std::stable_sort(3)

NAME
std::stable_sort - std::stable_sort

Synopsis
   Defined in header <algorithm>
   template< class RandomIt > (1)
   void stable_sort( RandomIt first, RandomIt last );
   template< class ExecutionPolicy, class RandomIt >
   void stable_sort( ExecutionPolicy&& policy, RandomIt first, (2) (since C++17)
   RandomIt last );
   template< class RandomIt, class Compare > (3)
   void stable_sort( RandomIt first, RandomIt last, Compare comp );
   template< class ExecutionPolicy, class RandomIt, class Compare >
   void stable_sort( ExecutionPolicy&& policy, RandomIt first, (4) (since C++17)
   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.
   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.
   1) Elements are compared using operator<.
   3) Elements are compared using the given comparison function comp.
   2,4) Same as (1,3), but executed according to policy. These overloads do not
   participate in overload resolution unless
   std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
   (until C++20)
   std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
   (since C++20) is true.

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 ValueSwappable and
   LegacyRandomAccessIterator.
   -
   The type of dereferenced RandomIt must meet the requirements of MoveAssignable and
   MoveConstructible.

Return value
   (none)

Complexity
   O(N·log(N)^2), where N = std::distance(first, last) applications of cmp. If
   additional memory is available, then the complexity is O(N·log(N)).

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.

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.

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

Example
// Run this code
 #include <algorithm>
 #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;
 }
 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)