# 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 equal 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);

The signature does not need to have const &, but the function object must not modify the objects passed to it.
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 `RandomAccessIterator`.
- The type of dereferenced `RandomIt` must meet the requirements of `MoveAssignable` and `MoveConstructible`.

### Return value

(none)

### Complexity

O(N·log2(N)), 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 three 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 {
        Employee(int age, std::string name) : age(age), name(name) { }
        int age;
        std::string name;  // Does not particpate in comparisons
    };

    bool operator<(const Employee &lhs, const Employee &rhs) {
        return lhs.age < rhs.age;
    }

    int main()
    {
        std::vector<Employee> v = {
            Employee(108, "Zaphod"),
            Employee(32, "Arthur"),
            Employee(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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