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Difference between revisions of "cpp/algorithm/swap"

From cppreference.com
< cpp‎ | algorithm
m (fmt)
(Uses {{mark constexpr since c++20}}.)
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{{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}}
  
 
{{dcl rev multi|num=1|until1=c++11|dcl1=
 
{{dcl rev multi|num=1|until1=c++11|dcl1=
 
template< class T >
 
template< class T >
 
void swap( T& a, T& b );
 
void swap( T& a, T& b );
−
|dcl2=
+
|notes2={{mark constexpr since c++20}}|dcl2=
 
template< class T >
 
template< class T >
−
void swap( T& a, T& b ) noexcept( /* see below */ );
+
void swap( T& a, T& b ) noexcept(/* see below */);
−
|since3=c++20|dcl3=
+
−
template< class T >
+
−
constexpr void swap( T& a, T& b ) noexcept( /* see below */ );
+
 
}}
 
}}
 
{{dcl rev multi|num=2|until1=c++11|dcl1=
 
{{dcl rev multi|num=2|until1=c++11|dcl1=
 
template< class T2, std::size_t N >
 
template< class T2, std::size_t N >
 
void swap( T2 (&a)[N], T2 (&b)[N] );
 
void swap( T2 (&a)[N], T2 (&b)[N] );
−
|dcl2=
+
|notes2={{mark constexpr since c++20}}|dcl2=
−
template< class T2, std::size_t N >
+
−
void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept( /* see below */ );
+
−
|since3=c++20|dcl3=
+
 
template< class T2, std::size_t N >
 
template< class T2, std::size_t N >
−
constexpr void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept( /* see below */ );
+
void swap( T2 (&a)[N], T2 (&b)[N] ) noexcept(/* see below */);
 
}}
 
}}
 
{{dcl end}}
 
{{dcl end}}
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Exchanges the given values.
 
Exchanges the given values.
  
−
@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}}.}}
+
@1@ Swaps the values {{c|a}} and {{c|b}}.
 +
{{rrev|since=c++17|
 +
{{cpp/enable if|{{c|std::is_move_constructible_v<T> && std::is_move_assignable_v<T>}} 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}}.}}
+
@2@ Swaps the arrays {{c|a}} and {{c|b}}. In effect calls {{c|std::swap_ranges(a, a + N, b)}}.
 +
{{rrev|since=c++17|
 +
{{cpp/enable if|{{c|std::is_swappable_v<T2>}} is {{c|true}}}}.
 +
}}
  
 
===Parameters===
 
===Parameters===
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===Complexity===
 
===Complexity===
−
@1@ Constant
+
@1@ Constant.
  
−
@2@ Linear in {{tt|N}}
+
@2@ Linear in {{c|N}}.
  
 
===Specializations===
 
===Specializations===
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{{tt|std::swap}} may be [[cpp/language/extending std|specialized in namespace std]] for program-defined types, but such specializations are not found by [[cpp/language/adl|ADL]] (the namespace std is not the associated namespace for the program-defined type).
 
{{tt|std::swap}} may be [[cpp/language/extending std|specialized in namespace std]] for program-defined types, but such specializations are not found by [[cpp/language/adl|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 {{named req|Swappable}} for details.  
+
The expected way to make a {{lsd|cpp/language/type#Program-defined type}} swappable is to provide a non-member function swap in the same namespace as the type: see {{named req|Swappable}} for details.  
  
 
The following overloads are already provided by the standard library:
 
The following overloads are already provided by the standard library:
−
 
 
{{dsc begin}}
 
{{dsc begin}}
 
{{dsc inc|cpp/utility/pair/dsc swap2}}
 
{{dsc inc|cpp/utility/pair/dsc swap2}}
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{{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=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=809|std=C++98|before=arrays could not be swapped|after=added overload {{v|2}}}}
 
{{dr list item|wg=lwg|dr=809|std=C++98|before=arrays could not be swapped|after=added overload {{v|2}}}}
−
{{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 item|wg=lwg|dr=2554|std=C++11|before=swapping multi-dimensional arrays can never<br>be {{c/core|noexcept}} due to name lookup problems|after=made to work}}
 
{{dr list end}}
 
{{dr list end}}
  

Revision as of 22:58, 25 March 2024

 
C++
 
Algorithm library
Constrained algorithms and algorithms on ranges (C++20)
Constrained algorithms, e.g. ranges::copy, ranges::sort, ...
Execution policies (C++17)
Non-modifying sequence operations
Batch operations
(C++17)
Search operations
(C++11)                (C++11)(C++11)

Modifying sequence operations
Copy operations
(C++11)
(C++11)
(C++11)
Swap operations
swap
Transformation operations
Generation operations
Removing operations
Order-changing operations
(until C++17)(C++11)
(C++20)(C++20)
Sampling operations
(C++17)

Sorting and related operations
Partitioning operations
Sorting operations
Binary search operations
(on partitioned ranges)
Set operations (on sorted ranges)
Merge operations (on sorted ranges)
Heap operations
Minimum/maximum operations
(C++11)
(C++17)
Lexicographical comparison operations
(C++20)
Permutation operations
C library
Numeric operations
Operations on uninitialized memory
 
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)

Contents

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:

(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++20)
 specializes the std::swap algorithm
(function template) [edit]
(C++23)
 specializes the std::swap algorithm
(function template) [edit]
(C++23)
 specializes the std::swap algorithm
(function template) [edit]
(C++23)
 specializes the std::swap algorithm
(function template) [edit]
(C++23)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++14)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++11)
 specializes the std::swap algorithm
(function template) [edit]
(C++17)
 specializes the std::swap algorithm
(function template) [edit]
(C++17)
 specializes the std::swap algorithm
(function) [edit]
(C++17)
 specializes the std::swap algorithm
(function template) [edit]
(C++23)
 specializes the std::swap algorithm
(function template) [edit]
(C++17)
 specializes the std::swap algorithm
(function) [edit]
(C++23)
 specializes the std::swap algorithm
(function) [edit]
(C++20)
 specializes the std::swap algorithm
(function) [edit]
(C++23)
 specializes the std::swap algorithm
(function) [edit]
(C++20)
 specializes the std::swap algorithm
(function) [edit]
(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 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

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