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

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Diagnostics library

Memory management library

Metaprogramming library (C++11)

General utilities library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library (C++17)

Concurrency support library (C++11)

Execution control library (C++26)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access begin cbegin   end cend rbegin crbegin   rend crend size ssize   data cdata reserve_hint (C++26)   empty

Range conversions std::from_range_t std::from_range (C++23)(C++23)     to (C++23)

Dangling iterator handling dangling borrowed_iterator_t borrowed_subrange_t

Range primitives
range_size_trange_difference_trange_value_t elements_of (C++23)	iterator_tconst_iterator_tsentinel_tconst_sentinel_t (C++23)(C++23)	range_reference_trange_const_reference_trange_rvalue_reference_trange_common_reference_t (C++23)

Range concepts
range borrowed_range    common_range	sized_range viewable_range    view	input_range output_range forward_range	bidirectional_range    random_access_range contiguous_range	approximately_sized_range (C++26) constant_range (C++23)

Views
view_interface	subrange

Range factories
empty_viewviews::empty	single_viewviews::single	basic_istream_viewviews::istream	iota_viewviews::iota	repeat_viewviews::repeat (C++23)(C++23)

Range adaptors
views::all_tviews::all as_rvalue_viewviews::as_rvalue (C++23)(C++23)   filter_viewviews::filter transform_viewviews::transform take_viewviews::take take_while_viewviews::take_while common_viewviews::common views::counted as_input_viewviews::as_input (C++26)(C++26)	ref_view drop_viewviews::drop drop_while_viewviews::drop_while lazy_split_viewviews::lazy_split split_viewviews::split join_viewviews::join join_with_viewviews::join_with (C++23)(C++23)   concat_viewviews::concat (C++26)(C++26) cache_latest_viewviews::cache_latest (C++26)(C++26)	owning_view reverse_viewviews::reverse as_const_viewviews::as_const (C++23)(C++23) elements_viewviews::elements keys_viewviews::keys values_viewviews::values enumerate_viewviews::enumerate (C++23)(C++23) zip_viewviews::zip (C++23)(C++23) zip_transform_viewviews::zip_transform (C++23)(C++23)	adjacent_viewviews::adjacent (C++23)(C++23) views::pairwise (C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23) views::pairwise_transform (C++23) chunk_viewviews::chunk (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23) stride_viewviews::stride (C++23)(C++23) cartesian_product_viewviews::cartesian_product (C++23)(C++23)

Range generators std::generator (C++23)

Range adaptor closure objects range_adaptor_closure (C++23)

Range adaptor objects

Helper items
copyable-box movable-box (until C++23)  (C++23)	simple-view non-propagating-cache

 

std::ranges::elements_view

Member functions

elements_view::elements_view

elements_view::base

elements_view::begin

elements_view::end

elements_view::size

elements_view::reserve_hint (C++26)

Nested classes

Iterator

elements_view::iterator::iterator

elements_view::iterator::base

elements_view::iterator::operator*

elements_view::iterator::operator[]

elements_view::iterator::operator++ elements_view::iterator::operator++(int) elements_view::iterator::operator-- elements_view::iterator::operator--(int) elements_view::iterator::operator+= elements_view::iterator::operator-=

operator==(elements_view::iterator) operator<(elements_view::iterator) operator>(elements_view::iterator) operator<=(elements_view::iterator) operator>=(elements_view::iterator) operator<=>(elements_view::iterator)

operator+(elements_view::iterator) operator-(elements_view::iterator)

Sentinel

elements_view::sentinel::sentinel

elements_view::sentinel::base

operator==(elements_view::iterator,elements_view::sentinel)

operator-(elements_view::iterator,elements_view::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 N^th 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.

See More

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.

Data members

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

Member functions

(constructor)	constructs a elements_view (public member function)
base	returns a copy of the underlying (adapted) view (public member function)
begin	returns an iterator to the beginning (public member function)
end	returns an iterator or a sentinel to the end (public member function)
size	returns the number of elements, provided only if the underlying (adapted) range satisfies sized_range (public member function)
reserve_hint (C++26)	returns the approximate size of the resulting approximately_sized_range (public member function)
Inherited from ranges::view_interface
empty	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>)
cbegin (C++23)	returns a constant iterator to the beginning of the range (public member function of std::ranges::view_interface<D>)
cend (C++23)	returns a sentinel for the constant iterator of the range (public member function of std::ranges::view_interface<D>)
operator bool	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>)
front	returns the first element in the derived view, provided if it satisfies forward_range (public member function of std::ranges::view_interface<D>)
back	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>)
operator[]	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>)

Nested classes

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

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.

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

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

ranges::keys_viewviews::keys (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)
ranges::values_viewviews::values (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)
ranges::zip_viewviews::zip (C++23)	a view consisting of tuples of references to corresponding elements of the adapted views (class template) (customization point object)
ranges::zip_transform_viewviews::zip_transform (C++23)	a view consisting of results of application of a transformation function to corresponding elements of the adapted views (class template) (customization point object)
slice	BLAS-like slice of a valarray: starting index, length, stride (class)