# std::is_copy_assignable, std::is_trivially_copy_assignable, std::is_nothrow_copy_assignable

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< cpp‎ | types

Defined in header `<type_traits>` |  |
template< class T > struct is_copy_assignable; | (1) | (since C++11)
template< class T > struct is_trivially_copy_assignable; | (2) | (since C++11)
template< class T > struct is_nothrow_copy_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&, const T&>::value.
2) Same as 1), but uses std::is_trivially_assignable <T&, const T&>
3) Same as 1), but uses std::is_nothrow_assignable <T&, const 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.

### Helper variable templates

template< class T > inline constexpr bool is_copy_assignable_v = is_copy_assignable<T>::value; |  | (since C++17)
template< class T > inline constexpr bool is_trivially_copy_assignable_v = is_trivially_copy_assignable<T>::value; |  | (since C++17)
template< class T > inline constexpr bool is_nothrow_copy_assignable_v = is_nothrow_copy_assignable<T>::value; |  | (since C++17)

## Inherited from std::integral_constant

### Member constants

value [static] | true if `T` is copy-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_copy_assignable
        : std::is_assignable< typename std::add_lvalue_reference<T>::type,
                              typename std::add_lvalue_reference<const T>::type> {};

    template< class T>
    struct is_trivially_copy_assignable
        : std::is_trivially_assignable< typename std::add_lvalue_reference<T>::type,
                                        typename std::add_lvalue_reference<const T>::type> {};

    template< class T>
    struct is_nothrow_copy_assignable
        : std::is_nothrow_assignable< typename std::add_lvalue_reference<T>::type,
                                      typename std::add_lvalue_reference<const T>::type> {};

### Notes

The trait `std::is_copy_assignable` is less strict than CopyAssignable because it does not check the type of the result of the assignment (which, for a CopyAssignable type, must be an lvalue of type `T`) and does not check the semantic requirement that the argument expression remains unchanged. It also does not check that `T` satisfies MoveAssignable, which is required of all CopyAssignable types.

### Example

Run this code

    #include <iostream>
    #include <utility>
    #include <type_traits>
    struct Foo { int n; };
    int main() {
        std::cout << std::boolalpha
                  << "Foo is trivially copy-assignable? "
                  << std::is_trivially_copy_assignable<Foo>::value << '\n'
                  << "int[2] is copy-assignable? "
                  << std::is_copy_assignable<int[2]>::value << '\n'
                  << "int is nothrow copy-assignable? "
                  << std::is_nothrow_copy_assignable<int>::value << '\n';
    }

Output:

    Foo is trivially copy-assignable? true
    int[2] is copy-assignable? false
    int is nothrow copy-assignable? true

### 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)
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)

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