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std::ranges::stable_sort

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

Sorts the elements in the range [first, last) in non-descending order. The order of equivalent elements is stable, i.e. guaranteed to be preserved.

A sequence is sorted with respect to a 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(proj, *(it + n)), std::invoke(proj, *it) evaluates to false.

1) Elements are compared using the given binary comparison function comp.

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 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 range to sort
r	-	the range to sort
comp	-	comparison to apply to the projected elements
proj	-	projection to apply to the elements

Return value

An iterator equal to last.

Complexity

\(\scriptsize N\cdot\log{(N)}\)N·log(N) comparisons, if extra memory is available; where \(\scriptsize N\)N is ranges::distance(first, last). \(\scriptsize N\cdot\log^2{(N)}\)N·log²(N) comparisons otherwise. Twice as many projections as the number of comparisons in both cases.

Notes

Feature-test macro	Value	Comment
`__cpp_lib_constexpr_algorithms`	202306L	`constexpr` stable sorting

Possible implementation

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

struct stable_sort_fn
{
    template<std::random_access_iterator I, std::sentinel_for<I> S,
             class Comp = ranges::less, class Proj = std::identity>
    requires std::sortable<I, Comp, Proj>
    constexpr //< since C++26
    I operator()(I first, S last, Comp comp = {}, Proj proj = {}) const
    {
        auto count = ranges::distance(first, last);
        auto mid = first + count / 2;
        auto last_it = first + count;
 
        if (count <= 1)
            return last_it;
 
        (*this)(first, mid, std::ref(comp), std::ref(proj));
        (*this)(mid, last_it, std::ref(comp), std::ref(proj));
 
        ranges::inplace_merge(first, mid, last_it);
 
        return last_it;
    }
 
    template<ranges::random_access_range R, class Comp = ranges::less,
             class Proj = std::identity>
    requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
    constexpr //< since C++26
    ranges::borrowed_iterator_t<R> operator()(R&& r, Comp comp = {}, Proj proj = {}) const
    {
        return (*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj));
    }
};
 
inline constexpr stable_sort_fn stable_sort {};

Example

#include <algorithm>
#include <array>
#include <functional>
#include <iomanip>
#include <iostream>
 
void print(auto const& seq)
{
    for (auto const& elem : seq)
        std::cout << elem << ' ';
    std::cout << '\n';
}
 
struct Particle
{
    std::string name; double mass; // MeV
    friend std::ostream& operator<<(std::ostream& os, Particle const& p)
    {
        return os << '\n' << std::left << std::setw(8) << p.name << " : " << p.mass;
    }
};
 
int main()
{
    std::array s {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};
 
    // sort using the default operator<
    std::ranges::stable_sort(s);
    print(s);
 
    // sort using a standard library compare function object
    std::ranges::stable_sort(s, std::ranges::greater());
    print(s);
 
    // sort using a custom function object
    struct
    {
        bool operator()(int a, int b) const
        {
            return a < b;
        }
    } customLess;
    std::ranges::stable_sort(s.begin(), s.end(), customLess);
    print(s);
 
    // sort using a lambda expression
    std::ranges::stable_sort(s, [](int a, int b) { return a > b; });
    print(s);
 
    // sort with projection
    Particle particles[]
    {
        {"Electron", 0.511}, {"Muon", 105.66}, {"Tau", 1776.86},
        {"Positron", 0.511}, {"Proton", 938.27}, {"Neutron", 939.57}
    };
    print(particles);
    std::ranges::stable_sort(particles, {}, &Particle::name); //< sort by name
    print(particles);
    std::ranges::stable_sort(particles, {}, &Particle::mass); //< sort by mass
    print(particles);
}

Output:

0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0
0 1 2 3 4 5 6 7 8 9
9 8 7 6 5 4 3 2 1 0
 
Electron : 0.511
Muon     : 105.66
Tau      : 1776.86
Positron : 0.511
Proton   : 938.27
Neutron  : 939.57
 
Electron : 0.511
Muon     : 105.66
Neutron  : 939.57
Positron : 0.511
Proton   : 938.27
Tau      : 1776.86
 
Electron : 0.511
Positron : 0.511
Muon     : 105.66
Proton   : 938.27
Neutron  : 939.57
Tau      : 1776.86

ranges::sort (C++20)	sorts a range into ascending order (niebloid)
ranges::partial_sort (C++20)	sorts the first N elements of a range (niebloid)
ranges::stable_partition (C++20)	divides elements into two groups while preserving their relative order (niebloid)
stable_sort	sorts a range of elements while preserving order between equal elements (function template)