std::sort() algorithm

od C++20

// (1)
template< class RandomIt >
constexpr void sort( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
constexpr void sort( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

od C++17

// (1)
template< class RandomIt >
void sort( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

do C++17

// (1)
template< class RandomIt >
void sort( RandomIt first, RandomIt last );

// (3)
template< class RandomIt, class Compare >
void sort( RandomIt first, RandomIt last, Compare comp );

Sorts the elements in the range [first; last) in ascending order.

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.

important

The order of equal elements is not guaranteed to be preserved.

• (1) Elements are compared using operator<.

• (2) Elements are compared using the given binary comparison function comp.

• (3, 4) Same as (1) and (2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if std::is_execution_policy_v<std::decay_t<ExecutionPolicy>> is true.  (do C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>> is true.  (od C++20)

Parameters

first last	The range of elements to sort.
policy	The execution policy to use. See execution policy for details.
cmp	Comparison function object (i.e. an object that satisfies the requirements of Compare). The signature of the comparison function should be equivalent to the following: bool cmp(const Type1 &a, const Type2 &b); The signature does not need to have const&, but must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so Type1& is not allowed, nor is Type1 unless for Type1 a move is equivalent to a copy (od C++11)) The types Type1 and Type2 must be such that an object of type RandomIt can be implicitly converted to both of them.

Type requirements

RandomIt	ValueSwappable LegacyRandomAccessIterator
Type of dereferenced RandomIt	MoveAssignable MoveConstructible
Compare	Compare

Return value

(none)

Complexity

Regardless of implementation, guaranteed 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.

Possible implementation

See implementations from libstdc++ and libc++.

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.

Examples

Main.cpp

#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