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

From cppreference.com

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

^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.

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 name	Definition
`base_` (private)	the underlying (adapted) view of type `V` ( 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)
Inherited from std::ranges::view_interface
empty	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>` )
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 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 if it satisfies `bidirectional_range` and `common_range` . (public member function of `std::ranges::view_interface<D>` )
operator[]	returns the `n` ^th element in the derived view. Provided 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

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)
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)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

Member functions
elements_view::elements_view
elements_view::base
elements_view::begin
elements_view::end
elements_view::size
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 )

ranges::keys_view views::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_view views::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_view views::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_view views::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)