std::sort(3)

💖 NAME 💖

okay bestie, welcome to the std::sort(3) page from the C++ Standard Library — yes, the manual name is right there, folded in like a little name tag 🫶.

std::sort — std::sort.

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✨ 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.

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🫶 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. 🌸

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🦋 TYPE REQUIREMENTS 🦋

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

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⭐ COMPLEXITY ⭐

Given N as last - first:

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

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💀 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.

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🛠️ POSSIBLE IMPLEMENTATION 🛠️

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

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🌈 NOTES 🌈

Before LWG713, the complexity requirement allowed sort() to be implemented using only Quicksort, which may need O(N^2) 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.

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🎀 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");
}

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💜 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

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🔥 DEFECT REPORTS 🔥

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

• LWG 713 — applied to C++98; behavior as published: the O(N·log(N)) time complexity was only required on the average; correct behavior: it is required for the worst case.

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👑 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)

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💖 source/version check, bestie: http://cppreference.com, 2024.06.10, and this iconic little page is std::sort(3) — footer folded in, facts intact, vibe immaculate. mwah 💋