std::is_swappable_with, std::is_swappable, std::is_nothrow_swappable_with, std::is_nothrow_swappable - cppreference.com

From cppreference.com

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 std::is_nothrow_swappable_with.

If T or U is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, 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.

If the program adds specializations for any of the templates described on this page, the behavior 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

	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>`

Notes

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.