# std::to_array

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< cpp‎ | container‎ | array

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 | Comment
--- | --- | --- | ---
`__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 (library fundamentals TS v2) | creates a std::array object whose size and optionally element type are deduced from the arguments (function template)

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