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Difference between revisions of "cpp/algorithm/swap"

From cppreference.com
< cpp‎ | algorithm
m ([string.view.synop]/1)
m (corrected indentation of example)
Line 126: Line 126:
 
int main()
 
int main()
 
{
 
{
−
  int a = 5, b = 3;
+
    int a = 5, b = 3;
  
−
  // before
+
    // before
−
  std::cout << a << ' ' << b << '\n';
+
    std::cout << a << ' ' << b << '\n';
  
−
  std::swap(a,b);
+
    std::swap(a,b);
  
−
  // after
+
    // after
−
  std::cout << a << ' ' << b << '\n';
+
    std::cout << a << ' ' << b << '\n';
 
}
 
}
 
  | output=
 
  | output=

Revision as of 01:02, 21 October 2020

 
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>
(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)

Contents

Parameters

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.

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++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++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++17)
 specializes the std::swap algorithm
(function) [edit]

Example

Run this code
#include <algorithm>
#include <iostream>
 
int main()
{
    int a = 5, b = 3;
 
    // before
    std::cout << a << ' ' << b << '\n';
 
    std::swap(a,b);
 
    // after
    std::cout << a << ' ' << b << '\n';
}

Output: 

5 3
3 5

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

See also

 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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