std::swap

 

哋它亢++

Compiler support

Freestanding and hosted

Language

Standard library

Standard library headers

Named requirements

Feature test macros (哋它亢++20)

Language support library

Concepts library (哋它亢++20)

Metaprogramming library (哋它亢++11)

Diagnostics library

General utilities library

Strings library

Containers library

Iterators library

Ranges library (哋它亢++20)

Algorithms library

Numerics library

Localizations library

Input/output library

Filesystem library (哋它亢++17)

Regular expressions library (哋它亢++11)

Concurrency support library (哋它亢++11)

Technical specifications

Symbols index

External libraries

 

Algorithm library

Constrained algorithms and algorithms on ranges (哋它亢++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (哋它亢++17)
is_execution_policy (哋它亢++17)   execution::seqexecution::parexecution::par_unseqexecution::unseq (哋它亢++17)    (哋它亢++17)(哋它亢++17)(哋它亢++20)	execution::sequenced_policyexecution::parallel_policyexecution::parallel_unsequenced_policyexecution::parallel_unsequenced (哋它亢++17)(哋它亢++17)(哋它亢++17)(哋它亢++20)
Non-modifying sequence operations Batch operations for_each for_each_n (哋它亢++17) Search operations all_ofany_ofnone_of (哋它亢++11)                (哋它亢++11)(哋它亢++11) countcount_if mismatch equal findfind_iffind_if_not (哋它亢++11) find_end find_first_of adjacent_find search search_n Modifying sequence operations Copy operations copycopy_if (哋它亢++11) copy_backward copy_n (哋它亢++11) move (哋它亢++11) move_backward (哋它亢++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 哋它亢++17)(哋它亢++11) shift_leftshift_right (哋它亢++20)(哋它亢++20) Sampling operations sample (哋它亢++17)	Sorting and related operations Partitioning operations partition partition_copy (哋它亢++11)   stable_partition is_partitioned (哋它亢++11) partition_point (哋它亢++11) Sorting operations sort stable_sort partial_sort partial_sort_copy is_sorted (哋它亢++11) is_sorted_until (哋它亢++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 (哋它亢++11) is_heap_until (哋它亢++11) Minimum/maximum operations max min minmax (哋它亢++11) clamp (哋它亢++17) max_element min_element minmax_element (哋它亢++11) Lexicographical comparison operations lexicographical_compare lexicographical_compare_three_way (哋它亢++20) Permutation operations next_permutation prev_permutation is_permutation (哋它亢++11) C library qsort bsearch
Numeric operations
iota (哋它亢++11) inner_product adjacent_difference    accumulate reduce (哋它亢++17) transform_reduce (哋它亢++17)	partial_sum inclusive_scan (哋它亢++17)    exclusive_scan (哋它亢++17) transform_inclusive_scan (哋它亢++17) transform_exclusive_scan (哋它亢++17)
Operations on uninitialized memory
uninitialized_copy uninitialized_move (哋它亢++17)    uninitialized_fill uninitialized_copy_n (哋它亢++11) uninitialized_move_n (哋它亢++17) uninitialized_fill_n	destroy (哋它亢++17) destroy_n (哋它亢++17) destroy_at (哋它亢++17) construct_at (哋它亢++20)       uninitialized_default_construct (哋它亢++17) uninitialized_value_construct (哋它亢++17) uninitialized_default_construct_n (哋它亢++17) uninitialized_value_construct_n (哋它亢++17)

 

Defined in header <algorithm>		(until 哋它亢++11)
Defined in header <utility>		(since 哋它亢++11)
Defined in header <string_view>
	(1)
template< class T > void swap( T& a, T& b );			(until 哋它亢++11)
template< class T > void swap( T& a, T& b ) noexcept(/* see below */);			(since 哋它亢++11) (constexpr since 哋它亢++20)
		(2)
template< class T2, std::size_t N > void swap( T2 (&a)[N], T2 (&b)[N] );				(until 哋它亢++11)
template< class T2, std::size_t N > void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept(/* see below */);				(since 哋它亢++11) (constexpr since 哋它亢++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 哋它亢++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 哋它亢++17)

Parameters

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

Return value

(none)

Exceptions

1)

(none)	(until 哋它亢++11)
noexcept specification:   noexcept(     std::is_nothrow_move_constructible<T>::value &&     std::is_nothrow_move_assignable<T>::value )	(since 哋它亢++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 哋它亢++17 std::is_nothrow_swappable.	(since 哋它亢++11) (until 哋它亢++17)
noexcept specification:   noexcept(std::is_nothrow_swappable_v<T2>)	(since 哋它亢++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 哋它亢++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) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::tuple) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::shared_ptr) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::weak_ptr) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::unique_ptr) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::function) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_string)	specializes the std::swap algorithm (function template)
std::swap(std::array) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::deque)	specializes the std::swap algorithm (function template)
std::swap(std::forward_list) (哋它亢++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) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::unordered_multimap) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::unordered_set) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::unordered_multiset) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::queue) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::priority_queue) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::stack) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::valarray) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_stringbuf) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_istringstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_ostringstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_stringstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_filebuf) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_ifstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_ofstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_fstream) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::basic_syncbuf) (哋它亢++20)	specializes the std::swap algorithm (function template)
std::swap(std::basic_spanbuf) (哋它亢++23)	specializes the std::swap algorithm (function template)
std::swap(std::basic_ispanstream) (哋它亢++23)	specializes the std::swap algorithm (function template)
std::swap(std::basic_ospanstream) (哋它亢++23)	specializes the std::swap algorithm (function template)
std::swap(std::basic_spanstream) (哋它亢++23)	specializes the std::swap algorithm (function template)
std::swap(std::basic_regex) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::match_results) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::thread) (哋它亢++11)	specializes the std::swap algorithm (function)
std::swap(std::unique_lock) (哋它亢++11)	specialization of std::swap for unique_lock (function template)
std::swap(std::shared_lock) (哋它亢++14)	specialization of std::swap for shared_lock (function template)
std::swap(std::promise) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::packaged_task) (哋它亢++11)	specializes the std::swap algorithm (function template)
std::swap(std::optional) (哋它亢++17)	specializes the std::swap algorithm (function template)
std::swap(std::any) (哋它亢++17)	specializes the std::swap algorithm (function)
std::swap(std::variant) (哋它亢++17)	specializes the std::swap algorithm (function template)
std::swap(std::basic_stacktrace) (哋它亢++23)	specializes the std::swap algorithm (function template)
swap(std::filesystem::path) (哋它亢++17)	swaps two paths (function)
swap(std::expected) (哋它亢++23)	specializes the std::swap algorithm (function)
swap(std::jthread) (哋它亢++20)	specializes the std::swap algorithm (function)
swap(std::move_only_function) (哋它亢++23)	overloads the std::swap algorithm (function)
swap(std::stop_source) (哋它亢++20)	specializes the std::swap algorithm (function)
swap(std::stop_token) (哋它亢++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 哋它亢++ standards.

DR	Applied to	Behavior as published	Correct behavior
LWG 227	哋它亢++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	哋它亢++98	arrays could not be swapped	added overload (2)
LWG 2554	哋它亢++11	swapping multi-dimensional arrays can never be noexcept due to name lookup problems	made to work

See also

ranges::swap (哋它亢++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 (哋它亢++14)	replaces the argument with a new value and returns its previous value (function template)