std::to_array(3)

NAME
std::to_array - std::to_array

Synopsis
   Defined in header <array>
   template< class T, std::size_t N >
   constexpr std::array<std::remove_cv_t<T>, N> to_array( T (&a)[N] (1) (since C++20)
   );
   template< class T, std::size_t N >
   constexpr std::array<std::remove_cv_t<T>, N> to_array( T (&&a)[N] (2) (since C++20)
   );
   Creates a std::array from the one dimensional built-in array 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 is 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 Value Std Feature
   __cpp_lib_to_array 201907L (C++20) std::to_array

Possible implementation
                        to_array (1)
   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>{});
   }
                        to_array (2)
   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 <array>
 #include <memory>
 #include <string_view>
 #include <type_traits>
 #include <utility>
 // creates a constexpr array of string_view's
 constexpr auto w1n = std::to_array<std::string_view>({
     "Mary", "Patricia", "Linda", "Barbara", "Elizabeth", "Jennifer"
 });
 static_assert(std::is_same_v<decltype(w1n), const std::array<std::string_view, 6>>);
 static_assert(w1n.size() == 6 and w1n[5] == "Jennifer");
 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, 0.0f}, {4, 0.1f}, {4, 0.1e23f}});
     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)