# std::ranges::set_symmetric_difference, std::ranges::set_symmetric_difference_result

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< cpp‎ | algorithm‎ | ranges

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 > requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2> constexpr set_symmetric_difference_result<I1, I2, O> set_symmetric_difference( I1 first1, S1 last1, I2 first2, S2 last2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (1) | (since C++20)
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 set_symmetric_difference_result<ranges::borrowed_iterator_t<R1>, ranges::borrowed_iterator_t<R2>, O> set_symmetric_difference( R1&& r1, R2&& r2, O result, Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {} ); | (2) | (since C++20)
Helper types |  |
template< class I1, class I2, class O > using set_symmetric_difference_result = ranges::in_in_out_result<I1, I2, O>; | (3) | (since C++20)

Computes symmetric difference of two sorted ranges: the elements that are found in either of the ranges, but not in both of them are copied to the range beginning at result. The resulting range is also sorted.

If some element is found `m` times in `[`first1`,` last1`)` and `n` times in `[`first2`,` last2`)`, it will be copied to result exactly `│m - n│` times. If `m > n`, then the last `m - n` of those elements are copied from `[`first1`,` last1`)`, otherwise the last `n - m` elements are copied from `[`first2`,` last2`)`. The resulting range cannot overlap with either of the input ranges.

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

{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_symmetric_difference_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_symmetric_difference_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)))
                {
                    *result = *first1;
                    ++first1;
                    ++result;
                }
                else if (std::invoke(comp, std::invoke(proj2, *first2),
                                           std::invoke(proj1, *first1)))
                {
                    *result = *first2;
                    ++first2;
                    ++result;
                }
                else
                {
                    ++first1;
                    ++first2;
                }
            }
            auto res1 {ranges::copy(std::move(first1), std::move(last1), std::move(result))};
            auto res2 {ranges::copy(std::move(first2), std::move(last2), std::move(res1.out))};
            return {std::move(res1.in), std::move(res2.in), std::move(res2.out)};
        }

        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_symmetric_difference_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_symmetric_difference_fn set_symmetric_difference {};

### Example

Run this code

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

    void visualize_this(const auto& v, int min = 1, int max = 9)
    {
        for (auto i {min}; i <= max; ++i)
        {
            std::ranges::binary_search(v, i) ? std::cout << i : std::cout << '.';
            std::cout << ' ';
        }
        std::cout << '\n';
    }

    int main()
    {
        const auto in1 = {1, 3, 4,    6, 7, 9};
        const auto in2 = {1,    4, 5, 6,    9};

        std::vector<int> out {};

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

        visualize_this(in1);
        visualize_this(in2);
        visualize_this(out);
    }

Output:

    1 . 3 4 . 6 7 . 9
    1 . . 4 5 6 . . 9
    . . 3 . 5 . 7 . .

### See also

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

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