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)