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std::ranges::views::values, std::ranges::values_view

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< cpp‎ | ranges
 
 
Ranges library
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(C++23)

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(until C++23)(C++23)


 
Defined in header <ranges>
template< class R >
using values_view = ranges::elements_view<R, 1>;
(1) (since C++20)
namespace views {

    inline constexpr auto values = ranges::elements<1>;

}
(2) (since C++20)

Takes a view of tuple-like values (e.g. std::tuple or std::pair), and produces a view with a value-type of the second element of the adapted view's value-type.

1) An alias for ranges::elements_view<R, 1>.
2) RangeAdaptorObject (and also RangeAdaptorClosureObject). The expression views::values(e) is expression-equivalent to values_view<views::all_t<decltype((e))>>{e} for any suitable subexpression e.

Contents

[edit] Notes

values_view can be useful for extracting values from associative containers, e.g.

std::map<int, std::string> map{{1, "alpha"}, {2, "beta"}};
for (auto const& value : std::views::values(map))
    std::cout << value << ' ';
// prints: alpha beta

[edit] Example

#include <iostream>
#include <map>
#include <ranges>
 
int main()
{
    const auto list = {std::pair{1, 11.1}, {2, 22.2}, {3, 33.3}};
    std::cout << "pair::second values in the list: ";
    for (double value : list | std::views::values)
        std::cout << value << ' ';
 
    std::map<char, int> map{{'A', 1}, {'B', 2}, {'C', 3}, {'D', 4}, {'E', 5}};
    auto odd = [](int x) { return 0 != (x & 1); };
    std::cout << "\nodd values in the map: ";
    for (int value : map | std::views::values | std::views::filter(odd))
        std::cout << value << ' ';
    std::cout << '\n';
}

Output:

pair::second values in the list: 11.1 22.2 33.3
odd values in the map: 1 3 5

[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 3563 C++20 keys_view is unable to participate in CTAD due to its use of views::all_t views::all_t removed

[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 tuple-like values and a number N and produces a view of Nth element of each tuple
(class template) (range adaptor object)[edit]
BLAS-like slice of a valarray: starting index, length, stride
(class) [edit]