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);
}