std::stable_sort

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

$O(N\cdotlog(N) 2)$ , where N = std::distance(first, last) applications of cmp. If additional memory is available, then the complexity is $O(N\cdotlog(N))$ .

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
{
    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

Language
Standard Library Headers
Freestanding and hosted implementations
Named requirements
Language support library
Concepts library (C++20)
Diagnostics library
Utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)
Filesystem library (C++17)
Technical Specifications

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