Difference between revisions of "cpp/algorithm/swap"

Revision as of 20:08, 24 August 2022

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

Exchanges the given values.

1) Swaps the values a and b. This overload does not participate in overload resolution unless 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 does not participate in overload resolution unless std::is_swappable_v<T2> is true.(since 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.	(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) [edit]
std::swap(std::tuple) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::shared_ptr) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::weak_ptr) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::unique_ptr) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::function) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_string)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::array) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::deque)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::forward_list) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::list)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::vector)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::map)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::multimap)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::set)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::multiset)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::unordered_map) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::unordered_multimap) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::unordered_set) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::unordered_multiset) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::queue) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::priority_queue) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::stack) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::valarray) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_stringbuf) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_istringstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_ostringstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_stringstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_filebuf) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_ifstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_ofstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_fstream) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_syncbuf) (C++20)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_spanbuf) (C++23)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_ispanstream) (C++23)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_ospanstream) (C++23)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_spanstream) (C++23)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_regex) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::match_results) (C++11)	specializes the `std::swap` algorithm (function template) [edit]
std::swap(std::thread) (C++11)	specializes the std::swap algorithm (function) [edit]
std::swap(std::unique_lock) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::shared_lock) (C++14)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::promise) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::packaged_task) (C++11)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::optional) (C++17)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::any) (C++17)	specializes the std::swap algorithm (function) [edit]
std::swap(std::variant) (C++17)	specializes the std::swap algorithm (function template) [edit]
std::swap(std::basic_stacktrace) (C++23)	specializes the std::swap algorithm (function template) [edit]
swap(std::filesystem::path) (C++17)	specializes the std::swap algorithm (function) [edit]
swap(std::expected) (C++23)	specializes the std::swap algorithm (function) [edit]
swap(std::jthread) (C++20)	specializes the std::swap algorithm (function) [edit]
swap(std::move_only_function) (C++23)	specializes the std::swap algorithm (function) [edit]
swap(std::stop_source) (C++20)	specializes the std::swap algorithm (function) [edit]
swap(std::stop_token) (C++20)	specializes the std::swap algorithm (function) [edit]

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 2554	C++11	swapping multi-dimensional arrays can never be `noexcept` due to name lookup problems	made to work

@@ Line 113: / Line 113: @@
 {{dsc inc | cpp/thread/thread/dsc swap2 | thread}}
 {{dsc inc | cpp/thread/unique_lock/dsc swap2}}
+{{dsc inc | cpp/thread/shared_lock/dsc swap2}}
 {{dsc inc | cpp/thread/promise/dsc swap2}}
 {{dsc inc | cpp/thread/packaged_task/dsc swap2}}
@@ Line 118: / Line 119: @@
 {{dsc inc | cpp/utility/any/dsc swap2}}
 {{dsc inc | cpp/utility/variant/dsc swap2}}
+{{dsc inc | cpp/utility/basic_stacktrace/dsc swap2}}
 {{dsc inc | cpp/filesystem/path/dsc swap2}}
+{{dsc inc | cpp/utility/expected/dsc swap2}}
+{{dsc inc | cpp/thread/thread/dsc swap2 | jthread}}
+{{dsc inc | cpp/utility/functional/move_only_function/dsc swap2}}
+{{dsc inc | cpp/thread/stop_source/dsc swap2}}
+{{dsc inc | cpp/thread/stop_token/dsc swap2}}
+<!--TODOs:
+swap(std::flat_map)
+swap(std::flat_set)
+swap(std::flat_multimap)
+swap(std::flat_multiset)
+swap(std::unexpected)
+swap(std::mdspan)
+-->
 {{dsc end}}

Compiler support
Freestanding and hosted
Language
Standard library
Standard library headers
Named requirements
Feature test macros (C++20)
Language support library
Concepts library (C++20)
Metaprogramming library (C++11)
Diagnostics library
General utilities library
Strings library
Containers library
Iterators library
Ranges library (C++20)
Algorithms library
Numerics library
Localizations library
Input/output library
Filesystem library (C++17)
Regular expressions library (C++11)
Concurrency support library (C++11)
Execution support library (C++26)
Technical specifications
Symbols index
External libraries

a, b	-	the values to be swapped
Type requirements
- `T` must meet the requirements of MoveAssignable and MoveConstructible.
- `T2` must meet the requirements of Swappable.

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

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Difference between revisions of "cpp/algorithm/swap"

Revision as of 20:08, 24 August 2022

Contents

Parameters

Return value

Exceptions

Complexity

Specializations

Example

Defect reports

See also

Navigation

Toolbox

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