std::set::swap(3)

💖 NAME 💖

Okay so bestie, you’re on the std::set::swap(3) page from the C++ Standard Libary manual — yes, that’s the exact manual name, typo and all, and we are not fixing it. 💅

std::set::swap — std::set::swap. That’s the name, and it’s highkey iconic: literally the function that swaps two sets. No cap.

✨ Synopsis ✨

void swap( set& other );                            (until C++17)
void swap( set& other ) noexcept(/* see below */);  (since C++17)

Okay so the vibe: it exchanges the contents of the container with those of other. It does not invoke any move, copy, or swap operations on individual elements. Not me expecting per-element drama, because there is none. 🫶

All iterators and references remain valid. The end() iterator is invalidated. That part matters, bestie — your regular iterators survive, but end() gets a glow-down. 😭

The Compare objects must be Swappable, and they are exchanged using an unqualified call to non-member swap.

If std::allocator_traits<allocator_type>::propagate_on_container_swap::value is true, then the allocators are exchanged using an unqualified call to non-member swap. Otherwise, they are not swapped (and if get_allocator() != other.get_allocator(), the behavior is undefined). (since C++11)

🌸 Parameters 🌸

• other — container to exchange the contents with. That’s the whole parameter, she’s simple and iconic. Periodt.

🎀 Return value 🎀

(none) — literally no return value, it just swaps and exits the group chat.

💀 Exceptions 💀

• Any exception thrown by the swap of the Compare objects. (until C++17)

noexcept specification:

noexcept(std::allocator_traits<Allocator>::is_always_equal::value
         && std::is_nothrow_swappable<Compare>::value)

(since C++17)

🔥 Complexity 🔥

Constant. Yes queen, constant energy — that’s the whole complexity, fr fr.

🧚 Example 🧚

// Run this code

#include <functional>
#include <iostream>
#include <set>

template<class Os, class Co>
Os& operator<<(Os& os, const Co& co)
{
    os << '{';
    for (auto const& i : co)
        os << ' ' << i;
    return os << " } ";
}

int main()
{
    std::set<int> a1{3, 1, 3, 2}, a2{5, 4, 5};

    auto it1 = std::next(a1.begin());
    auto it2 = std::next(a2.begin());

    const int& ref1 = *(a1.begin());
    const int& ref2 = *(a2.begin());

    std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';
    a1.swap(a2);
    std::cout << a1 << a2 << *it1 << ' ' << *it2 << ' ' << ref1 << ' ' << ref2 << '\n';

    // Note that every iterator referring to an element in one container before the swap
    // refers to the same element in the other container after the swap. Same is true
    // for references.

    struct Cmp : std::less<int>
    {
        int id{};
        Cmp(int i) : id{i} {}
    };

    std::set<int, Cmp> s1{{2, 2, 1, 1}, Cmp{6}}, s2{{4, 4, 3, 3}, Cmp{9}};

    std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';
    s1.swap(s2);
    std::cout << s1 << s2 << s1.key_comp().id << ' ' << s2.key_comp().id << '\n';

    // So, comparator objects (Cmp) are also exchanged after the swap.
}

🍓 Output 🍓

{ 1 2 3 } { 4 5 } 2 5 1 4
{ 4 5 } { 1 2 3 } 2 5 1 4
{ 1 2 } { 3 4 } 6 9
{ 3 4 } { 1 2 } 9 6

👑 See also 👑

• std::swap(std::set) specializes the std::swap algorithm — function template. iykyk.

🏷️ Category 🏷️

• conditionally noexcept

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Source/version/date tea: http://cppreference.com, 2024.06.10, page std::set::swap(3) — mwah 💋