table of contents
std::to_array(3) | C++ Standard Libary | std::to_array(3) |
NAME¶
std::to_array - std::to_array
Synopsis¶
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 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>
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 creates a std::array object whose size and optionally
(library fundamentals TS v2) element type are deduced from the arguments
(function template)
2024.06.10 | http://cppreference.com |