std::ranges:: set_intersection, std::ranges:: set_intersection_result

Constructs a sorted range beginning at result consisting 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.

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.

The function-like entities described on this page are algorithm function objects (informally known as 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.

Parameters

Return value

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

Complexity

At most \(\scriptsize 2\cdot(N_1+N_2)-1\) 2·(N 1 +N 2 )-1 comparisons and applications of each projection, where \(\scriptsize N_1\) N 1 and \(\scriptsize N_2\) N 2 are ranges:: distance ( first1, last1 ) and ranges:: distance ( first2, last2 ) , respectively.

Possible implementation

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

#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");
}

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

See also