std::is_move_assignable, std::is_trivially_move_assignable, std::is_nothrow_move_assignable

Defined in header `<type_traits>`
template< class T > struct is_move_assignable;	(1)	(since C++11)
template< class T > struct is_trivially_move_assignable;	(2)	(since C++11)
template< class T > struct is_nothrow_move_assignable;	(3)	(since C++11)

Type trait	The value of the member constant `value`
Type trait	`T` is a referenceable type	`T` is not a referenceable type
(1)	std::is_assignable<T&, T&&>::value	false
(2)	std::is_trivially_assignable<T&, T&&>::value
(3)	std::is_nothrow_assignable<T&, T&&>::value

If T 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.

template< class T > inline constexpr bool is_move_assignable_v = is_move_assignable<T>::value;		(since C++17)
template< class T > inline constexpr bool is_trivially_move_assignable_v = is_trivially_move_assignable<T>::value;		(since C++17)
template< class T > inline constexpr bool is_nothrow_move_assignable_v = is_nothrow_move_assignable<T>::value;		(since C++17)

[edit]

Inherited from std::integral_constant

Member constants

value

[static]

true if T is move-assignable, 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>

Possible implementation

template<class T>
struct is_move_assignable
    : std::is_assignable<typename std::add_lvalue_reference<T>::type,
                         typename std::add_rvalue_reference<T>::type> {};
 
template<class T>
struct is_trivially_move_assignable
    : std::is_trivially_assignable<typename std::add_lvalue_reference<T>::type,
                                   typename std::add_rvalue_reference<T>::type> {};
 
template<class T>
struct is_nothrow_move_assignable
    : std::is_nothrow_assignable<typename std::add_lvalue_reference<T>::type,
                                 typename std::add_rvalue_reference<T>::type> {};

Notes

The trait std::is_move_assignable is less strict than MoveAssignable because it does not check the type of the result of the assignment (which, for a MoveAssignable type, must be T&), nor the semantic requirement that the target's value after the assignment is equivalent to the source's value before the assignment.

The type does not have to implement a move assignment operator in order to satisfy this trait; see MoveAssignable for details.

Example

Run this code

#include <iostream>
#include <string>
#include <type_traits>
 
struct Foo { int n; };
 
struct NoMove
{
    // prevents implicit declaration of default move assignment operator
    // however, the class is still move-assignable because its
    // copy assignment operator can bind to an rvalue argument
    NoMove& operator=(const NoMove&) { return *this; }
};
 
int main()
{
    std::cout << std::boolalpha
              << "std::string is nothrow move-assignable? "
              << std::is_nothrow_move_assignable<std::string>::value << '\n'
              << "int[2] is move-assignable? "
              << std::is_move_assignable<int[2]>::value << '\n'
              << "Foo is trivially move-assignable? "
              << std::is_trivially_move_assignable<Foo>::value << '\n'
              << "NoMove is move-assignable? "
              << std::is_move_assignable<NoMove>::value << '\n'
              << "NoMove is nothrow move-assignable? "
              << std::is_nothrow_move_assignable<NoMove>::value << '\n';
}

Output:

std::string is nothrow move-assignable? true
int[2] is move-assignable? false
Foo is trivially move-assignable? true
NoMove is move-assignable? true
NoMove is nothrow move-assignable? false

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 2196	C++11	the behavior was unclear if T&& cannot be formed	the value produced is false in this case

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is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)	checks if a type has an assignment operator for a specific argument (class template) [edit]
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)	checks if a type has a copy assignment operator (class template) [edit]

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std::is_move_assignable, std::is_trivially_move_assignable, std::is_nothrow_move_assignable

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Helper variable templates