std::stable_sort() algorithm

since C++26

// (1)
template< class RandomIt >
constexpr void stable_sort( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
constexpr void stable_sort( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

since C++17

// (1)
template< class RandomIt >
void stable_sort( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void stable_sort( RandomIt first, RandomIt last, Compare comp );

// (3)
template< class ExecutionPolicy, class RandomIt >
void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last );

// (4)
template< class ExecutionPolicy, class RandomIt, class Compare >
void stable_sort( ExecutionPolicy&& policy, RandomIt first, RandomIt last, Compare comp );

until C++17

// (1)
template< class RandomIt >
void stable_sort( RandomIt first, RandomIt last );

// (2)
template< class RandomIt, class Compare >
void stable_sort( RandomIt first, RandomIt last, Compare comp );

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<.

• (2) Elements are compared using the given binary comparison function comp.

• (3, 4) Same as (1) and (2), but executed according to policy.

  Overload Resolution

  These overloads participate in overload resolution only if 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.
cmp	Comparison function object (i.e. an object that satisfies the requirements of Compare). 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 must not modify arguments. Must accept all values of type (possibly const) Type and Type2, regardless of value category (so 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 implicitly converted to both of them.

Type requirements

RandomIt	ValueSwappable LegacyRandomAccessIterator
Type of dereferenced RandomIt	MoveAssignable MoveConstructible
Compare	Compare

Return value

(none)

Complexity

Given N as std::distance(first, last)

O(N * pow(log(N), 2)) applications of comp.

If additional memory is available, then the complexity is O(N * log(N)).

Regardless of implementation, guaranteed O(N * log(N)) comparisons, where N is std::distance(first, last).

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.

Examples

Main.cpp

#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