# std::sort

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void sort( RandomIt first, RandomIt last ); | (1) | (constexpr since C++20)
template< class ExecutionPolicy, class RandomIt > void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); | (2) | (since C++17)
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, RandomIt first, RandomIt last, Compare comp ); | (4) | (since C++17)

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 all following conditions are satisfied:

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.
- The type of *first is not MoveAssignable.

(since C++11)

### Parameters

first, last | - | the range of elements to sort
policy | - | the execution policy to use

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

### 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 (C++20) | sorts a range into ascending order (algorithm function object)

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