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std::swap

From cppreference.com
< cpp‎ | algorithm
 
 
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)
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
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>
template< class T >
void swap( T& a, T& b );
(1) (conditionally noexcept since C++11)
(constexpr since C++20)
template< class T2, std::size_t N >
void swap( T2 (&a)[N], T2 (&b)[N] );
(2) (conditionally noexcept 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

[edit] 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.

[edit] Return value

(none)

[edit] 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:  
(since C++17)

[edit] Complexity

1) Constant.
2) Linear in N.

[edit] 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:

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]
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]
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]
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]
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]
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) [edit]
specialization of std::swap for unique_lock
(function template) [edit]
specialization of std::swap for shared_lock
(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) [edit]
specializes the std::swap algorithm
(function template) [edit]
specializes the std::swap algorithm
(function template) [edit]
swaps two paths
(function) [edit]
specializes the std::swap algorithm
(function) [edit]
specializes the std::swap algorithm
(function) [edit]
overloads the std::swap algorithm
(function) [edit]
specializes the std::swap algorithm
(function) [edit]
specializes the std::swap algorithm
(function) [edit]

[edit] Example

#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

[edit] 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

[edit] See also

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]