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 nondescending 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 nonnegative 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
.
operator<
.comp
.policy
. These overloads do not participate in overload resolution unless std::is_execution_policy_v<std::decay_t<ExecutionPolicy>>
(until C++20) std::is_execution_policy_v<std::remove_cvref_t<ExecutionPolicy>>
(since C++20) is true.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:
While the signature does not need to have 
Type requirements  
RandomIt must meet the requirements of ValueSwappable and LegacyRandomAccessIterator. 

The type of dereferenced RandomIt must meet the requirements of MoveAssignable and MoveConstructible. 
(none).
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)).
The overloads with a template parameter named ExecutionPolicy
report errors as follows:
ExecutionPolicy
is one of the standard policies, std::terminate
is called. For any other ExecutionPolicy
, the behavior is implementationdefined. std::bad_alloc
is thrown. 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.
See also the implementations in libstdc++ and libc++.
#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
sorts a range into ascending order (function template) 

sorts the first N elements of a range (function template) 

divides elements into two groups while preserving their relative order (function template) 

(C++20)  sorts a range of elements while preserving order between equal elements (niebloid) 
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