# std::swap

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< cpp‎ | algorithm

Defined in header `<algorithm>` |  | (until C++11)
Defined in header `<utility>` |  | (since C++11)
Defined in header `<string_view>` |  |
template< class T > void swap( T& a, T& b ); | (1) | (conditionally noexcept since C++11) (constexpr since C++20)
template< class T2, std::size_t N > void swap( T2 (&a)[N], T2 (&b)[N] ); | (2) | (conditionally noexcept since C++11) (constexpr since C++20)

Exchanges the given values.

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. Equivalent to 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)

### Parameters

a, b | - | the values to be swapped
Type requirements
- `T` must meet the requirements of CopyConstructible and CopyAssignable (until C++11) MoveConstructible and MoveAssignable (since C++11).
- `T2` must meet the requirements of Swappable.

### Return value

(none)

### Exceptions

1)

(none) | (until C++11)
`noexcept` specification: noexcept( std::is_nothrow_move_constructible <T>::value && std::is_nothrow_move_assignable <T>::value ) | (since C++11)

2)

`noexcept` specification: noexcept(noexcept(swap(*a, *b))) 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++11) (until C++17)
`noexcept` specification: noexcept( std::is_nothrow_swappable_v <T2>) | (since C++17)

### Complexity

1) Constant.
2) Linear in N.

### 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) | specializes the std::swap algorithm (function template)
std::swap(std::shared_lock) (C++14) | specializes the std::swap algorithm (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) | specializes the std::swap algorithm (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) | specializes 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

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

### Defect reports

The following behavior-changing defect reports were applied retroactively to previously published C++ standards.

DR | Applied to | Behavior as published | Correct behavior
--- | --- | --- | ---
LWG 227 | C++98 | `T` was not required to be CopyConstructible or DefaultConstructible (a temporary object of type `T` might not be able to be constructed) | `T` is also required to be CopyConstructible
LWG 809 | C++98 | arrays could not be swapped | added overload (2)
LWG 2554 | C++11 | swapping multi-dimensional arrays can never be noexcept due to name lookup problems | 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)

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