std::ranges::stable_sort() algorithm
- od C++20
- Simplified
- Detailed
// (1)
I stable_sort( I first, S last, Comp comp = {}, Proj proj = {} );
// (2)
ranges::borrowed_iterator_t<R> stable_sort( R&& r, Comp comp = {}, Proj proj = {} );
The type of arguments are generic and have following constraints:
I
-std::random_access_iterator
S
-std::sentinel_for<I>
R
-std::ranges::random_access_range
Comp
- (none)Proj
- (none)
The Proj
and Comp
template arguments have, the following default types for all overloads: std::identity
, ranges::less
.
Additionally, each overload has the following constraints:
- (1) -
sortable<I, Comp, Proj>
- (2) -
sortable<ranges::iterator_t<R>, Comp, Proj>
(The std::
namespace was ommitted here for readability)
// (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>
constexpr I stable_sort( I first, S last, Comp comp = {}, Proj proj = {} );
// (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 = {} );
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 source range, as if usingranges::begin(r)
asfirst
andranges::end(r)
aslast
.
The function-like entities described on this page are niebloids.
Parameters
first last | The range to sort. |
r | The range to sort. |
comp | Comparison object to apply to the projected elements. |
proj | Projection to apply to the elements. |
Return value
An iterator equal to last
.
Complexity
Given N
as ranges::distance(first, last)
:
N * log^2(N) comparisons and twice as many projections.
N * log (N) comparisons and twice as many projections, if extra memory is available;
Exceptions
(none)
Possible implementation
stable_sort(1) and stable_sort(2)
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 {};
Examples
#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);
}
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
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