std::to_array

 

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std::array

Member types
Member functions
Element access array::at array::operator[]    array::front array::back array::data Operations array::fill array::swap	Iterators array::beginarray::cbegin array::endarray::cend array::rbeginarray::crbegin    array::rendarray::crend	Capacity array::empty array::size array::max_size
Non-member functions
operator==operator<=> (C++20) get(std::array) swap(std::array) to_array (C++20)	operator|=operator<operator>operator<=operator>= (until C++20)(until C++20)(until C++20)(until C++20)(until C++20)
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Defined in header <array>
template<class T, std::size_t N > constexprstd::array<std::remove_cv_t<T>, N> to_array( T (&a)[N]);	(1)	(since C++20)
template<class T, std::size_t N > constexprstd::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>constexprstd::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>constexprstd::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>constexprstd::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>constexprstd::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 constexprauto w1n = std::to_array<std::string_view>({"Mary", "Patricia", "Linda", "Barbara", "Elizabeth", "Jennifer"}); static_assert(std::is_same_v<decltype(w1n), conststd::array<std::string_view, 6>>); static_assert(w1n.size()==6 and w1n[5]=="Jennifer");   int main(){// copies a string literalauto a1 = std::to_array("foo"); static_assert(a1.size()==4);   // deduces both element type and lengthauto 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 happensauto 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::arrayauto 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)