std::ranges::stable_sort(3)

NAME
std::ranges::stable_sort - std::ranges::stable_sort

Synopsis
   Defined in header <algorithm>
   Call signature
   template< std::random_access_iterator I, std::sentinel_for<I>
   S,
             class Comp = ranges::less, class Proj = std::identity (since C++20)
   > (1) (constexpr since
   requires std::sortable<I, Comp, Proj> C++26)
       I stable_sort( I first, S last, Comp comp = {}, Proj proj =
   {} );
   template< ranges::random_access_range R, class Comp =
   ranges::less,
                        (since C++20)
             class Proj = std::identity > (2) (constexpr since
   requires std::sortable<ranges::iterator_t<R>, Comp, Proj> C++26)
   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 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 Std Feature
   __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 sorts a range into ascending order
   (C++20) (niebloid)
   ranges::partial_sort sorts the first N elements of a range
   (C++20) (niebloid)
   ranges::stable_partition divides elements into two groups while preserving their
   (C++20) relative order
                        (niebloid)
                        sorts a range of elements while preserving order between
   stable_sort equal elements
                        (function template)