,
  auto&&... rest) {
  if constexpr (sizeof...(rest) == 0) {
    return std::apply([](auto&&... elements) {
        return std::array { std::forward<decltype(elements)>(elements)... };
    }, std::tuple_cat(a, b));
  } else {
    return array_cat(a, array_cat(b, std::forward<decltype(rest)>(rest)...));
  }
}

constexpr std::array a { 1, 2};
constexpr std::array b { 3, 4};
constexpr std::array c { 5, 6};
static_assert(array_cat(a, b) == std::array { 1, 2, 3, 4});
static_assert(array_cat(a, b, c) == std::array { 1, 2, 3, 4, 5, 6});