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std::stable_sort(3)	       C++ Standard Libary	     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  unde-
     fined:

	  * 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 de-
     tails.
		       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 equiv-
     alent 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  regard-
     less 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  con-
     verted 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(NA.log(N)) comparisons using
	operator<
	(until C++20)
	std::less{}
	(since	C++20) if enough extra memory is available, otherwise \(\script-
     size O(N \cdot
	\log^{2}(N))\)O(NA.log2
	(N)) comparisons.
	3,4) \(\scriptsize O(N \cdot \log(N))\)O(NA.log(N)) applications of  the
     comparator
	comp  if  enough  extra memory is available, otherwise \(\scriptsize O(N
     \cdot
	\log^{2}(N))\)O(NA.log2
	(N)) applications.

Exceptions
	The overloads with a template parameter named ExecutionPolicy report er-
     rors 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-de-
     fined.
	  * 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 be-
     tween equal
	(C++20) 	    elements
			    (niebloid)

http://cppreference.com 	   2024.06.10		     std::stable_sort(3)
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