NAME

std::ranges::sort - std::ranges::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 >
requires std::sortable<I, Comp, Proj> (1) (since C++20)
constexpr I

sort( I first, S last, Comp comp = {}, Proj proj = {} );
template< ranges::random_access_range R, class Comp =
ranges::less,

class Proj = std::identity > (2) (since C++20)
requires std::sortable<ranges::iterator_t<R>, Comp, Proj>
constexpr ranges::borrowed_iterator_t<R>

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 not 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 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 may not be specified when calling any of them.
* None of them is visible to argument-dependent lookup.
* When one of them is found by normal unqualified lookup for the name to the left
of the function-call operator, it inhibits argument-dependent lookup.

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

\(\scriptsize \mathcal{O}(N\cdot\log{(N)})\)𝓞(N·log(N)) comparisons and
projections, where N = ranges::distance(first, last).

Possible implementation

Note that typical implementations use introsort. See also the implementation in MSVC
STL and libstdc++.

struct 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 I
operator()( I first, S last, Comp comp = {}, Proj proj = {} ) const {
if (first == last)
return first;

I last_iter = ranges::next(first, last);
ranges::make_heap(first, last_iter, std::ref(comp), std::ref(proj));
ranges::sort_heap(first, last_iter, std::ref(comp), std::ref(proj));

return last_iter;
}

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>
operator()( R&& r, Comp comp = {}, Proj proj = {} ) const {
return (*this)(ranges::begin(r), ranges::end(r), std::move(comp), std::move(proj));
} };

inline constexpr sort_fn sort{};

Notes

std::sort uses std::iter_swap to swap elements, whereas ranges::sort instead uses
ranges::iter_swap (which performs ADL for iter_swap, unlike std::iter_swap)

Example

// Run this code

#include <algorithm>
#include <array>
#include <functional>
#include <iomanip>
#include <iostream>

void print(auto comment, auto const& seq, char term = ' ') {
for (std::cout << comment << '\n'; auto const& elem : seq)
std::cout << elem << term;
std::cout << '\n';
}

struct Particle {
std::string name; double mass; // MeV
template<class Os> friend
Os& operator<< (Os& os, Particle const& p) {
return os << std::left << std::setw(8) << p.name << " : " << p.mass << ' ';
}
};

int main()
{
std::array s {5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

namespace ranges = std::ranges;

ranges::sort(s);
print("Sort using the default operator<", s);

ranges::sort(s, ranges::greater());
print("Sort using a standard library compare function object", s);

struct {
bool operator()(int a, int b) const { return a < b; }
} customLess;
ranges::sort(s.begin(), s.end(), customLess);
print("Sort using a custom function object", s);

ranges::sort(s, [](int a, int b) { return a > b; });
print("Sort using a lambda expression", s);

Particle particles[] {
{"Electron", 0.511}, {"Muon", 105.66}, {"Tau", 1776.86},
{"Positron", 0.511}, {"Proton", 938.27}, {"Neutron", 939.57},
};
ranges::sort(particles, {}, &Particle::name);
print("\nSort by name using a projection", particles, '\n');
ranges::sort(particles, {}, &Particle::mass);
print("Sort by mass using a projection", particles, '\n');
}

Output:

Sort using the default operator<
0 1 2 3 4 5 6 7 8 9
Sort using a standard library compare function object
9 8 7 6 5 4 3 2 1 0
Sort using a custom function object
0 1 2 3 4 5 6 7 8 9
Sort using a lambda expression
9 8 7 6 5 4 3 2 1 0

Sort by name using a projection
Electron : 0.511
Muon : 105.66
Neutron : 939.57
Positron : 0.511
Proton : 938.27
Tau : 1776.86

Sort by mass using a projection
Electron : 0.511
Positron : 0.511
Muon : 105.66
Proton : 938.27
Neutron : 939.57
Tau : 1776.86