std::swap

 

C++

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Named requirements

Feature test macros (C++20)

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Concepts library (C++20)

Metaprogramming library (C++11)

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Strings library

Containers library

Iterators library

Ranges library (C++20)

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Filesystem library (C++17)

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Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
is_execution_policy (C++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (C++17)    (C++17)(C++17)(C++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (C++17)(C++17)(C++17)(C++20)
Non-modifying sequence operations Batch operations for_each for_each_n (C++17) Search operations all_ofany_ofnone_of (C++11)                (C++11)(C++11) countcount_if mismatch equal findfind_iffind_if_not (C++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (C++11) copy_backward copy_n (C++11) move (C++11) move_backward (C++11) Swap operations swap iter_swap swap_ranges Transformation operations replacereplace_if transform replace_copyreplace_copy_if Generation operations fill fill_n generate generate_n Removing operations removeremove_if unique remove_copyremove_copy_if unique_copy Order-changing operations reverse reverse_copy rotate rotate_copy random_shuffleshuffle (until C++17)(C++11) shift_leftshift_right (C++20)(C++20) Sampling operations sample (C++17)	Sorting and related operations Partitioning operations partition partition_copy (C++11)   stable_partition is_partitioned (C++11) partition_point (C++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (C++11) is_sorted_until (C++11) nth_element Binary search operations (on partitioned ranges) lower_bound upper_bound equal_range binary_search Set operations (on sorted ranges) includes set_union set_intersection set_difference set_symmetric_difference Merge operations (on sorted ranges) merge inplace_merge Heap operations push_heap pop_heap make_heap sort_heap is_heap (C++11) is_heap_until (C++11) Minimum/maximum operations max min minmax (C++11) clamp (C++17) max_element min_element minmax_element (C++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (C++20) Permutation operations next_permutation prev_permutation is_permutation (C++11) C library qsort bsearch
Numeric operations
iota (C++11) inner_product adjacent_difference    accumulate reduce (C++17) transform_reduce (C++17)	partial_sum inclusive_scan (C++17)    exclusive_scan (C++17) transform_inclusive_scan (C++17) transform_exclusive_scan (C++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (C++17)    uninitialized_fill uninitialized_copy_n (C++11) uninitialized_move_n (C++17) uninitialized_fill_n	destroy (C++17) destroy_n (C++17) destroy_at (C++17) construct_at (C++20)       uninitialized_default_construct (C++17) uninitialized_value_construct (C++17) uninitialized_default_construct_n (C++17) uninitialized_value_construct_n (C++17)

 

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

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)