std::swap() algorithm

od C++20

// (1)
template< class T >
constexpr void swap( T& a, T& b ) noexcept(/* see below */);

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

od C++11

// (1)
template< class T >
void swap( T& a, T& b ) noexcept(/* see below */);

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

do C++11

// (1)
template< class T >
void swap( T& a, T& b );

// (2)
template< class T2, std::size_t N >
void swap( T2 (&a)[N], T2 (&b)[N] );

Exchanges the given values.

• (1) Swaps the values a and b.

  Overload Resolution

  This overload does not participate in overload resolution unless std::is_move_constructible_v<T> && std::is_move_assignable_v<T> is true. (od C++17)

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

  Overload Resolution

  This overload does not participate in overload resolution unless std::is_swappable_v<T2> is true. (od C++17)

Parameters

a b	The values to be swapped.

Type requirements

T	MoveConstructible, MoveAssignable  (od C++11) CopyConstructible, CopyAssignable  (do C++11)
T2	Swappable

Return value

(none)

Complexity

• (1) - Constant.
• (1) - Linear in N.

Exceptions

Before C++11 no exceptions were thrown.

• (1)

  od C++11

  noexcept specification:

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

• (2)

  od C++17

  noexcept specification:

  noexcept(std::is_nothrow_swappable_v<T2>)

  do C++17

  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.

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 none other ExecutionPolicy, the behavior is implementation-defined.
• If the algorithm fails to allocate memory, std::bad_alloc is thrown.

Specializations

do C++20

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).

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

Examples

Main.cpp

#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