# std::stable_sort

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< cpp‎ | algorithm

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

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)
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)

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