std::ranges::set_union,std::ranges::set_union_result(3)

NAME
std::ranges::set_union,std::ranges::set_union_result - std::ranges::set_union,std::ranges::set_union_result

Synopsis
   Defined in header <algorithm>
   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
   > (1) (since
   requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2> C++20)
   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 = {} );
   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>, (since
   ranges::iterator_t<R2>, (2) C++20)
                        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 = {} );

Helper types
   template< class I1, class I2, class O > (3) (since
   using set_union_result = ranges::in_in_out_result<I1, I2, O>; C++20)
   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).
   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 result, preserving order,
   and then exactly max(n - m, 0) elements will be copied from [first2, last2) to
   result, also preserving order.
   The behavior is undefined if
     * the input ranges are not sorted with respect to comp and proj1 or proj2,
       respectively, or
     * the resulting range overlaps with either of the input ranges.
   1) Elements are compared using the given binary comparison function comp.
   2) Same as (1), but uses r1 as the first range and r2 as the second range, as if
   using ranges::begin(r1) as first1, ranges::end(r1) as last1, ranges::begin(r2) as
   first2, and ranges::end(r2) as last2.
   The function-like entities described on this page are niebloids, that is:
     * Explicit template argument lists cannot be specified when calling any of them.
     * None of them are visible to argument-dependent lookup.
     * When any of them are found by normal unqualified lookup as the name to the left
       of the function-call operator, argument-dependent lookup is inhibited.
   In practice, they may be implemented as function objects, or with special compiler
   extensions.

Parameters
   first1, last1 - iterator-sentinel pair denoting the first input sorted range
   first2, last2 - iterator-sentinel pair denoting the second input sorted range
   r1 - the first input sorted range
   r2 - the second input sorted range
   result - the beginning of the output range
   comp - comparison to apply to the projected elements
   proj1 - projection to apply to the elements in the first range
   proj2 - projection to apply to the elements in the second range

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

Complexity
   At most \(\scriptsize 2\cdot(N_1+N_2)-1\)2·(N
   1+N
   2)-1 comparisons and applications of each projection, where \(\scriptsize N_1\)N
   1 and \(\scriptsize N_2\)N
   2 are ranges::distance(first1, last1) and ranges::distance(first2, last2),
   respectively.

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 \(\scriptsize (N_1+N_2)\)(N
   1+N
   2) values and ranges::set_union may produce less.

Possible implementation
struct set_union_fn {
    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 ranges::set_union_result<I1, I2, O>
        operator()(I1 first1, S1 last1, I2 first2, S2 last2,
                   O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        for (; !(first1 == last1 or first2 == last2); ++result)
        {
            if (std::invoke(comp, std::invoke(proj1, *first1),
                        std::invoke(proj2, *first2)))
            {
                *result = *first1;
                ++first1;
            }
            else if (std::invoke(comp, std::invoke(proj2, *first2),
                        std::invoke(proj1, *first1)))
            {
                *result = *first2;
                ++first2;
            }
            else
            {
                *result = *first1;
                ++first1;
                ++first2;
            }
        }
        auto res1 = ranges::copy(std::move(first1), std::move(last1), std::move(result));
        auto res2 = ranges::copy(std::move(first2), std::move(last2), std::move(res1.out));
        return {std::move(res1.in), std::move(res2.in), std::move(res2.out)};
    }
    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 ranges::set_union_result<ranges::borrowed_iterator_t<R1>,
                        ranges::borrowed_iterator_t<R2>, O>
        operator()(R1&& r1, R2&& r2, O result, Comp comp = {},
                   Proj1 proj1 = {}, Proj2 proj2 = {}) const
    {
        return (*this)(ranges::begin(r1), ranges::end(r1),
                       ranges::begin(r2), ranges::end(r2),
                       std::move(result), std::move(comp),
                       std::move(proj1), std::move(proj2));
    } }; inline constexpr set_union_fn set_union {};

Example
// Run this code
 #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());
 }

Output:
 { 1 2 3 4 5 } ∪ { 3 4 5 6 7 } =
 { 1 2 3 4 5 6 7 }
 { 1 2 3 4 5 5 5 } ∪ { 3 4 5 6 7 } =
 { 1 2 3 4 5 5 5 6 7 }

See also
   ranges::set_difference computes the difference between two sets
   (C++20) (niebloid)
   ranges::set_intersection computes the intersection of two sets
   (C++20) (niebloid)
   ranges::set_symmetric_difference computes the symmetric difference between two sets
   (C++20) (niebloid)
   ranges::merge merges two sorted ranges
   (C++20) (niebloid)
   ranges::includes returns true if one sequence is a subsequence of
   (C++20) another
                        (niebloid)
   set_union computes the union of two sets
                        (function template)