std::ranges::partial_sort_copy,std::ranges::partial_sort_copy_result(3)

NAME
std::ranges::partial_sort_copy,std::ranges::partial_sort_copy_result - std::ranges::partial_sort_copy,std::ranges::partial_sort_copy_result

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::input_iterator I1, std::sentinel_for<I1> S1,
             std::random_access_iterator I2, std::sentinel_for<I2> S2,
             class Comp = ranges::less, class Proj1 = std::identity,
             class Proj2 = std::identity >
   requires std::indirectly_copyable<I1, I2> &&
            std::sortable<I2, Comp, Proj2> &&
            std::indirect_strict_weak_order<Comp, std::projected<I1, (1) (since
   Proj1>, C++20)
                std::projected<I2, Proj2>>
   constexpr partial_sort_copy_result<I1, I2>
       partial_sort_copy( I1 first, S1 last, I2 result_first, S2
   result_last,
                        Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 =
   {} );
   template< ranges::input_range R1, ranges::random_access_range R2,
             class Comp = ranges::less, class Proj1 = std::identity,
             class Proj2 = std::identity >
   requires std::indirectly_copyable<ranges::iterator_t<R1>,
   ranges::iterator_t<R2>> &&
            std::sortable<ranges::iterator_t<R2>, Comp, Proj2> &&
            std::indirect_strict_weak_order<Comp, (2) (since
   std::projected<ranges::iterator_t<R1>, C++20)
                Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>>
   constexpr partial_sort_copy_result<ranges::borrowed_iterator_t<R1>,
                        ranges::borrowed_iterator_t<R2>>
       partial_sort_copy( R1&& r, R2&& result_r,
                        Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 =
   {} );

Helper types
   template< class I, class O > (3) (since
   using partial_sort_copy_result = ranges::in_out_result<I, O>; C++20)
   Copies the first N elements from the source range [first, last), as if it was
   partially sorted with respect to comp and proj1, into the destination range
   [result_first, result_first + N), where \(\scriptsize N = \min{(L_1, L_2)}\)N =
   min(L₁, L₂), \(\scriptsize L_1\)L₁ is equal to ranges::distance(first, last), and
   \(\scriptsize L_2\)L₂ is equal to ranges::distance(result_first, result_last).
   The order of equal elements is not guaranteed to be preserved.
   1) The source range elements are projected using the function object proj1, and the
   destination elements are projected using the function object proj2.
   2) Same as (1), but uses r as the source range and result_r as the destination
   range, as if using ranges::begin(r) as first, ranges::end(r) as last,
   ranges::begin(result_r) as result_first, and ranges::end(result_r) as result_last.
   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
   first, last - iterator-sentinel defining the source range to copy from
   r - the source range to copy from
   result_first, result_last - iterator-sentinel defining the destination range
   result_r - the destination range
   comp - comparison to apply to the projected elements
   proj1 - projection to apply to the elements of source range
   proj2 - projection to apply to the elements of destination range

Return value
   An object equal to {last, result_first + N}.

Complexity
   At most \(\scriptsize L_1 \cdot \log{(N)}\)L₁*log(N) comparisons and \(\scriptsize 2
   \cdot L_1 \cdot \log{(N)}\)2*L₁*log(N) projections.

