# std::stable_sort

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  |
template< class RandomIt > void stable_sort( RandomIt first, RandomIt last ); | (1) |
template< class ExecutionPolicy, class RandomIt > void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last ); | (2) | (since C++17)
template< class RandomIt, class Compare > void stable_sort( RandomIt first, RandomIt last, Compare comp ); | (3) |
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 ascending order. The order of equivalent elements is guaranteed to be preserved.

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

### 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.

### 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

partial_sort | sorts the first N elements of a range (function template)
sort | sorts a range into ascending order (function template)

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