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

< cpp‎ | types
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Type categories
Type properties
(C++11)(until C++20)
(C++11)(deprecated in C++20)
Type trait constants
Constant evaluation context
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11)(until C++20)(C++17)
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)
1) 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_assignable<T&, T&&>::value.
2) Same as (1), but uses std::is_trivially_assignable<T&, T&&>
3) Same as (1), but uses std::is_nothrow_assignable<T&, T&&>

T shall 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.


[edit] Helper variable templates

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)

Inherited from std::integral_constant

Member constants

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)
returns value
(public member function)

Member types

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

[edit] 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> {};

[edit] 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.

[edit] Example

#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 trivally move-assignable? "
              << std::is_trivially_move_assignable<Foo>::value << '\n';
    std::cout << std::boolalpha
              << "NoMove is move-assignable? "
              << std::is_move_assignable<NoMove>::value << '\n'
              << "NoMove is nothrow move-assignable? "
              << std::is_nothrow_move_assignable<NoMove>::value << '\n';


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

[edit] See also

checks if a type has a assignment operator for a specific argument
(class template) [edit]
checks if a type has a copy assignment operator
(class template) [edit]