std::ranges...s::subrange)(3)

💖 NAME 💖

std::ranges::get(std::ranges::subrange) — std::ranges::get(std::ranges::subrange). Bestie, it's the get overload set that pulls the iterator or sentinel out of a std::ranges::subrange. That's the whole name. No cap. 🫶

This is from the C++ Standard Libary manual — yes, that's the manual name, we're folding it in right here at the top instead of copying the header line. 💅

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✨ Synopsis ✨

Defined in header <ranges>
template< std::size_t N, class I, class S, ranges::subrange_kind K
>
                                                                           (1)
(since C++20)
       requires ((N == 0 && std::copyable<I>) || N == 1)

constexpr auto get( const ranges::subrange<I, S, K>& r );
template< std::size_t N, class I, class S, ranges::subrange_kind K
>
                                                                           (2)
(since C++20)
       requires (N < 2)

constexpr auto get( ranges::subrange<I, S, K>&& r );
namespace std { using ranges::get; }
(3) (since C++20)

1. Obtains the iterator or sentinel from a subrange lvalue (or a const rvalue) when N == 0 or N == 1, respectively. It is mainly provided for structured binding support. Periodt. 🎀
2. Same as (1), except that it takes a non-const subrange rvalue. Subtle, but iconic. ✨
3. (1,2) are imported into namespace std, which simplifies their usage and makes every subrange with a copyable iterator a pair-like type. iykyk. 💜

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🎀 Parameters 🎀

• r — a subrange. That's it. That's the parameter list, bestie. 💅

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🌸 Return value 🌸

1. An iterator or sentinel copy constructed from the stored one when N == 0 or N == 1, respectively. copy constructed, not invented, okay? 🫶
2. Same as (1), except that the iterator is move constructed if N == 0 and I does not satisfy copyable. Move construction said "let me handle this." 🦋

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🛠️ Possible implementation 🛠️

template<std::size_t N, class I, class S, std::ranges::subrange_kind
K>
    requires ((N == 0 && std::copyable<I>) || N == 1)
constexpr auto get(const std::ranges::subrange<I, S, K>& r)
{
    if constexpr (N == 0)
        return r.begin();
    else
        return r.end();
}

template<std::size_t N, class I, class S, std::ranges::subrange_kind
K>
    requires (N < 2)
constexpr auto get(std::ranges::subrange<I, S, K>&& r)
{
    if constexpr (N == 0)
        return r.begin(); // may perform move construction
    else
        return r.end();
}

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🌈 Example 🌈

// Run this code — yes bestie, go run it. 🏃‍♀️💨

        #include <array>
        #include <iostream>
        #include <iterator>
        #include <ranges>

        int main()
        {
            std::array a{1, -2, 3, -4};

            std::ranges::subrange sub_a{std::next(a.begin()),
       std::prev(a.end())};
            std::cout << *std::ranges::get<0>(sub_a) << ' ' // == *(begin(a) +
       1)
                      << *get<1>(sub_a) << '\n';            // == *(end(a) - 1)

            *get<0>(sub_a) = 42; // OK
        //  *get<2>(sub_a) = 13; // hard error: index can only be 0 or 1
        }

💻 Output 💻

-2 -4

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💀 Defect reports 💀

The following behavior-changing defect reports were applied retroactively to previously published C++ standards. History really said "let me fix that." 😭

• LWG 3589 — applied to C++20. Behavior as published: the overload for const lvalue was ill-formed if N == 0 and I does not model copyable. Correct behavior: it is removed from the overload set. Slay. 💅

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🦋 See also 🦋

• Structured binding (C++17) binds the specified names to sub-objects or tuple elements of the initializer. the moment structured bindings ate. 🎀
• get(std::tuple) (C++11) tuple accesses specified element (function template). ✨
• get(std::pair) (C++11) accesses an element of a pair (function template). 💖
• get(std::array) (C++11) accesses an element of an array (function template). 🌸

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okay that's the whole page, bestie 💅 — source/version/date: http://cppreference.com, 2024.06.10, and the manual is the C++ Standard Libary one. mwah 🎀✨