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std::to_array

From cppreference.com
< 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. 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 would be ill-formed when std::is_constructible_v<T, T&> is false.
2) For every i in 0, ..., N - 1, copy-initializes result's correspond element with std::move(a[i]). This overload would be ill-formed when std::is_move_constructible_v<T> is false.

Both overloads would be ill-formed when std::is_array_v<T> is true.

Contents

[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 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 the class template argument deduction creating a std::array of one 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" }

[edit] 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>{});
}

[edit] Example

#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);
}


[edit] See also

Creates a std::array object whose size and optionally element type are deduced from the arguments
(function template) [edit]