# std::set_union

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< cpp‎ | algorithm

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

Constructs a sorted union beginning at `d_first` consisting of the set of 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.

1) Elements are compared using `operator<` and the ranges must be sorted with respect to the same.
3) Elements are compared using the given binary comparison function `comp` and the ranges must be sorted with respect to the same.
2,4) Same as (1,3), but executed according to `policy`. This overload only participates in overload resolution if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true

### Parameters

first1, last1 | - | the first input sorted range
first2, last2 | - | the second input sorted range
policy | - | the execution policy to use. See execution policy for details.
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.
- `ForwardIt1, ForwardIt2, ForwardIt3` must meet the requirements of LegacyForwardIterator.
- `OutputIt` must meet the requirements of LegacyOutputIterator.

### 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).

### 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.

### 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 between 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

First version
    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);
    }
Second version
    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

Example with vectors :

Run this code

    #include <vector>
    #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';
        }
        {
            std::vector<int> v1 = {1, 2, 3, 4, 5, 5, 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';
        }
    }

Output:

    1 2 3 4 5 6 7
    1 2 3 4 5 5 5 6 7

### See also

includes | returns true if one set is a subset of another (function template)
merge | merges two sorted ranges (function template)
set_difference | computes the difference between two sets (function template)
set_intersection | computes the intersection of two sets (function template)
set_symmetric_difference | computes the symmetric difference between two sets (function template)

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