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

From cppreference.com
< cpp‎ | ranges
 
 
 
 
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.

Contents

[edit] 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.

[edit] Member functions

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

[edit] Nested classes

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

[edit] 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.

[edit] Example

#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

[edit] 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

[edit] See also

takes a view consisting of pair-like values and produces a view of the first elements of each pair
(class template) (range adaptor object) [edit]
takes a view consisting of pair-like values and produces a view of the second elements of each pair
(class template) (range adaptor object) [edit]
BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]