Possible implementation
 struct partial_sort_copy_fn
 {
     template<std::input_iterator I1, std::sentinel_for<I1> S1,
              std::random_access_iterator I2, std::sentinel_for<I2> S2,
              class Comp = ranges::less, class Proj1 = std::identity,
              class Proj2 = std::identity>
     requires std::indirectly_copyable<I1, I2> && std::sortable<I2, Comp, Proj2> &&
              std::indirect_strict_weak_order<Comp, std::projected<I1, Proj1>,
              std::projected<I2, Proj2>>
     constexpr ranges::partial_sort_copy_result<I1, I2>
         operator()(I1 first, S1 last, I2 result_first, S2 result_last,
                    Comp comp = {}, Proj1 proj1 = {}, Proj2 proj2 = {}) const
     {
         if (result_first == result_last)
             return {std::move(ranges::next(std::move(first), std::move(last))),
                     std::move(result_first)};
         auto out_last{result_first};
         // copy first N elements
         for (; !(first == last or out_last == result_last); ++out_last, ++first)
             *out_last = *first;
         // convert N copied elements into a max-heap
         ranges::make_heap(result_first, out_last, comp, proj2);
         // process the rest of the input range (if any), preserving the heap property
         for (; first != last; ++first)
         {
             if (std::invoke(comp, std::invoke(proj1, *first),
                        std::invoke(proj2, *result_first)))
             {
                 // pop out the biggest item and push in a newly found smaller one
                 ranges::pop_heap(result_first, out_last, comp, proj2);
                 *(out_last - 1) = *first;
                 ranges::push_heap(result_first, out_last, comp, proj2);
             }
         }
         // first N elements in the output range is still
         // a heap - convert it into a sorted range
         ranges::sort_heap(result_first, out_last, comp, proj2);
         return {std::move(first), std::move(out_last)};
     }
     template<ranges::input_range R1, ranges::random_access_range R2,
              class Comp = ranges::less, class Proj1 = std::identity,
              class Proj2 = std::identity>
     requires std::indirectly_copyable<ranges::iterator_t<R1>, ranges::iterator_t<R2>> &&
              std::sortable<ranges::iterator_t<R2>, Comp, Proj2> &&
              std::indirect_strict_weak_order<Comp, std::projected<ranges::iterator_t<R1>,
              Proj1>, std::projected<ranges::iterator_t<R2>, Proj2>>
     constexpr ranges::partial_sort_copy_result<ranges::borrowed_iterator_t<R1>,
               ranges::borrowed_iterator_t<R2>>
         operator()(R1&& r, R2&& result_r, Comp comp = {},
                    Proj1 proj1 = {}, Proj2 proj2 = {}) const
     {
         return (*this)(ranges::begin(r), ranges::end(r),
                        ranges::begin(result_r), ranges::end(result_r),
                        std::move(comp), std::move(proj1), std::move(proj2));
     }
 };
 inline constexpr partial_sort_copy_fn partial_sort_copy {};

Example
// Run this code
 #include <algorithm>
 #include <forward_list>
 #include <functional>
 #include <iostream>
 #include <ranges>
 #include <string_view>
 #include <vector>
 void print(std::string_view rem, std::ranges::input_range auto const& v)
 {
     for (std::cout << rem; const auto& e : v)
         std::cout << e << ' ';
     std::cout << '\n';
 }
 int main()
 {
     const std::forward_list source{4, 2, 5, 1, 3};
     print("Write to the smaller vector in ascending order: ", "");
     std::vector dest1{10, 11, 12};
     print("const source list: ", source);
     print("destination range: ", dest1);
     std::ranges::partial_sort_copy(source, dest1);
     print("partial_sort_copy: ", dest1);
     print("Write to the larger vector in descending order:", "");
     std::vector dest2{10, 11, 12, 13, 14, 15, 16};
     print("const source list: ", source);
     print("destination range: ", dest2);
     std::ranges::partial_sort_copy(source, dest2, std::greater{});
     print("partial_sort_copy: ", dest2);
 }

Output:
 Write to the smaller vector in ascending order:
 const source list: 4 2 5 1 3
 destination range: 10 11 12
 partial_sort_copy: 1 2 3
 Write to the larger vector in descending order:
 const source list: 4 2 5 1 3
 destination range: 10 11 12 13 14 15 16
 partial_sort_copy: 5 4 3 2 1 15 16

See also
   ranges::partial_sort sorts the first N elements of a range
   (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)
   ranges::sort_heap turns a max heap into a range of elements sorted in ascending
   (C++20) order
                        (niebloid)
   ranges::make_heap creates a max heap out of a range of elements
   (C++20) (niebloid)
   ranges::push_heap adds an element to a max heap
   (C++20) (niebloid)
   ranges::pop_heap removes the largest element from a max heap
   (C++20) (niebloid)
   partial_sort_copy copies and partially sorts a range of elements
                        (function template)