std::swap(3)

💖 NAME 💖

std::swap — std::swap

Okay bestie, this is the std::swap(3) page from the C++ Standard Libary — yes, that's the header vibe — and the name is literally std::swap. We're swapping things. It's giving glow-up for values. ✨

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✨ Synopsis ✨

Lowkey the headers changed over time, so here's the whole map, no cap:

Defined in header <algorithm>        (until C++11)
Defined in header <utility>          (since C++11)
Defined in header <string_view>

template< class T >                                (until C++11)
void swap( T& a, T& b );

template< class T >                                (since C++11)
void swap( T& a, T& b ) noexcept(/* see below */)
                                                   (constexpr since C++20);

template< class T2, std::size_t N >                (1)
                                                   (until C++11)
void swap( T2 (&a)[N], T2 (&b)[N] );

template< class T2, std::size_t N >                (2)
                                                   (since C++11)
void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept(/* see below */)
                                                   (constexpr since C++20);

So the whole point: exchanges the given values. Iconic, honestly. 🫶

1) Swaps the values a and b.

This overload participates in overload resolution only if std::is_move_constructible_v<T> && std::is_move_assignable_v<T> is true. (since C++17)

2) Swaps the arrays a and b. In effect calls std::swap_ranges(a, a + N, b).

This overload participates in overload resolution only if std::is_swappable_v<T2> is true. (since C++17)

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🫶 Parameters 🫶

• a, b — the values to be swapped

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💅 Type requirements 💅

• T must meet the requirements of CopyConstructible and CopyAssignable (until C++11), and MoveConstructible and MoveAssignable (since C++11).
• T2 must meet the requirements of Swappable.

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🎀 Return value 🎀

(none) — literally nothing comes back, bestie. The swap is the moment. 💅

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🔥 Exceptions 🔥

1)

(until C++11) (none)

(since C++11) noexcept specification:

noexcept(
    std::is_nothrow_move_constructible<T>::value &&
    std::is_nothrow_move_assignable<T>::value
)

2)

noexcept specification:

noexcept(noexcept(swap(*a, *b)))

(since C++11) (until C++17) The lookup for the identifier swap in the exception specification finds this function template in addition to anything found by the usual lookup rules, making the exception specification equivalent to C++17 std::is_nothrow_swappable.

(since C++17) noexcept specification:

noexcept(std::is_nothrow_swappable_v<T2>)

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🦋 Complexity 🦋

1) Constant.

2) Linear in N.

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👑 Specializations 👑

std::swap may be specialized in namespace std for program-defined types, but such specializations are not found by ADL (the namespace std is not the associated namespace for the program-defined type). (until C++20)

The expected way to make a program-defined type swappable is to provide a non-member function swap in the same namespace as the type: see Swappable for details.

The following overloads are already provided by the standard library:

