std::ranges...:values_view(3)

NAME 🎀

okay so welcome to std::ranges...:values_view(3) from the C++ Standard Libary — we're talking about std::ranges::views::values,std::ranges::values_view, and yes bestie, that comma is right there in the name. It's giving "grab the second element of tuple-like things" and honestly it ate. 💅

Synopsis ✨

Defined in header <ranges>:

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

    inline constexpr auto values = ranges::elements<1>; (2) (since C++20)

}

So here's the tea, no cap: this 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. It's an alias for ranges::elements_view<R, 1>.
2. It's a RangeAdaptorObject (and also a RangeAdaptorClosureObject). The expression views::values(e) is expression-equivalent to values_view<views::all_t<decltype((e))>>{e} for any suitable subexpression e.

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

That's lowkey iconic when you just want the values out of an associative container, iykyk. 🦋

Example 🔥

// Run this code — hit it, bestie:

#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

Defect reports 🩹

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. Not me being dramatic, this is literally the official list:

• DR: LWG 3563
• Applied to: C++20
• Behavior as published: keys_view is unable to participate in CTAD due to its use of views::all_t
• Correct behavior: views::all_t removed

See also 👀

• 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::elements_view / views::elements (C++20) — takes a view consisting of tuple-like values and a number N and produces a view of N^th element of each tuple (class template) (range adaptor object)
• slice — BLAS-like slice of a valarray: starting index, length, stride (class)

mwah 💋 — source/version: http://cppreference.com, date: 2024.06.10, and yes the page is still std::ranges...:values_view(3). That's the whole vibe. Bye bestie! 🫶🌈