std::sort(3)

NAME
std::sort - std::sort

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.

Parameters
   first, last - the range of elements to sort
   policy - the execution policy to use. See execution policy for details.
                  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);
   comp -
                  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
                      the \(\scriptsize O(N \cdot it is required for
   LWG 713 C++98 \log(N))\)O(N·log(N)) time complexity was only the worst case
                      required on the average

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)