std::ranges::merge,std::ranges::merge_result(3)

NAME
std::ranges::merge,std::ranges::merge_result - std::ranges::merge,std::ranges::merge_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 C++20)
   requires std::mergeable<I1, I2, O, Comp, Proj1, Proj2>
   constexpr merge_result<I1, I2, O>
       merge( 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>,
   ranges::iterator_t<R2>, (2) (since C++20)
                        O, Comp, Proj1, Proj2>
   constexpr merge_result<ranges::borrowed_iterator_t<R1>,
                        ranges::borrowed_iterator_t<R2>, O>
       merge( R1&& r1, R2&& r2, O result, Comp comp = {},
              Proj1 proj1 = {}, Proj2 proj2 = {} );

Helper types
   template< class I1, class I2, class O > (3) (since C++20)
   using merge_result = ranges::in_in_out_result<I1, I2, O>;
   Merges two sorted ranges [[first1, last1) and [first2, last2) into one sorted range
   beginning at result.
   A sequence is said to be sorted with respect to the comparator comp if for any
   iterator it pointing to the sequence and any non-negative integer n such that it + n
   is a valid iterator pointing to an element of the sequence, std::invoke(comp,
   std::invoke(proj2, *(it + n)), std::invoke(proj1, *it))) evaluates to false.
   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 behavior is undefined if the destination range overlaps either of the input
   ranges (the input ranges may overlap each other).
   This merge function is stable, which means that for equivalent elements in the
   original two ranges, the elements from the first range (preserving their original
   order) precede the elements from the second range (preserving their original order).
   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 - the first input sorted range
   first2, last2 - 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 N − 1 comparisons and applications of each projection, where N =
   ranges::distance(first1, last1) + ranges::distance(first2, last12).

Notes
   This algorithm performs a similar task as ranges::set_union 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 max(n, m)
   ones only. So ranges::merge outputs exactly N values and ranges::set_union may
   produce fewer.

Possible implementation
struct merge_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::merge_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(proj2, *first2), std::invoke(proj1, *first1)))
                *result = *first2, ++first2;
            else
                *result = *first1, ++first1;
        }
        auto ret1{ranges::copy(std::move(first1), std::move(last1), std::move(result))};
        auto ret2{ranges::copy(std::move(first2), std::move(last2), std::move(ret1.out))};
        return {std::move(ret1.in), std::move(ret2.in), std::move(ret2.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::merge_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 merge_fn merge {};

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 << "} +\n{ ";
     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::merge(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::merge(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 3 4 4 5 5 6 7 }
 { 1 2 3 4 5 5 5 } +
 { 3 4 5 6 7 } =
 { 1 2 3 3 4 4 5 5 5 5 6 7 }

See also
   ranges::inplace_merge merges two ordered ranges in-place
   (C++20) (niebloid)
   ranges::is_sorted checks whether a range is sorted into ascending order
   (C++20) (niebloid)
   ranges::set_union computes the union of two sets
   (C++20) (niebloid)
   ranges::sort sorts a range into ascending order
   (C++20) (niebloid)
   ranges::stable_sort sorts a range of elements while preserving order between equal
   (C++20) elements
                        (niebloid)
   merge merges two sorted ranges
                        (function template)