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std::stable_sort

Defined in header `<algorithm>`
	(1)
template< class RandomIt > void stable_sort( RandomIt first, RandomIt last );		(until C++26)
template< class RandomIt > constexpr void stable_sort( RandomIt first, RandomIt last );		(since C++26)
template< class ExecutionPolicy, class RandomIt > void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );	(2)	(since C++17)
	(3)
template< class RandomIt, class Compare > void stable_sort( RandomIt first, RandomIt last, Compare comp );		(until C++26)
template< class RandomIt, class Compare > constexpr void stable_sort( RandomIt first, RandomIt last, Compare comp );		(since C++26)
template< class ExecutionPolicy, class RandomIt, class Compare > void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );	(4)	(since C++17)

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>>` is `true`.	(until C++20)
`std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>` is `true`.	(since C++20)

Parameters

first, last	-	the range of elements to sort
policy	-	the execution policy to use. See execution policy for details.
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.
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.

Feature-test macro	Value	Comment
`__cpp_lib_constexpr_algorithms`	202306L	`constexpr` stable sorting

Possible implementation

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

Example

#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

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