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std::stable_sort

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< cpp‎ | algorithm
 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
Defined in header <algorithm>
template< class RandomIt >
void stable_sort( RandomIt first, RandomIt last );
 (1) (constexpr since C++26)
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) (constexpr since C++26)
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 non-descending order. The order of equivalent elements is guaranteed to be preserved.

1) Elements are sorted with respect to operator<(until C++20)std::less{}(since C++20).
3) Elements are sorted with respect to comp.
2,4) Same as (1,3), but executed according to policy.
These overloads participate in overload resolution only if all following conditions are satisfied:

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)

If any of the following conditions is satisfied, the behavior is undefined:

(until C++11)
(since C++11)

Contents

[edit] Parameters

first, last - the pair of iterators defining the range of elements to sort
policy - the execution policy to use
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 LegacyRandomAccessIterator.
-
Compare must meet the requirements of Compare.

[edit] Complexity

Given N as last - first:

1,2) O(N·log(N)) comparisons using operator<(until C++20)std::less{}(since C++20) if enough extra memory is available, otherwise O(N·log2
(N)) comparisons.
3,4) O(N·log(N)) applications of the comparator comp if enough extra memory is available, otherwise O(N·log2
(N)) applications.

[edit] 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.

[edit] Possible implementation

See also the implementations in libstdc++ and libc++.

[edit] 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.

Feature-test macro Value Std Feature
__cpp_lib_constexpr_algorithms 202306L (C++26) constexpr stable sorting, overloads (1), (3)

[edit] Example

Run this code
#include <algorithm>
#include <array>
#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;
}
 
#if __cpp_lib_constexpr_algorithms >= 202306L
consteval auto get_sorted()
{
    auto v = std::array{3, 1, 4, 1, 5, 9};
    std::stable_sort(v.begin(), v.end());
    return v;
}
static_assert(std::ranges::is_sorted(get_sorted()));
#endif
 
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

[edit] See also

 sorts a range into ascending order
(function template) [edit]
sorts the first N elements of a range
(function template) [edit]
divides elements into two groups while preserving their relative order
(function template) [edit]
(C++20)
 sorts a range of elements while preserving order between equal elements
(algorithm function object)[edit]
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