std::is_swappable_with, std::is_swappable, std::is_nothrow_swappable_with, std::is_nothrow_swappable

Defined in header <type_traits>
template< class T, class U >
struct is_swappable_with;
(1) (since C++17)
template< class T >
struct is_swappable;
(2) (since C++17)
template< class T, class U >
struct is_nothrow_swappable_with;
(3) (since C++17)
template< class T >
struct is_nothrow_swappable;
(4) (since C++17)
1) If the expressions swap(std::declval<T>(), std::declval<U>()) and swap(std::declval<U>(), std::declval<T>()) are both well-formed in unevaluated context after using std::swap; (see Swappable), provides the member constant value equal true. Otherwise, value is false. Access checks are performed as if from a context unrelated to either type.
2) If T is not a referenceable type (i.e., possibly cv-qualified void or a function type with a cv-qualifier-seq or a ref-qualifier), provides a member constant value equal to false. Otherwise, provides a member constant value equal to std::is_swappable_with<T&, T&>::value
3) Same as (1), but evaluations of both expressions from (1) are known not to throw exceptions
4) Same as (2), but uses is_nothrow_swappable_with.

T and U shall each be a complete type, (possibly cv-qualified) void, or an array of unknown bound. Otherwise, the behavior is undefined.

If an instantiation of a template above depends, directly or indirectly, on an incomplete type, and that instantiation could yield a different result if that type were hypothetically completed, the behavior is undefined.

The behavior of a program that adds specializations for any of the templates described on this page is undefined.

Helper variable templates

template <class T, class U> 
inline constexpr bool is_swappable_with_v = is_swappable_with<T, U>::value;
(since C++17)
template <class T> 
inline constexpr bool is_swappable_v = is_swappable<T>::value;
(since C++17)
template <class T, class U>
inline constexpr bool is_nothrow_swappable_with_v = is_nothrow_swappable_with<T, U>::value;
(since C++17)
template <class T>
inline constexpr bool is_nothrow_swappable_v = is_nothrow_swappable<T>::value;
(since C++17)

Inherited from std::integral_constant

Member constants

true if T is swappable with U , false otherwise
(public static member constant)

Member functions

operator bool
converts the object to bool, returns value
(public member function)
returns value
(public member function)

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>


This trait does not check anything outside the immediate context of the swap expressions: if the use of T or U would trigger template specializations, generation of implicitly-defined special member functions etc, and those have errors, the actual swap may not compile even if std::is_swappable_with<T,U>::value compiles and evaluates to true.


See also

swaps the values of two objects
(function template)
checks if a type has a move assignment operator
(class template)
specifies that a type can be swapped or that two types can be swapped with each other

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