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

 

C++

Language

Standard Library Headers

Freestanding and hosted implementations

Named requirements

Language support library

Concepts library (C++20)

Diagnostics library

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)

Atomic operations library (C++11)

Thread support library (C++11)

Technical Specifications

 

Ranges library

Range access
begin cbegin end cend	rbegin crbegin rend crend	size ssize data cdata	empty
Range primitives
iterator_tsentinel_t	range_size_trange_difference_t	range_value_t	range_reference_trange_rvalue_reference_t
Dangling iterator handling
dangling	borrowed_iterator_t	borrowed_subrange_t
Range concepts
range borrowed_range sized_range	common_range view viewable_range	input_range output_range forward_range	bidirectional_range random_access_range contiguous_range
Views
view_interface	subrange
Semiregular wrapper
semiregular_box
Factories
empty_viewviews::empty	single_viewviews::single	iota_viewviews::iota	basic_istream_viewistream_view
Adaptors
views::all_tviews::all common_viewviews::common join_viewviews::join split_viewviews::split	take_viewviews::take drop_viewviews::drop take_while_viewviews::take_while drop_while_viewviews::drop_while	transform_viewviews::transform reverse_viewviews::reverse filter_viewviews::filter ref_view	views::counted elements_viewviews::elements keys_viewviews::keys values_viewviews::values

 

std::ranges::elements_view

Member functions

elements_view::elements_view

elements_view::base

elements_view::begin

elements_view::end

elements_view::size

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         view<V> &&         __HasTupleElement<ranges::range_value_t<V>, N> &&         __HasTupleElement<std::remove_reference_t<ranges::range_reference_t<V>>, N> &&         __ReturnableElement<<ranges::range_reference_t<V>, N> class elements_view : public ranges::view_interface<elements_view<V, N>>;	(1)	(since C++20)
namespace views {     inline constexpr /*unspecified*/ elements = /*unspecified*/; }	(2)	(since C++20)
Helper concepts
template< class T, std::size_t N > concept __HasTupleElement =  // 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 __ReturnableElement =  // 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) 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, either are both potentially-throwing or are both not potentially-throwing (i.e. noexcept(e) == noexcept(f)), and either are both constant subexpressions or are both not constant subexpressions.

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	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 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	the iterator type (public member class)
sentinel	the sentinel type (public member class)

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: std::views::elements<1>(vt)) { 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)