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std::ranges::views::elements, std::ranges::elements_view

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< cpp‎ | ranges
 
 
Ranges library
Range primitives
Range concepts
Views

Range factories
Range adaptors
 
std::ranges::elements_view
Member functions
Nested classes
Iterator
Sentinel
 
Defined in header <ranges>
template< ranges::input_range V, std::size_t N >

    requires ranges::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 >
    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
(3)
template< class T, std::size_t N >

concept /*has-tuple-element*/ =
    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>&>;

    };
(until C++23)
(exposition only*)
template< class T, std::size_t N >

concept /*has-tuple-element*/ =

    /*tuple-like*/<T> && N < std::tuple_size_v <T>
(since C++23)
(exposition only*)
template< class T, std::size_t N >

concept returnable-element =
     std::is_reference_v <T> || std::move_constructible <

                                       std::tuple_element_t <N, T>>;
(4) (exposition only*)
1) Accepts a view of tuple-like values, and issues a view with a value type of the Nth element of the adapted view's value-type.
2) Every specialization of views::elements is a RangeAdaptorObject. 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, see tuple-like (since C++23).
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.

Contents

[edit] Data members

Member Description
V base_ the underlying view
(exposition-only member object*)

[edit] Member functions

constructs a elements_view
(public member function) [edit]
returns a copy of the underlying (adapted) view
(public member function) [edit]
returns an iterator to the beginning
(public member function) [edit]
returns an iterator or a sentinel to the end
(public member function) [edit]
returns the number of elements, provided only if the underlying (adapted) range satisfies sized_range
(public member function) [edit]
returns the approximate size of the resulting approximately_sized_range
(public member function) [edit]
Inherited from std::ranges::view_interface
returns whether the derived view is empty, provided only if it satisfies sized_range or forward_range
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
returns a constant iterator to the beginning of the range
(public member function of std::ranges::view_interface<D>) [edit]
(C++23)
returns a sentinel for the constant iterator of the range
(public member function of std::ranges::view_interface<D>) [edit]
returns whether the derived view is not empty, provided only if ranges::empty is applicable to it
(public member function of std::ranges::view_interface<D>) [edit]
returns the first element in the derived view, provided if it satisfies forward_range
(public member function of std::ranges::view_interface<D>) [edit]
returns the last element in the derived view, provided only if it satisfies bidirectional_range and common_range
(public member function of std::ranges::view_interface<D>) [edit]
returns the nth element in the derived view, provided only if it satisfies random_access_range
(public member function of std::ranges::view_interface<D>) [edit]

[edit] Nested classes

the iterator type
(exposition-only member class template*)
the sentinel type
(exposition-only member class template*)

[edit] Helper templates

template<class T, std::size_t N>

constexpr bool enable_borrowed_range<std::ranges::elements_view<T, N>> =

    ranges::enable_borrowed_range <T>;
(since C++20)

This specialization of ranges::enable_borrowed_range makes elements_view satisfy borrowed_range when the underlying view satisfies it.

[edit] Example

Run this code
#include <iostream>
#include <ranges>
#include <string>
#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

[edit] 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

[edit] See also

takes a view consisting of pair-like values and produces a view of the first elements of each pair
(class template) (range adaptor object)[edit]
takes a view consisting of pair-like values and produces a view of the second elements of each pair
(class template) (range adaptor object)[edit]
a view consisting of tuples of references to corresponding elements of the adapted views
(class template) (customization point object)[edit]
a view consisting of results of application of a transformation function to corresponding elements of the adapted views
(class template) (customization point object)[edit]
BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]

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