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) 
swap(std::declval<T>(), std::declval<U>())
and swap(std::declval<U>(), std::declval<T>())
are both wellformed 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. T
is not a referenceable type (i.e., possibly cvqualified void
or a function type with a cvqualifierseq or a refqualifier), provides a member constant value
equal to false
. Otherwise, provides a member constant value
equal to std::is_swappable_with<T&, T&>::value
is_nothrow_swappable_with
.T
and U
shall each be a complete type, (possibly cvqualified) 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.
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) 
value
[static]  true if T is swappable with U , false otherwise (public static member constant) 
operator bool  converts the object to bool , returns value (public member function) 
operator()
(C++14)  returns value (public member function) 
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 implicitlydefined 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
.
swaps the values of two objects (function template) 

(C++11)(C++11)(C++11)  checks if a type has a move assignment operator (class template) 
(C++20)  specifies that a type can be swapped or that two types can be swapped with each other (concept) 
© cppreference.com
Licensed under the Creative Commons AttributionShareAlike Unported License v3.0.
https://en.cppreference.com/w/cpp/types/is_swappable