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

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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`
--- | ---
`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.

### 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

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

### See also

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)
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)

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