std::sort

Since C++17

Header: <algorithm>

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

# Declarations

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

(constexpr since C++20)

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

(since C++17)

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

(constexpr since C++20)

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

(since C++17)

# 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.

# 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

#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");
}

# Defect reports

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