• std::swap(std::pair) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::tuple) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::shared_ptr) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::weak_ptr) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::unique_ptr) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::function) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_string) — specializes the std::swap algorithm (function template)
• std::swap(std::array) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::deque) — specializes the std::swap algorithm (function template)
• std::swap(std::forward_list) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::list) — specializes the std::swap algorithm (function template)
• std::swap(std::vector) — specializes the std::swap algorithm (function template)
• std::swap(std::map) — specializes the std::swap algorithm (function template)
• std::swap(std::multimap) — specializes the std::swap algorithm (function template)
• std::swap(std::set) — specializes the std::swap algorithm (function template)
• std::swap(std::multiset) — specializes the std::swap algorithm (function template)
• std::swap(std::unordered_map) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::unordered_multimap) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::unordered_set) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::unordered_multiset) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::queue) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::priority_queue) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::stack) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::valarray) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_stringbuf) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_istringstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_ostringstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_stringstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_filebuf) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_ifstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_ofstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_fstream) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_syncbuf) (C++20) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_spanbuf) (C++23) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_ispanstream) (C++23) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_ospanstream) (C++23) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_spanstream) (C++23) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_regex) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::match_results) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::thread) (C++11) — specializes the std::swap algorithm (function)
• std::swap(std::unique_lock) (C++11) — specialization of std::swap for unique_lock (function template)
• std::swap(std::shared_lock) (C++14) — specialization of std::swap for shared_lock (function template)
• std::swap(std::promise) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::packaged_task) (C++11) — specializes the std::swap algorithm (function template)
• std::swap(std::optional) (C++17) — specializes the std::swap algorithm (function template)
• std::swap(std::any) (C++17) — specializes the std::swap algorithm (function)
• std::swap(std::variant) (C++17) — specializes the std::swap algorithm (function template)
• std::swap(std::basic_stacktrace) (C++23) — specializes the std::swap algorithm (function template)
• swap(std::filesystem::path) (C++17) — swaps two paths (function)
• swap(std::expected) (C++23) — specializes the std::swap algorithm (function)
• swap(std::jthread) (C++20) — specializes the std::swap algorithm (function)
• swap(std::move_only_function) (C++23) — overloads the std::swap algorithm (function)
• swap(std::stop_source) (C++20) — specializes the std::swap algorithm (function)
• swap(std::stop_token) (C++20) — specializes the std::swap algorithm (function)

✨ Example ✨

okay bestie, this is the little demo and it’s honestly iconic. Run this code and watch std::swap do its thing:

// Run this code

#include <algorithm>
#include <iostream>

namespace Ns
{
    class A
    {
        int id {};

        friend void swap(A& lhs, A& rhs)
        {
            std::cout << "swap(" << lhs << ", " << rhs << ")\n";
            std::swap(lhs.id, rhs.id);
        }

        friend std::ostream& operator<<(std::ostream& os, A const& a)
        {
            return os << "A::id=" << a.id;
        }

    public:
        A(int i) : id {i} {}
        A(A const&) = delete;
        A& operator = (A const&) = delete;
    };
}

int main()
{
    int a = 5, b = 3;
    std::cout << a << ' ' << b << '\n';
    std::swap(a, b);
    std::cout << a << ' ' << b << '\n';

    Ns::A p {6}, q {9};
    std::cout << p << ' ' << q << '\n';
//  std::swap(p, q); // error, type requirements are not satisfied
    swap(p, q);      // OK, ADL finds the appropriate friend `swap`
    std::cout << p << ' ' << q << '\n';
}

Output:

5 3
3 5
A::id=6 A::id=9
swap(A::id=6, A::id=9)
A::id=9 A::id=6

A quick vibe check on what you’re seeing: the plain ints trade places first, no drama. Then Ns::A shows up with its id values, and the call std::swap(p, q); is commented out because — per the comment — it’s an error, since the type requirements are not satisfied. But swap(p, q); is OK because ADL finds the appropriate friend swap. The friend prints its little message, swaps the ids, and everybody claps. 💅

👑 Defect reports 👑

These behavior-changing defect reports got applied retroactively to previously published C++ standards — receipts below, no cap:

• LWG 227 — applied to C++98; behavior as published: T was not required to be CopyConstructible or DefaultConstructible (a temporary object of type T might not be able to be constructed); correct behavior: T is also required to be CopyConstructible.
• LWG 809 — applied to C++98; behavior as published: arrays could not be swapped; correct behavior: added overload (2).
• LWG 2554 — applied to C++11; behavior as published: swapping multi-dimensional arrays can never be noexcept due to name lookup problems; correct behavior: made to work.

🦋 See also 🦋

• ranges::swap (C++20) — swaps the values of two objects (customization point object).
• iter_swap — swaps the elements pointed to by two iterators (function template).
• swap_ranges — swaps two ranges of elements (function template).
• exchange (C++14) — replaces the argument with a new value and returns its previous value (function template).

🎀 Category 🎀

• conditionally noexcept

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okay that’s the whole page, bestie 🫶 — source/version/date vibes: cppreference.com, 2024.06.10, std::swap(3). mwah 💋✨