NAME¶

std::sort - std::sort

Synopsis¶

Defined in header <algorithm>
template< class RandomIt > (1) (constexpr since C++20)
void sort( RandomIt first, RandomIt last );
template< class ExecutionPolicy, class RandomIt >

void sort( ExecutionPolicy&& policy, (2) (since C++17)

RandomIt first, RandomIt last );
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp (3) (constexpr since C++20)
);
template< class ExecutionPolicy, class RandomIt, class
Compare >

void sort( ExecutionPolicy&& policy, (4) (since C++17)

RandomIt first, RandomIt last, Compare comp
);

Sorts the elements in the range [first, last) in non-descending order. The order of
equal elements is not guaranteed to be preserved.

1) Elements are sorted with respect to
operator<
(until C++20)
std::less{}
(since C++20).
3) Elements are sorted with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if

std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true. (until
C++20)
std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true. (since
C++20)

If any of the following conditions is satisfied, the behavior is undefined:

* The type of *first is not Swappable. (until C++11)
* RandomIt is not ValueSwappable.
* The type of *first is not MoveConstructible. (since C++11)
* The type of *first is not MoveAssignable.

Parameters¶

first, last - the range of elements to sort
policy - the execution policy to use. See execution policy for details.
comparison function object (i.e. an object that satisfies the
requirements of Compare) which returns true if the first argument is
less than (i.e. is ordered before) the second.

The signature of the comparison function should be equivalent to the
following:

bool cmp(const Type1& a, const Type2& b);
comp -
While the signature does not need to have const&, the function must
not modify the objects passed to it and must be able to accept all
values of type (possibly const) Type1 and Type2 regardless of value
category (thus, Type1& is not allowed
, nor is Type1 unless for Type1 a move is equivalent to a copy
(since C++11)).
The types Type1 and Type2 must be such that an object of type
RandomIt can be dereferenced and then implicitly converted to both of
them.

Type requirements¶

-
RandomIt must meet the requirements of LegacyRandomAccessIterator.
-
Compare must meet the requirements of Compare.

Complexity¶

Given \(\scriptsize N\)N as last - first:

1,2) \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) comparisons using
operator<
(until C++20)
std::less{}
(since C++20).
3,4) \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) applications of the comparator
comp.

Exceptions¶

The overloads with a template parameter named ExecutionPolicy report errors as
follows:

* If execution of a function invoked as part of the algorithm throws an exception
and ExecutionPolicy is one of the standard policies, std::terminate is called.
For any other ExecutionPolicy, the behavior is implementation-defined.
* If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Possible implementation¶

See also the implementations in libstdc++ and libc++.

Notes¶

Before LWG713, the complexity requirement allowed sort() to be implemented using
only Quicksort, which may need \(\scriptsize O(N^2)\)O(N2
) comparisons in the worst case.

Introsort can handle all cases with \(\scriptsize O(N \cdot \log(N))\)O(N·log(N))
comparisons (without incurring additional overhead in the average case), and thus is
usually used for implementing sort().

libc++ has not implemented the corrected time complexity requirement until LLVM 14.

Example¶

// Run this code

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

int main()
{
std::array<int, 10> s{5, 7, 4, 2, 8, 6, 1, 9, 0, 3};

auto print = [&s](std::string_view const rem)
{
for (auto a : s)
std::cout << a << ' ';
std::cout << ": " << rem << '\n';
};

std::sort(s.begin(), s.end());
print("sorted with the default operator<");

std::sort(s.begin(), s.end(), std::greater<int>());
print("sorted with the standard library compare function object");

struct
{
bool operator()(int a, int b) const { return a < b; }
}
customLess;

std::sort(s.begin(), s.end(), customLess);
print("sorted with a custom function object");

std::sort(s.begin(), s.end(), [](int a, int b)
{
return a > b;
});
print("sorted with a lambda expression");
}

Output:¶

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

Defect reports

The following behavior-changing defect reports were applied retroactively to
previously published C++ standards.

DR Applied to Behavior as published Correct behavior
the \(\scriptsize O(N \cdot it is required for
LWG 713 C++98 \log(N))\)O(N·log(N)) time complexity was only the worst case
required on the average

See also¶

partial_sort sorts the first N elements of a range
(function template)
stable_sort sorts a range of elements while preserving order between equal elements
(function template)
ranges::sort sorts a range into ascending order
(C++20) (niebloid)