std::ranges:: partial_sort_copy, std::ranges:: partial_sort_copy_result

From cppreference.com

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, Proj1 > , 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 = { } ) ;	(1)	(since C++20)
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 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 = { } ) ;	(2)	(since C++20)
Helper types
template < class I, class O > using partial_sort_copy_result = ranges:: in_out_result < I, O > ;	(3)	(since 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 $N = min(L₁, L₂)$ , $\scriptsize L_1$ $L₁$ 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 algorithm function objects (informally known as 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.

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 $L₁•log(N)$ comparisons and $2•L₁•log(N)$ 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

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::partial_sort (C++20)	sorts the first N elements of a range (algorithm function object)
ranges::sort (C++20)	sorts a range into ascending order (algorithm function object)
ranges::stable_sort (C++20)	sorts a range of elements while preserving order between equal elements (algorithm function object)
ranges::sort_heap (C++20)	turns a max heap into a range of elements sorted in ascending order (algorithm function object)
ranges::make_heap (C++20)	creates a max heap out of a range of elements (algorithm function object)
ranges::push_heap (C++20)	adds an element to a max heap (algorithm function object)
ranges::pop_heap (C++20)	removes the largest element from a max heap (algorithm function object)
partial_sort_copy	copies and partially sorts a range of elements (function template)