std::ranges::set_union, std::ranges::set_union_result

Since C++20

Header: <algorithm>

Constructs a sorted union beginning at result consisting of the set of elements present in one or both sorted input ranges [first1,last1) and [first2,last2).

# Declarations

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_union_result<I1, I2, O>
set_union( I1 first1, S1 last1, I2 first2, S2 last2,
O result, Comp comp = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );

(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_union_result<ranges::borrowed_iterator_t<R1>,
ranges::borrowed_iterator_t<R2>, O>
set_union( R1&& r1, R2&& r2, O result, Comp comp = {},
Proj1 proj1 = {}, Proj2 proj2 = {} );

(since C++20)

Helper types

template< class I1, class I2, class O >
using set_union_result = ranges::in_in_out_result<I1, I2, O>;

(since C++20)

# Parameters

# Return value

{last1, last2, result_last}, where result_last is the end of the constructed range.

# Notes

This algorithm performs a similar task as ranges::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, ranges::merge would output all n+m occurrences whereas ranges::set_union would output std::max(n, m) ones only. So ranges::merge outputs exactly (\scriptsize (N_1+N_2))(N1+N2) values and ranges::set_union may produce less.

# Example

#include <algorithm>
#include <iostream>
#include <iterator>
#include <vector>
 
void print(const auto& in1, const auto& in2, auto first, auto last)
{
    std::cout << "{ ";
    for (const auto& e : in1)
        std::cout << e << ' ';
    std::cout << "} ∪ { ";
    for (const auto& e : in2)
        std::cout << e << ' ';
    std::cout << "} =\n{ ";
    while (!(first == last))
        std::cout << *first++ << ' ';
    std::cout << "}\n\n";
}
 
int main()
{
    std::vector<int> in1, in2, out;
 
    in1 = {1, 2, 3, 4, 5};
    in2 = {      3, 4, 5, 6, 7};
    out.resize(in1.size() + in2.size());
    const auto ret = std::ranges::set_union(in1, in2, out.begin());
    print(in1, in2, out.begin(), ret.out);
 
    in1 = {1, 2, 3, 4, 5, 5, 5};
    in2 = {      3, 4, 5, 6, 7};
    out.clear();
    out.reserve(in1.size() + in2.size());
    std::ranges::set_union(in1, in2, std::back_inserter(out));
    print(in1, in2, out.cbegin(), out.cend());
}

# See also