std::ranges::stable_sort

Header: <algorithm>

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.

# Declarations

Call signature

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

(since C++20) (constexpr since C++26)

# Parameters

# Return value

An iterator equal to last.

# Notes

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

# 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);
}

# See also