std::to_array(3)

NAME
std::to_array - std::to_array

Synopsis
   Defined in header <array>
   template<class T, std::size_t N> (1) (since C++20)
   constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&a)[N]);
   template<class T, std::size_t N> (2) (since C++20)
   constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&&a)[N]);
   Creates a std::array from the one dimensional built-in array a. The elements of the
   std::array are copy-initialized from the corresponding element of a. Copying or
   moving multidimensional built-in array is not supported.
   1) For every i in 0, ..., N - 1, copy-initializes result's correspond element with
   a[i]. This overload is ill-formed when std::is_constructible_v<T, T&> is false.
   2) For every i in 0, ..., N - 1, move-initializes result's correspond element with
   std::move(a[i]). This overload is ill-formed when std::is_move_constructible_v<T> is
   false.
   Both overloads are ill-formed when std::is_array_v<T> is true.

Parameters
   a - the built-in array to be converted the std::array

Type requirements
   -
   T must meet the requirements of CopyConstructible in order to use overload (1).
   -
   T must meet the requirements of MoveConstructible in order to use overload (2).

Return value
   1) std::array<std::remove_cv_t<T>, N>{ a[0], ..., a[N - 1] }
   2) std::array<std::remove_cv_t<T>, N>{ std::move(a[0]), ..., std::move(a[N - 1]) }

Notes
   There are some occasions where class template argument deduction of std::array
   cannot be used while to_array being available:
     * to_array can be used when the element type of the std::array is manually
       specified and the length is deduced, which is preferable when implicit
       conversion is wanted.
     * to_array can copy a string literal, while class template argument deduction
       constructs a std::array of a single pointer to its first character.
 std::to_array<long>({3, 4}); // OK: implicit conversion
 // std::array<long>{3, 4}; // error: too few template arguments
 std::to_array("foo"); // creates std::array<char, 4>{ 'f', 'o', 'o', '\0' }
 std::array{"foo"}; // creates std::array<const char*, 1>{ +"foo" }
   Feature-test macro: __cpp_lib_to_array

Possible implementation

First version
   namespace detail {
   template <class T, std::size_t N, std::size_t... I>
   constexpr std::array<std::remove_cv_t<T>, N>
       to_array_impl(T (&a)[N], std::index_sequence<I...>)
   {
       return { {a[I]...} };
   }
   }
   template <class T, std::size_t N>
   constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&a)[N])
   {
       return detail::to_array_impl(a, std::make_index_sequence<N>{});
   }

Second version
   namespace detail {
   template <class T, std::size_t N, std::size_t... I>
   constexpr std::array<std::remove_cv_t<T>, N>
       to_array_impl(T (&&a)[N], std::index_sequence<I...>)
   {
       return { {std::move(a[I])...} };
   }
   }
   template <class T, std::size_t N>
   constexpr std::array<std::remove_cv_t<T>, N> to_array(T (&&a)[N])
   {
       return detail::to_array_impl(std::move(a), std::make_index_sequence<N>{});
   }

Example
// Run this code
 #include <type_traits>
 #include <utility>
 #include <array>
 #include <memory>
 int main()
 {
     // copies a string literal
     auto a1 = std::to_array("foo");
     static_assert(a1.size() == 4);
     // deduces both element type and length
     auto a2 = std::to_array({ 0, 2, 1, 3 });
     static_assert(std::is_same_v<decltype(a2), std::array<int, 4>>);
     // deduces length with element type specified
     // implicit conversion happens
     auto a3 = std::to_array<long>({ 0, 1, 3 });
     static_assert(std::is_same_v<decltype(a3), std::array<long, 3>>);
     auto a4 = std::to_array<std::pair<int, float>>(
         { { 3, .0f }, { 4, .1f }, { 4, .1e23f } });
     static_assert(a4.size() == 3);
     // creates a non-copyable std::array
     auto a5 = std::to_array({ std::make_unique<int>(3) });
     static_assert(a5.size() == 1);
     // error: copying multidimensional arrays is not supported
     // char s[2][6] = { "nice", "thing" };
     // auto a6 = std::to_array(s);
 }

See also
   make_array Creates a std::array object whose size and optionally
   (library fundamentals TS v2) element type are deduced from the arguments
                        (function template)