std::is_move_assignable, std::is_trivially_move_assignable, std::is_nothrow_move_assignable - cppreference.com

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

From cppreference.com
< cpp‎ | types

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&&>
2) 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.

### [edit] Helper variable templates

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

### [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, in 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

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 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';
    }

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

### [edit] See also

is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11) | checks if a type has a 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]
std::experimental::is_move_assignable_v (library fundamentals TS) | variable template alias of `std::is_move_assignable::value` (variable template) [edit]
std::experimental::is_trivially_move_assignable_v (library fundamentals TS) | variable template alias of `std::is_trivially_move_assignable::value` (variable template) [edit]
std::experimental::is_nothrow_move_assignable_v (library fundamentals TS) | variable template alias of `std::is_nothrow_move_assignable::value` (variable template) [edit]

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