std::ranges::views::elements, std::ranges::elements_view
Defined in header <ranges> |
||
|---|---|---|
template< ranges::input_range V, std::size_t N >
requires
view<V> &&
/*has-tuple-element*/<ranges::range_value_t<V>, N> &&
/*has-tuple-element*/<std::remove_reference_t<ranges::range_reference_t<V>>, N> &&
/*returnable-element*/<ranges::range_reference_t<V>, N>
class elements_view : public ranges::view_interface<elements_view<V, N>>; |
(1) | (since C++20) |
namespace views {
template< std::size_t N >
inline constexpr /*unspecified*/ elements = /*unspecified*/;
} |
(2) | (since C++20) |
| Call signature | ||
template< ranges::viewable_range R >
requires /* see below */
constexpr ranges::view auto elements<N>( R&& r ); |
(since C++20) | |
| Helper concepts | ||
template< class T, std::size_t N >
concept /*has-tuple-element*/ = // exposition only
requires(T t) {
typename std::tuple_size<T>::type;
requires N < std::tuple_size_v<T>;
typename std::tuple_element_t<N, T>;
{ std::get<N>(t) } -> std::convertible_to<const std::tuple_element_t<N, T>&>;
}; |
(3) | (since C++20) |
template< class T, std::size_t N >
concept /*returnable-element*/ = // exposition only
std::is_reference_v<T> || std::move_constructible<std::tuple_element_t<N, T>>; |
(4) | (since C++20) |
1) Accepts a
view of
tuple-like values, and issues a view with a
value-type of the
N-th element of the adapted view's value-type.
2) Every specialization of
views::elements is a
range adaptor object. The expression
views::elements<M>(e) is
expression-equivalent to
elements_view<views::all_t<decltype((e))>, M>{e} for any suitable subexpression
e and constant expression
M.
3) Ensures that the elements of the underlying view are
tuple-like values.
4) Ensures that dangling references cannot be returned.
elements_view models the concepts random_access_range, bidirectional_range, forward_range, input_range, common_range, and sized_range when the underlying view V models respective concepts.
Expression-equivalent
Expression e is expression-equivalent to expression f, if.
-
eandfhave the same effects, and - either both are constant subexpressions or else neither is a constant subexpression, and
- either both are potentially-throwing or else neither is potentially-throwing (i.e.
noexcept(e) == noexcept(f)).
Member functions
|
(C++20)
|
constructs a elements_view (public member function) |
|
(C++20)
|
returns a copy of the underlying (adapted) view (public member function) |
|
(C++20)
|
returns an iterator to the beginning (public member function) |
|
(C++20)
|
returns an iterator or a sentinel to the end (public member function) |
|
(C++20)
|
returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range. (public member function) |
Inherited from |
|
|
(C++20)
|
Returns whether the derived view is empty. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>) |
|
(C++20)
|
Returns whether the derived view is not empty. Provided if ranges::empty is applicable to it. (public member function of std::ranges::view_interface<D>) |
|
(C++20)
|
Returns the first element in the derived view. Provided if it satisfies forward_range. (public member function of std::ranges::view_interface<D>) |
|
(C++20)
|
Returns the last element in the derived view. Provided if it satisfies bidirectional_range and common_range. (public member function of std::ranges::view_interface<D>) |
|
(C++20)
|
Returns the nth element in the derived view. Provided if it satisfies random_access_range. (public member function of std::ranges::view_interface<D>) |
Nested classes
|
(C++20)
|
the iterator type (exposition-only member class template) |
|
(C++20)
|
the sentinel type (exposition-only member class template) |
Helper templates
template<class T, std::size_t N>
inline constexpr bool enable_borrowed_range<std::ranges::elements_view<T, N>> =
std::ranges::enable_borrowed_range<T>; |
(since C++20) |
This specialization of std::ranges::enable_borrowed_range makes elements_view satisfy borrowed_range when the underlying view satisfies it.
Example
#include <iostream>
#include <ranges>
#include <tuple>
#include <vector>
int main()
{
const std::vector<std::tuple<int, char, std::string>> vt {
{1, 'A', "α"},
{2, 'B', "β"},
{3, 'C', "γ"},
{4, 'D', "δ"},
{5, 'E', "ε"},
};
for (int const e: std::views::elements<0>(vt)) { std::cout << e << ' '; }
std::cout << '\n';
for (char const e: vt | std::views::elements<1>) { std::cout << e << ' '; }
std::cout << '\n';
for (std::string const& e: std::views::elements<2>(vt)) { std::cout << e << ' '; }
std::cout << '\n';
}
Output:
1 2 3 4 5 A B C D E α β γ δ ε
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 3494 | C++20 | elements_view was never a borrowed_range |
it is a borrowed_range if its underlying view is |
| LWG 3502 | C++20 | dangling reference could be obtained from elements_view |
such usage is forbidden |
See also
|
(C++20)
|
takes a view consisting of pair-like values and produces a view of the first elements of each pair (class template) (range adaptor object) |
|
(C++20)
|
takes a view consisting of pair-like values and produces a view of the second elements of each pair (class template) (range adaptor object) |
| BLAS-like slice of a valarray: starting index, length, stride (class) |
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