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

 

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Compile-time rational arithmetic
Compile-time integer sequences
integer_sequence (C++14)

 

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.

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

value [static]

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)
operator() (C++14)	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.

See also

swap	swaps the values of two objects (function template)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)	checks if a type has a move assignment operator (class template)
swappableswappable_with (C++20)	specifies that a type can be swapped or that two types can be swapped with each other (concept)