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.

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

Parameters

Return value

Iterator past the end of the constructed range.

Complexity

Given N1 as std::distance(first1, last1) and N2 as std::distance(first2, last2):

1,2) At most 2⋅(N1+N2)-1 comparisons using operator<(until C++20)std::less{}(since C++20).

3,4) At most 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

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);
}

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

#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';
}

Defect reports

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

See also