std::ranges::stable_sort() algorithm

since C++20

Simplified

// (1)
I stable_sort( I first, S last, Comp comp = {}, Proj proj = {} );

// (2)
ranges::borrowed_iterator_t<R> stable_sort( R&& r, Comp comp = {}, Proj proj = {} );

The type of arguments are generic and have following constraints:

• I - std::random_access_iterator
• S - std::sentinel_for<I>
• R - std::ranges::random_access_range
• Comp - (none)
• Proj - (none)

The Proj and Comp template arguments have, the following default types for all overloads: std::identity, ranges::less.

Additionally, each overload has the following constraints:

• (1) - sortable<I, Comp, Proj>
• (2) - sortable<ranges::iterator_t<R>, Comp, Proj>

(The std:: namespace was ommitted here for readability)

Detailed

// (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>
constexpr I stable_sort( I first, S last, Comp comp = {}, Proj proj = {} );

// (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 = {} );

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 source 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.

Parameters

first last	The range to sort.
r	The range to sort.
comp	Comparison object to apply to the projected elements.
proj	Projection to apply to the elements.

Return value

An iterator equal to last.

Complexity

Given N as ranges::distance(first, last):

N * log^2(N) comparisons and twice as many projections.

N * log (N) comparisons and twice as many projections, if extra memory is available;

Exceptions

(none)

Possible implementation

stable_sort(1) and stable_sort(2)

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 {};

Examples

Main.cpp

#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