# std::ranges::stable_sort

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< cpp‎ | algorithm‎ | ranges

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

Run this code

    #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

### See also

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)

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