std::ranges::inplace_merge

Defined in header `<algorithm>`
Call signature
template < std::bidirectional_iterator I, std::sentinel_for <I> S, class Comp = ranges::less, class Proj = std::identity > requires std::sortable <I, Comp, Proj> I inplace_merge( I first, I middle, S last, Comp comp = { }, Proj proj = { } ) ;	(1)	(since C++20) (constexpr since C++26)
template < ranges::bidirectional_range R, class Comp = ranges::less, class Proj = std::identity > requires std::sortable < ranges::iterator_t <R>, Comp, Proj> ranges::borrowed_iterator_t <R> inplace_merge( R&& r, ranges::iterator_t <R> middle, Comp comp = { }, Proj proj = { } ) ;	(2)	(since C++20) (constexpr since C++26)

Merges two consecutive sorted ranges [first, middle) and [middle, last) into one sorted range [first, last)

A sequence is said to be sorted with respect to the comparator comp and projection proj 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 (proj, * (it + n) ), std::invoke (proj, *it) ) ) evaluates to false

This merge function is stable

1) Elements are compared using the given binary comparison function comp and projection object proj

2) Same as (1), but uses r as the range, as if using ranges::begin(r) as first, and ranges::end(r) as 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

Parameters

first	-	the beginning of the first sorted range
middle	-	the end of the first range and the beginning of the second range
last	-	the end of the second sorted range
r	-	the range of elements to merge inplace
comp	-	comparison to apply to the projected elements
proj	-	projection to apply to the elements in the range

Return value

An iterator equal to last.

Complexity

Exactly N − 1 comparisons, if additional memory buffer is available, where N = ranges::distance (first, last) . Otherwise, $𝓞(N•log(N))$

Notes

This function attempts to allocate a temporary buffer. If the allocation fails, the less efficient algorithm is chosen.

Feature-test macro	Value	Std	Feature
`__cpp_lib_constexpr_algorithms`	`202306L`	(C++26)	constexpr stable sorting

Possible implementation

This implementation only shows the slower algorithm used when no additional memory is available. See also the implementation in MSVC STL and libstdc++

struct inplace_merge_fn
{
    template<std::bidirectional_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity>
    requires std::sortable<I, Comp, Proj>
    constexpr I operator()(I first, I middle, S last, Comp comp = {}, Proj proj = {}) const
    {
        I last_it = ranges::next(middle, last);
        inplace_merge_slow(first, middle, last_it,
                           ranges::distance(first, middle),
                           ranges::distance(middle, last_it),
                           std::ref(comp), std::ref(proj));
        return last_it;
    }
 
    template<ranges::bidirectional_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr ranges::borrowed_iterator_t<R>
        operator()(R&& r, ranges::iterator_t<R> middle,
                   Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), std::move(middle), ranges::end(r),
                       std::move(comp), std::move(proj));
    }
 
private:
    template<class I, class Comp, class Proj>
    static constexpr void inplace_merge_slow(I first, I middle, I last,
                                             std::iter_difference_t<I> n1,
                                             std::iter_difference_t<I> n2,
                                             Comp comp, Proj proj)
    {
        if (n1 == 0 || n2 == 0)
            return;
        if (n1 + n2 == 2 && comp(proj(*middle), proj(*first)))
        {
            ranges::iter_swap(first, middle);
            return;
        }
 
        I cut1 = first, cut2 = middle;
        std::iter_difference_t<I> d1{}, d2{};
 
        if (n1 > n2)
        {
            d1 = n1 / 2;
            ranges::advance(cut1, d1);
            cut2 = ranges::lower_bound(middle, last, *cut1,
                                       std::ref(comp), std::ref(proj));
            d2 = ranges::distance(middle, cut2);
        }
        else
        {
            d2 = n2 / 2;
            ranges::advance(cut2, d2);
            cut1 = ranges::upper_bound(first, middle, *cut2,
                                       std::ref(comp), std::ref(proj));
            d1 = ranges::distance(first, cut1);
        }
 
        I new_middle = ranges::rotate(cut1, middle, cut2);
        inplace_merge_slow(first, cut1, new_middle, d1, d2,
                           std::ref(comp), std::ref(proj));
        inplace_merge_slow(new_middle, cut2, last, n1 - d1, n2 - d2,
                           std::ref(comp), std::ref(proj));
    }
};
 
inline constexpr inplace_merge_fn inplace_merge {};

Example

Run this code

#include <algorithm>
#include <complex>
#include <functional>
#include <iostream>
#include <iterator>
#include <vector>
 
void print(auto const& v, auto const& rem, int middle = -1)
{
    for (int i{}; auto n : v)
        std::cout << (i++ == middle ? "│ " : "") << n << ' ';
    std::cout << rem << '\n';
}
 
template<std::random_access_iterator I, std::sentinel_for<I> S>
requires std::sortable<I>
void merge_sort(I first, S last)
{
    if (last - first > 1)
    {
        I middle{first + (last - first) / 2};
        merge_sort(first, middle);
        merge_sort(middle, last);
        std::ranges::inplace_merge(first, middle, last);
    }
}
 
int main()
{
    // custom merge-sort demo
    std::vector v{8, 2, 0, 4, 9, 8, 1, 7, 3};
    print(v, ": before sort");
    merge_sort(v.begin(), v.end());
    print(v, ": after sort\n");
 
    // merging with comparison function object and projection
    using CI = std::complex<int>;
    std::vector<CI> r{{0,1}, {0,2}, {0,3}, {1,1}, {1,2}};
    const auto middle{std::ranges::next(r.begin(), 3)};
    auto comp{std::ranges::less{}};
    auto proj{[](CI z) { return z.imag(); }};
 
    print(r, ": before merge", middle - r.begin());
    std::ranges::inplace_merge(r, middle, comp, proj);
    print(r, ": after merge");
}

Output:

8 2 0 4 9 8 1 7 3 : before sort
0 1 2 3 4 7 8 8 9 : after sort
 
(0,1) (0,2) (0,3) │ (1,1) (1,2) : before merge
(0,1) (1,1) (0,2) (1,2) (0,3) : after merge

Compiler support
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Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Diagnostics library
Memory management library
Metaprogramming library (C++11)
General utilities library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Strings library
Text processing library
Numerics library
Date and time library
Input/output library
Filesystem library (C++17)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

ranges::merge (C++20)	merges two sorted ranges (algorithm function object)
ranges::set_union (C++20)	computes the union of two sets (algorithm function object)
ranges::is_sorted (C++20)	checks whether a range is sorted into ascending order (algorithm function object)
ranges::sort (C++20)	sorts a range into ascending order (algorithm function object)
inplace_merge	merges two ordered ranges in-place (function template)