std::sort

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< cpp‎ | algorithm
 
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
(C++11)(C++11)(C++11)
(C++17)
Modifying sequence operations
Partitioning operations
Sorting operations
(C++11)
Binary search operations
Set operations (on sorted ranges)
Heap operations
(C++11)
Minimum/maximum operations
(C++11)
(C++17)

Permutations
Numeric operations
Operations on uninitialized storage
(C++17)
(C++17)
(C++17)
C library
 
Defined in header <algorithm>
(1)
template< class RandomIt >
void sort( RandomIt first, RandomIt last );
(until C++20)
template< class RandomIt >
constexpr void sort( RandomIt first, RandomIt last );
(since C++20)
template< class ExecutionPolicy, class RandomIt >

void sort( ExecutionPolicy&& policy,

           RandomIt first, RandomIt last );
(2) (since C++17)
(3)
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );
(until C++20)
template< class RandomIt, class Compare >
constexpr void sort( RandomIt first, RandomIt last, Compare comp );
(since C++20)
template< class ExecutionPolicy, class RandomIt, class Compare >

void sort( ExecutionPolicy&& policy,

           RandomIt first, RandomIt last, Compare comp );
(4) (since C++17)

Sorts the elements in the range [firstlast) in non-descending order. The order of equal 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, comp(*(it + n), *it) (or *(it + n) < *it) evaluates to false.

1) Elements are compared using operator<.
3) Elements are compared using the given binary comparison function comp.
2,4) Same as (1,3), but executed according to policy. These overloads do not participate in overload resolution unless

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)

Parameters

first, last - the range of elements to sort
policy - the execution policy to use. See execution policy for details.
comp - 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);

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 ValueSwappable and LegacyRandomAccessIterator.
-
The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible.
-
Compare must meet the requirements of Compare.

Return value

(none)

Complexity

O(N·log(N)) comparisons, where N is std::distance(first, last).

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.

Notes

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

Introsort can handle all cases with 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.

Possible implementation

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

Example

#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
LWG 713 C++98 the O(N·log(N)) time complexity was only required on the average it is required for the worst case

See also

sorts the first N elements of a range
(function template)
sorts a range of elements while preserving order between equal elements
(function template)
sorts a range into ascending order
(niebloid)