std::stable_sort(3)

💅 NAME 💅

Bestie, welcome to std::stable_sort(3) from the C++ Standard Library manual. The icon itself: std::stable_sort — std::stable_sort. Yes, that's literally what the NAME line says, and honestly? We're keeping it.

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✨ Synopsis ✨

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)

This sorts the elements in the range [first, last) in non-descending order. The order of equivalent elements is guaranteed to be preserved. That's the whole stable-sort fantasy, and it ate.

• 1) Elements are sorted with respect to operator< (until C++20) or 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:
  • 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 — no cap, this is the danger zone:

• The type of *first is not Swappable. (until C++11)
• RandomIt is not ValueSwappable.
• The type of *first is not MoveConstructible. (since C++11)
• The type of *first is not MoveAssignable.

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🌸 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.

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🦋 Type requirements 🦋

• RandomIt must meet the requirements of LegacyRandomAccessIterator.
• Compare must meet the requirements of Compare.

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🔥 Complexity 🔥

Given N as last - first:

• 1,2) O(N·log(N)) comparisons using operator< (until C++20) or 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.

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😭 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.

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🛠️ Possible implementation 🛠️

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

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👑 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 vibes:

• __cpp_lib_constexpr_algorithms — 202306L — (C++26) — constexpr stable sorting, overloads (1), (3).

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🍓 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

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🌈 See also 🌈

• sort — sorts a range into ascending order (function template).
• partial_sort — sorts the first N elements of a range (function template).
• stable_partition — divides elements into two groups while preserving their relative order (function template).
• ranges::stable_sort (C++20) — sorts a range of elements while preserving order between equal elements (niebloid).

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And that's the whole std::stable_sort(3) page, bestie! Source/version: http://cppreference.com, date: 2024.06.10. Periodt. 💅✨