Difference between revisions of "cpp/algorithm/swap"

Revision as of 23:32, 10 October 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 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 2554	C++11	swapping multi-dimensional arrays can never be `noexcept` due to name lookup problems	made to work

@@ Line 2: / Line 2: @@
 {{cpp/algorithm/navbar}}
 {{dcl begin}}
-{{dcl header | algorithm | notes={{mark until c++11}}}}
+{{dcl header|algorithm|notes={{mark until c++11}}}}
-{{dcl header | utility | notes={{mark since c++11}}}}
+{{dcl header|utility|notes={{mark since c++11}}}}
-{{dcl header | string_view | notes={{mark since c++17}}}}
+{{dcl header|string_view|notes={{mark since c++17}}}}
-{{dcl rev multi | num=1| until1=c++11 | dcl1=
+{{dcl rev multi|num=1|until1=c++11|dcl1=
 template< class T >
 void swap( T& a, T& b );
@@ Line 16: / Line 16: @@
 constexpr void swap( T& a, T& b ) noexcept(/* see below */);
 }}
-{{dcl rev multi | num=2 | since1=c++11 | until1=c++20 |dcl1=
+{{dcl rev multi|num=2|since1=c++11|until1=c++20|dcl1=
 template< class T2, std::size_t N >
-void swap( T2 (&a)[N], T2 (&b)[N]) noexcept(/* see below */);
+void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept(/* see below */);
 |dcl2=
 template< class T2, std::size_t N >
-constexpr void swap( T2 (&a)[N], T2 (&b)[N]) noexcept(/* see below */);
+constexpr void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept(/* see below */);
 }}
 {{dcl end}}
@@ Line 29: / Line 27: @@
 Exchanges the given values.
-@1@ Swaps the values {{tt|a}} and {{tt|b}}. {{rev inl|since=c++17|This overload does not participate in overload resolution unless {{c|std::is_move_constructible_v<T> && std::is_move_assignable_v<T>}} is {{c|true}}.}}
+@1@ Swaps the values {{c|a}} and {{c|b}}. {{rev inl|since=c++17|This overload does not participate in overload resolution unless {{c|std::is_move_constructible_v<T> && std::is_move_assignable_v<T>}} is {{c|true}}.}}
-@2@ Swaps the arrays {{tt|a}} and {{tt|b}}. In effect calls {{c|std::swap_ranges(a, a+N, b)}}. {{rev inl|since=c++17|This overload does not participate in overload resolution unless  {{c|std::is_swappable_v<T2>}} is {{c|true}}.}}
+@2@ Swaps the arrays {{c|a}} and {{c|b}}. In effect calls {{c|std::swap_ranges(a, a + N, b)}}. {{rev inl|since=c++17|This overload does not participate in overload resolution unless  {{c|std::is_swappable_v<T2>}} is {{c|true}}.}}
 ===Parameters===
 {{par begin}}
-{{par | a, b | the values to be swapped}}
+{{par|a, b|the values to be swapped}}
 {{par hreq}}
-{{par req | {{tt|T}} must meet the requirements of {{rev inl|until=c++11|{{named req|CopyConstructible}} and {{named req|CopyAssignable}}}}{{rev inl|since=c++11|{{named req|MoveConstructible}} and {{named req|MoveAssignable}}}}}}
+{{par req|{{tt|T}} must meet the requirements of {{rev inl|until=c++11|{{named req|CopyConstructible}} and {{named req|CopyAssignable}}}}{{rev inl|since=c++11|{{named req|MoveConstructible}} and {{named req|MoveAssignable}}}}}}
-{{par req named | T2 | Swappable }}
+{{par req named|T2|Swappable}}
 {{par end}}
@@ Line 138: / Line 136: @@
 ===Example===
 {{example
- | <!-- should demo proper use of swap with generic types, as in void swap(T& other) {using std::swap; swap(member, other.member);} , not just trivialities -->
+|<!-- should demo proper use of swap with generic types, as in void swap(T& other) {using std::swap; swap(member, other.member);} , not just trivialities -->
- | code=
+|code=
 #include <algorithm>
 #include <iostream>
-namespace Ns {
+namespace Ns
-class A {
+{
-     int id{};
+    class A
+     {
-    friend void swap(A& lhs, A& rhs) {
+        int id{};
-         std::cout << "swap(" << lhs << ", " << rhs << ")\n";
-        std::swap(lhs.id, rhs.id);
+        friend void swap(A& lhs, A& rhs)
-    }
+         {
+            std::cout << "swap(" << lhs << ", " << rhs << ")\n";
-    friend std::ostream& operator<< (std::ostream& os, A const& a) {
+            std::swap(lhs.id, rhs.id);
-         return os << "A::id=" << a.id;
+        }
-    }
+        friend std::ostream& operator<< (std::ostream& os, A const& a)
-public:
+         {
-    A(int i) : id{i} { }
+            return os << "A::id=" << a.id;
-    A(A const&) = delete;
+        }
-    A& operator = (A const&) = delete;
-};
+    public:
+        A(int i) : id{i} {}
+        A(A const&) = delete;
+        A& operator = (A const&) = delete;
+    };
 }
@@ Line 167: / Line 169: @@
      int a = 5, b = 3;
      std::cout << a << ' ' << b << '\n';
-     std::swap(a,b);
+     std::swap(a, b);
      std::cout << a << ' ' << b << '\n';
@@ Line 173: / Line 175: @@
      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`
+     swap(p, q);       // OK, ADL finds the appropriate friend `swap`
      std::cout << p << ' ' << q << '\n';
 }
- | output=
+|output=
 3
 5
@@ Line 184: / Line 186: @@
 }}
-=== Defect reports ===
+===Defect reports===
 {{dr list begin}}
-{{dr list item|wg=lwg|dr=2554|std=C++11|before=swapping multi-dimensional arrays can never be {{tt|noexcept}} due to name lookup problems|after=made to work}}
+{{dr list item|wg=lwg|dr=227|std=C++98|before={{tt|T}} was not required to be {{named req|CopyConstructible}} or {{named req|DefaultConstructible}}<br>(a temporary object of type {{tt|T}} might not be able to be constructed)|after={{tt|T}} is also required to<br>be {{named req|CopyConstructible}}}}
+{{dr list item|wg=lwg|dr=2554|std=C++11|before=swapping multi-dimensional arrays can never<br>be {{tt|noexcept}} due to name lookup problems|after=made to work}}
 {{dr list end}}
 ===See also===
 {{dsc begin}}
-{{dsc inc | cpp/utility/ranges/dsc swap}}
+{{dsc inc|cpp/utility/ranges/dsc swap}}
-{{dsc inc | cpp/algorithm/dsc iter_swap}}
+{{dsc inc|cpp/algorithm/dsc iter_swap}}
-{{dsc inc | cpp/algorithm/dsc swap_ranges}}
+{{dsc inc|cpp/algorithm/dsc swap_ranges}}
-{{dsc inc | cpp/utility/dsc exchange}}
+{{dsc inc|cpp/utility/dsc exchange}}
 {{dsc end}}
 {{langlinks|de|es|fr|it|ja|pt|ru|zh}}

Compiler support
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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 CopyConstructible and CopyAssignable(until C++11)MoveConstructible and MoveAssignable(since C++11)
- `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 23:32, 10 October 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)