std::ranges...tion_result(3)

💅 std::ranges::set_intersection + std::ranges::set_intersection_result 💅

okay so you've opened page std::ranges...tion_result(3) of the C++ Standard Libary manual — yes bestie, the page spells the manual name like that — and we are about to get into some sorted-range main character behavior. ✨

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🌸 NAME 🌸

std::ranges::set_intersection,std::ranges::set_intersection_result - std::ranges::set_intersection,std::ranges::set_intersection_result — the name line literally says it twice, so both of these are the moment. No cap. 💅

✨ Synopsis ✨

Defined in header <algorithm>

📞 Call signature 📞

template< std::input_iterator I1, std::sentinel_for<I1> S1,

          std::input_iterator I2, std::sentinel_for<I2> S2,
          std::weakly_incrementable O, class Comp = ranges::less,
          class Proj1 = std::identity, class Proj2 = std::identity >
 (since
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
 (1) C++20)
    constexpr set_intersection_result<I1, I2, O>
        set_intersection( I1 first1, S1 last1, I2 first2, S2 last2,
                          O result, Comp comp = {},

                          Proj1 proj1 = {}, Proj2 proj2 = {} );
template< ranges::input_range R1, ranges::input_range R2,

          std::weakly_incrementable O, class Comp = ranges::less,
          class Proj1 = std::identity, class Proj2 = std::identity >
requires std::mergeable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
 (2) (since
                            O, Comp, Proj1, Proj2>
 C++20)
constexpr set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                  ranges::borrowed_iterator_t<R2>, O>
    set_intersection( R1&& r1, R2&& r2, O result, Comp comp = {},

                      Proj1 proj1 = {}, Proj2 proj2 = {} );

🧬 Helper types 🧬

template< class I1, class I2, class O >
(3) (since
using set_intersection_result = ranges::in_in_out_result<I1, I2, O>;
C++20)

okay so what it does: it constructs a sorted range beginning at result made of elements that are found in both sorted input ranges [first1, last1) and [first2, last2). If some element is found m times in [first1, last1) and n times in [first2, last2), the first min(m, n) elements will be copied from the first range to result. The order of equivalent elements is preserved. periodt. 🫶

🚨 bestie, undefined behavior zone: the behavior is undefined if: - the input ranges are not sorted with respect to comp and proj1 or proj2, respectively, or - the resulting range overlaps with either of the input ranges.

1. Elements are compared using the given binary comparison function comp.
2. Same as (1), but uses r1 as the first range and r2 as the second range, as if using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as first2, and ranges::end(r2) as last2.

The function-like entities described on this page are niebloids, that is:

• Explicit template argument lists cannot be specified when calling any of them.
• None of them are visible to argument-dependent lookup.
• When any of them are found by normal unqualified lookup as the name to the left of the function-call operator, argument-dependent lookup is inhibited.

In practice, they may be implemented as function objects, or with special compiler extensions. ✨

💖 Parameters 💖

• first1, last1 — iterator-sentinel pair denoting the first input sorted range
• first2, last2 — iterator-sentinel pair denoting the second input sorted range
• r1 — the first sorted input range
• r2 — the second sorted input range
• result — the beginning of the output range
• comp — comparison to apply to the projected elements
• proj1 — projection to apply to the elements in the first range
• proj2 — projection to apply to the elements in the second range

🫶 Return value 🫶

Returns {last1, last2, result_last}, where result_last is the end of the constructed range.

🔥 Complexity 🔥

At most 2·(N_1+N_2)-1 comparisons and applications of each projection, where N_1 and N_2 are ranges::distance(first1, last1) and ranges::distance(first2, last2), respectively. (The page's math rendering also shows \(\scriptsize 2\cdot(N_1+N_2)-1\), and the plain-text fallback is 2·(N 1+N 2)-1 — same number, same vibe.) 🔥

🧚 Possible implementation 🧚

this is the possible implementation exactly as the page prints it — including that first operator() return type saying ranges::set_union_result<I1, I2, O>; yes, bestie, that's what's in the text. 👀

struct set_intersection_fn
{
    template<std::input_iterator I1, std::sentinel_for<I1> S1,
             std::input_iterator I2, std::sentinel_for<I2> S2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 =
std::identity>
    requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
    constexpr ranges::set_union_result<I1, I2, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        while (!(first1 == last1 or first2 == last2))
        {
            if (std::invoke(comp, std::invoke(proj1, *first1),
                                  std::invoke(proj2, *first2)))
                ++first1;
            else if (std::invoke(comp, std::invoke(proj2, *first2),
                                       std::invoke(proj1, *first1)))
                ++first2;
            else
                *result = *first1, ++first1, ++first2, ++result;
        }
        return {ranges::next(std::move(first1), std::move(last1)),
                ranges::next(std::move(first2), std::move(last2)),
                std::move(result)};
    }

    template<ranges::input_range R1, ranges::input_range R2,
             std::weakly_incrementable O, class Comp = ranges::less,
             class Proj1 = std::identity, class Proj2 =
std::identity>
    requires std::mergeable<ranges::iterator_t<R1>,
ranges::iterator_t<R2>,
                            O, Comp, Proj1, Proj2>
    constexpr
ranges::set_intersection_result<ranges::borrowed_iterator_t<R1>,
                                                  ranges::borrowed_iterator_t<R2>,
O>
        operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    }
};

inline constexpr set_intersection_fn set_intersection {};

🎀 Example 🎀

// Run this code

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>

void print(const auto& v, const auto& rem)
{
    std::cout << "{ ";
    for (const auto& e : v)
        std::cout << e << ' ';
    std::cout << '}' << rem;
}

int main()
{
    const auto in1 = {1, 2, 2, 3, 4, 5, 6};
    const auto in2 = {2, 2, 3, 3, 5, 7};
    std::vector<int> out {};

    std::ranges::set_intersection(in1, in2, std::back_inserter(out));

    print(in1, " ∩ "), print(in2, " = "), print(out, "\n");
}

Output:

{ 1 2 2 3 4 5 6 } ∩ { 2 2 3 3 5 7 } = { 2 2 3 5 }

👑 See also 👑

• ranges::set_union — computes the union of two sets (C++20) (niebloid)
• ranges::set_difference — computes the difference between two sets (C++20) (niebloid)
• ranges::set_symmetric_difference — computes the symmetric difference between two sets (C++20) (niebloid)
• ranges::includes — returns true if one sequence is a subsequence of another (C++20) (niebloid)
• set_intersection — computes the intersection of two sets (function template)

okay bye bestie 💖 this page came from http://cppreference.com, dated 2024.06.10, and it's std::ranges...tion_result(3) if you're citing it. slay. ✨