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

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< cpp‎ | ranges

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.

`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

### 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)

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