std::to_array
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std::array
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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)
constexpr std::array <std::remove_cv_t <T>, N> to_array( T (&a)[N] );
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)
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. 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.
[edit] 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).
[edit] 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]) }
[edit] Notes
There are some occasions where class template argument deduction of std::array cannot be used while to_array is available:
-
to_arraycan be used when the element type of thestd::arrayis manually specified and the length is deduced, which is preferable when implicit conversion is wanted. -
to_arraycan copy a string literal, while class template argument deduction constructs astd::arrayof 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円'} 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
|
[edit] 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>{}); } |
[edit] 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); }
[edit] See also
(library fundamentals TS v2)
(function template) [edit]