std::stable_sort(3)

NAME
std::stable_sort - std::stable_sort

Synopsis
   Defined in header <algorithm>
   template< class RandomIt > (1) (constexpr since C++26)
   void stable_sort( RandomIt first, RandomIt last );
   template< class ExecutionPolicy, class RandomIt >
   void stable_sort( ExecutionPolicy&& policy, (2) (since C++17)
                     RandomIt first, RandomIt last );
   template< class RandomIt, class Compare >
   void stable_sort( RandomIt first, RandomIt last, Compare (3) (constexpr since C++26)
   comp );
   template< class ExecutionPolicy, class RandomIt, class
   Compare >
   void stable_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
   equivalent elements is 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) if enough extra memory is available, otherwise \(\scriptsize O(N \cdot
   \log^{2}(N))\)O(N·log2
   (N)) comparisons.
   3,4) \(\scriptsize O(N \cdot \log(N))\)O(N·log(N)) applications of the comparator
   comp if enough extra memory is available, otherwise \(\scriptsize O(N \cdot
   \log^{2}(N))\)O(N·log2
   (N)) applications.

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
   This function attempts to allocate a temporary buffer equal in size to the sequence
   to be sorted. If the allocation fails, the less efficient algorithm is chosen.
         Feature-test macro Value Std Feature
   __cpp_lib_constexpr_algorithms 202306L (C++26) constexpr stable sorting, overloads
                        (1), (3)

Example
// Run this code
 #include <algorithm>
 #include <array>
 #include <iostream>
 #include <string>
 #include <vector>
 struct Employee
 {
     int age;
     std::string name; // Does not participate in comparisons
 };
 bool operator<(const Employee& lhs, const Employee& rhs)
 {
     return lhs.age < rhs.age;
 }
 #if __cpp_lib_constexpr_algorithms >= 202306L
 consteval auto get_sorted()
 {
     auto v = std::array{3, 1, 4, 1, 5, 9};
     std::stable_sort(v.begin(), v.end());
     return v;
 }
 static_assert(std::ranges::is_sorted(get_sorted()));
 #endif
 int main()
 {
     std::vector<Employee> v{{108, "Zaphod"}, {32, "Arthur"}, {108, "Ford"}};
     std::stable_sort(v.begin(), v.end());
     for (const Employee& e : v)
         std::cout << e.age << ", " << e.name << '\n';
 }

Output:
 32, Arthur
 108, Zaphod
 108, Ford

See also
   sort sorts a range into ascending order
                       (function template)
   partial_sort sorts the first N elements of a range
                       (function template)
                       divides elements into two groups while preserving their relative
   stable_partition order
                       (function template)
   ranges::stable_sort sorts a range of elements while preserving order between equal
   (C++20) elements
                       (niebloid)