# std::set_symmetric_difference

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class InputIt1, class InputIt2, class OutputIt > OutputIt set_symmetric_difference ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3 > ForwardIt3 set_symmetric_difference ( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, ForwardIt3 d_first ); | (2) | (since C++17)
template< class InputIt1, class InputIt2, class OutputIt, class Compare > OutputIt set_symmetric_difference ( InputIt1 first1, InputIt1 last1, InputIt2 first2, InputIt2 last2, OutputIt d_first, Compare comp ); | (3) | (constexpr since C++20)
template< class ExecutionPolicy, class ForwardIt1, class ForwardIt2, class ForwardIt3, class Compare > ForwardIt3 set_symmetric_difference ( ExecutionPolicy&& policy, ForwardIt1 first1, ForwardIt1 last1, ForwardIt2 first2, ForwardIt2 last2, ForwardIt3 d_first, Compare comp ); | (4) | (since C++17)

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 d_first. The output range is also sorted.

If `[`first1`,` last1`)` contains m elements that are equivalent to each other and `[`first2`,` last2`)` contains n elements that are equivalent to them, then std::abs(m - n) of those elements will be copied to the output range, preserving order:

- if m > n, the final m - n of these elements from `[`first1`,` last1`)`.
- if m < n, the final n - m of these elements from `[`first2`,` last2`)`.

1) If `[`first1`,` last1`)` or `[`first2`,` last2`)` is not sorted with respect to operator<(until C++20) std::less {}(since C++20), the behavior is undefined.
3) If `[`first1`,` last1`)` or `[`first2`,` last2`)` is not sorted with respect to comp, the behavior is undefined.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

std::is_execution_policy_v < std::decay_t <ExecutionPolicy>> is true. | (until C++20)
std::is_execution_policy_v < std::remove_cvref_t <ExecutionPolicy>> is true. | (since C++20)

If the output range overlaps with `[`first1`,` last1`)` or `[`first2`,` last2`)`, the behavior is undefined.

### Parameters

first1, last1 | - | the first sorted range of elements
first2, last2 | - | the second sorted range of elements
d_first | - | the beginning of the output range
policy | - | the execution policy to use

comp

-

comparison function object (i.e. an object that satisfies the requirements of Compare) which returns ​true if the first argument is less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the following:

bool cmp(const Type1& a, const Type2& b);

While the signature does not need to have const&, the function must not modify the objects passed to it and must be able to accept all values of type (possibly const) `Type1` and `Type2` regardless of value category (thus, `Type1&` is not allowed, nor is `Type1` unless for `Type1` a move is equivalent to a copy(since C++11)).
The types Type1 and Type2 must be such that objects of types InputIt1 and InputIt2 can be dereferenced and then implicitly converted to both Type1 and Type2. ​

Type requirements
- `InputIt1, InputIt2` must meet the requirements of LegacyInputIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.
- `ForwardIt1, ForwardIt2, ForwardIt3` must meet the requirements of LegacyForwardIterator.
- `Compare` must meet the requirements of Compare.

### Return value

Iterator past the end of the constructed range.

### Complexity

Given \(\scriptsize N_1\)N1 as std::distance (first1, last1) and \(\scriptsize N_2\)N2 as std::distance (first2, last2):

1,2) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N1+N2)-1 comparisons using operator<(until C++20) std::less {}(since C++20).
3,4) At most \(\scriptsize 2 \cdot (N_1+N_2)-1\)2⋅(N1+N2)-1 applications of the comparison function comp.

### Exceptions

The overloads with a template parameter named `ExecutionPolicy` report errors as follows:

- If execution of a function invoked as part of the algorithm throws an exception and `ExecutionPolicy` is one of the standard policies, std::terminate is called. For any other `ExecutionPolicy`, the behavior is implementation-defined.
- If the algorithm fails to allocate memory, std::bad_alloc is thrown.

### Possible implementation

set_symmetric_difference (1)

    template<class InputIt1, class InputIt2, class OutputIt>
    OutputIt set_symmetric_difference(InputIt1 first1, InputIt1 last1,
                                      InputIt2 first2, InputIt2 last2, OutputIt d_first)
    {
        while (first1 != last1)
        {
            if (first2 == last2)
                return std::copy(first1, last1, d_first);

            if (*first1 < *first2)
                *d_first++ = *first1++;
            else
            {
                if (*first2 < *first1)
                    *d_first++ = *first2;
                else
                    ++first1;
                ++first2;
            }
        }
        return std::copy(first2, last2, d_first);
    }

set_symmetric_difference (3)

    template<class InputIt1, class InputIt2, class OutputIt, class Compare>
    OutputIt set_symmetric_difference(InputIt1 first1, InputIt1 last1,
                                      InputIt2 first2, InputIt2 last2,
                                      OutputIt d_first, Compare comp)
    {
        while (first1 != last1)
        {
            if (first2 == last2)
                return std::copy(first1, last1, d_first);

            if (comp(*first1, *first2))
                *d_first++ = *first1++;
            else
            {
                if (comp(*first2, *first1))
                    *d_first++ = *first2;
                else
                    ++first1;
                ++first2;
            }
        }
        return std::copy(first2, last2, d_first);
    }

### Example

Run this code

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

    int main()
    {
        std::vector<int> v1{1, 2, 3, 4, 5, 6, 7, 8};
        std::vector<int> v2{5, 7, 9, 10};
        std::sort(v1.begin(), v1.end());
        std::sort(v2.begin(), v2.end());

        std::vector<int> v_symDifference;

        std::set_symmetric_difference(v1.begin(), v1.end(), v2.begin(), v2.end(),
                                      std::back_inserter(v_symDifference));

        for (int n : v_symDifference)
            std::cout << n << ' ';
        std::cout << '\n';
    }

Output:

    1 2 3 4 6 8 9 10

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 291 | C++98 | it was unspecified how to handle equivalent elements in the input ranges | specified

### See also

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

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