std::set_union

template< class InputIt1, class InputIt2, class OutputIt >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
                    InputIt2 first2, InputIt2 last2,
                    OutputIt d_first );

template< class InputIt1, class InputIt2,
          class OutputIt, class Compare >
OutputIt set_union( InputIt1 first1, InputIt1 last1,
                    InputIt2 first2, InputIt2 last2,
                    OutputIt d_first, Compare comp );

Constructs a sorted range beginning at d_first consisting of all elements present in one or both sorted ranges [first1, last1) and [first2, last2).

If some element is found m times in [first1, last1) and n times in [first2, last2), then all m elements will be copied from [first1, last1) to d_first, preserving order, and then exactly std::max(n-m, 0) elements will be copied from [first2, last2) to d_first, also preserving order.

The resulting range cannot overlap with either of the input ranges.

Parameters

Return value

Iterator past the end of the constructed range.

Complexity

At most 2·(N₁+N₂-1) comparisons, where N₁= std::distance(first1, last1) and N₂= std::distance(first2, last2).

Notes

This algorithm performs a similar task as std::merge does. Both consume two sorted input ranges and produce a sorted output with elements from both inputs. The difference bewteen these two algorithms is with handling values from both input ranges which compare equivalent (see notes on LessThanComparable). If any equivalent values appeared n times in the first range and m times in the second, std::merge would output all n+m occurrences whereas std::set_union would output std::max(n, m) ones only. So std::merge outputs exactly std::distance(first1, last1) + std::distance(first2, last2) values and std::set_union may produce less.

Possible implementation

template<class InputIt1, class InputIt2, class OutputIt>
OutputIt set_union(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first)
{
    for (; first1 != last1; ++d_first) {
        if (first2 == last2)
            return std::copy(first1, last1, d_first);
        if (*first2 < *first1) {
            *d_first = *first2++;
        } else {
            *d_first = *first1;
            if (!(*first1 < *first2))
                ++first2;
            ++first1;
        }
    }
    return std::copy(first2, last2, d_first);
}

template<class InputIt1, class InputIt2,
         class OutputIt, class Compare>
OutputIt set_union(InputIt1 first1, InputIt1 last1,
                   InputIt2 first2, InputIt2 last2,
                   OutputIt d_first, Compare comp)
{
    for (; first1 != last1; ++d_first) {
        if (first2 == last2)
            return std::copy(first1, last1, d_first);
        if (comp(*first2, *first1)) {
            *d_first = *first2++;
        } else {
            *d_first = *first1;
            if (!comp(*first1, *first2))
                ++first2;
            ++first1;
        }
    }
    return std::copy(first2, last2, d_first);
}

Example

#include <vector>
#include <set>
#include <iostream>
#include <algorithm>
#include <iterator>
 
int main()
{
    std::vector<int> v1 = {1, 2, 3, 4, 5}; 
    std::vector<int> v2 = {      3, 4, 5, 6, 7}; 
    std::vector<int> dest1;
 
    std::set_union(v1.begin(), v1.end(),
                   v2.begin(), v2.end(),                  
                   std::back_inserter(dest1));
 
    for (const auto &i : dest1) {
        std::cout << i << ' ';
    }   
    std::cout << '\n';
}

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See also