std::ranges::stable_sort

 

C++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros(C++20)

Language support library

Concepts library(C++20)

Diagnostics library

Memory management library

Metaprogramming library(C++11)

General utilities library

Containers library

Iterators library

Ranges library(C++20)

Algorithms library

Strings library

Text processing library

Numerics library

Date and time library

Input/output library

Filesystem library(C++17)

Concurrency support library(C++11)

Execution control library(C++26)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges(C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

Constrained algorithms

All names in this menu belong to namespace std::ranges

Non-modifying sequence operations

all_ofany_ofnone_of for_each for_each_n    countcount_if adjacent_find mismatch equal lexicographical_compare    containscontains_subrange (C++23)(C++23) findfind_iffind_if_not find_lastfind_last_iffind_last_if_not (C++23)(C++23)(C++23)   find_end find_first_of search search_n starts_with (C++23) ends_with (C++23)

Modifying sequence operations

copycopy_if copy_n copy_backward    move move_backward shift_leftshift_right (C++23)(C++23)   fill fill_n generate generate_n    transform swap_ranges    shuffle sample removeremove_if replacereplace_if reverse rotate remove_copyremove_copy_if    unique replace_copyreplace_copy_if reverse_copy rotate_copy unique_copy

Partitioning operations

is_partitioned    partition_point    partition    partition_copy    stable_partition

Sorting operations

is_sorted is_sorted_until     sort stable_sort         partial_sort       nth_element partial_sort_copy

Binary search operations (on sorted ranges)

lower_bound         upper_bound         binary_search      equal_range

Set operations (on sorted ranges)

merge inplace_merge       set_difference set_intersection    set_union          includes set_symmetric_difference

Heap operations

is_heap is_heap_until       make_heap           sort_heap push_heap pop_heap

Minimum/maximum operations

max max_element         min min_element         minmax minmax_element     clamp

Permutation operations

is_permutation      next_permutation    prev_permutation

Fold operations

fold_left (C++23) fold_left_first (C++23)   fold_right (C++23) fold_right_last (C++23)   fold_left_with_iter (C++23) fold_left_first_with_iter (C++23)

Numeric operations iota (C++23)             Random number generation generate_random (C++26)

Operations on uninitialized storage

construct_at    destroy destroy_n destroy_at uninitialized_copy    uninitialized_move uninitialized_fill uninitialized_copy_n    uninitialized_move_n uninitialized_fill_n uninitialized_default_construct uninitialized_default_construct_n uninitialized_value_construct uninitialized_value_construct_n

Return types

in_fun_result in_in_result in_found_result    in_out_result in_in_out_result    in_out_out_result    min_max_result out_value_result (C++23) in_value_result (C++23)

 

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> requires std::sortable<I, Comp, Proj>     I stable_sort( I first, S last, Comp comp ={}, Proj proj ={});	(1)	(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 ={});	(2)	(since C++20) (constexpr 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 algorithm function objects (informally known as 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.

Parameters

first, last	-	the iterator-sentinel pair defining the range of elements 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	(C++26)	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++26ranges::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));}};   inlineconstexpr stable_sort_fn stable_sort{};

Example

Run this code

#include <algorithm>#include <array>#include <functional>#include <iomanip>#include <iostream>   void print(constauto& seq){for(constauto& elem : seq)std::cout<< elem <<' ';std::cout<<'\n';}   struct Particle {std::string name;double mass;// MeVfriendstd::ostream& operator<<(std::ostream& os, const Particle& 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 objectstruct{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);//< sorts by name print(particles); std::ranges::stable_sort(particles, {}, &Particle::mass);//< sorts 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 (algorithm function object)
ranges::partial_sort (C++20)	sorts the first N elements of a range (algorithm function object)
ranges::stable_partition (C++20)	divides elements into two groups while preserving their relative order (algorithm function object)
stable_sort	sorts a range of elements while preserving order between equal elements (function template)