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

 

C++

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Freestanding and hosted

Language

Standard library headers

Named requirements

Feature test macros (C++20)

Language support library

Concepts library (C++20)

Metaprogramming library (C++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (C++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (C++17)

Regular expressions library (C++11)

Concurrency support library (C++11)

Technical specifications

Symbols index

External libraries

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range conversions
to (C++23)
Range primitives
range_size_trange_difference_t range_value_t	range_reference_trange_const_reference_trange_rvalue_reference_t   (C++23)	iterator_tconst_iterator_tsentinel_t (C++23)
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range    sized_range constant_range (C++23)	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Factories
empty_viewviews::empty iota_viewviews::iota	single_viewviews::single basic_istream_viewviews::istream	repeat_viewviews::repeat (C++23)(C++23)
Adaptors
ref_view views::all_tviews::all common_viewviews::common owning_view views::counted reverse_viewviews::reverse zip_viewviews::zip (C++23)(C++23) as_const_viewviews::as_const (C++23)(C++23) as_rvalue_viewviews::as_rvalue (C++23)(C++23)	take_viewviews::take take_while_viewviews::take_while drop_viewviews::drop drop_while_viewviews::drop_while zip_transform_viewviews::zip_transform (C++23)(C++23) adjacent_transform_viewviews::adjacent_transform (C++23)(C++23)   cartesian_product_viewviews::cartesian_product (C++23)(C++23)	elements_viewviews::elements keys_viewviews::keys values_viewviews::values transform_viewviews::transform adjacent_viewviews::adjacent (C++23)(C++23) slide_viewviews::slide (C++23)(C++23) stride_viewviews::stride (C++23)(C++23)	filter_viewviews::filter join_viewviews::join split_viewviews::split lazy_split_viewviews::lazy_split join_with_viewviews::join_with (C++23)(C++23) chunk_viewviews::chunk (C++23)(C++23) chunk_by_viewviews::chunk_by (C++23)(C++23)
Range adaptor objects
Range adaptor closure objects
Helper items
copyable-box	simple-view

 

std::ranges::elements_view

Member functions

elements_view::elements_view (C++20)

elements_view::base (C++20)

elements_view::begin (C++20)

elements_view::end (C++20)

elements_view::size (C++20)

Nested classes

Iterator

elements_view::iterator::iterator (C++20)

elements_view::iterator::base (C++20)

elements_view::iterator::operator* (C++20)

elements_view::iterator::operator[] (C++20)

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-= (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

operator==(elements_view::iterator) operator<(elements_view::iterator) operator>(elements_view::iterator) operator<=(elements_view::iterator) operator>=(elements_view::iterator) operator<=>(elements_view::iterator) (C++20)(C++20)(C++20)(C++20)(C++20)(C++20)

operator+(elements_view::iterator) operator-(elements_view::iterator) (C++20)(C++20)

Sentinel

elements_view::sentinel::sentinel (C++20)

elements_view::sentinel::base (C++20)

operator==(elements_view::iterator,elements_view::sentinel) (C++20)

operator-(elements_view::iterator,elements_view::sentinel) (C++20)

 

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

• e and f have 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

(constructor) (C++20)	constructs a elements_view (public member function)
base (C++20)	returns a copy of the underlying (adapted) view (public member function)
begin (C++20)	returns an iterator to the beginning (public member function)
end (C++20)	returns an iterator or a sentinel to the end (public member function)
size (C++20)	returns the number of elements. Provided only if the underlying (adapted) range satisfies sized_range. (public member function)
Inherited from std::ranges::view_interface
empty (C++20)	Returns whether the derived view is empty. Provided if it satisfies sized_range or forward_range. (public member function of std::ranges::view_interface<D>)
operator bool (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>)
front (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>)
back (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>)
operator[] (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

iterator (C++20)	the iterator type (exposition-only member class template)
sentinel (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

Run this code

#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

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)
slice	BLAS-like slice of a valarray: starting index, length, stride (class)