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

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Defined in header <type_traits>
template< class T >
struct is_copy_assignable;
 (1) (since 哋它亢++11)
template< class T >
struct is_trivially_copy_assignable;
 (2) (since 哋它亢++11)
template< class T >
struct is_nothrow_copy_assignable;
 (3) (since 哋它亢++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, const T&>::value false
(2) std::is_trivially_assignable<T, const T&>::value
(3) std::is_nothrow_assignable<T, const 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_copy_assignable_v =

    is_copy_assignable<T>::value;
 (since 哋它亢++17)
template< class T >

inline constexpr bool is_trivially_copy_assignable_v =

    is_trivially_copy_assignable<T>::value;
 (since 哋它亢++17)
template< class T >

inline constexpr bool is_nothrow_copy_assignable_v =

    is_nothrow_copy_assignable<T>::value;
 (since 哋它亢++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()
(哋它亢++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 <type_traits>
#include <utility>
 
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

(哋它亢++11)(哋它亢++11)(哋它亢++11)
 checks if a type has an assignment operator for a specific argument
(class template)
(哋它亢++11)(哋它亢++11)(哋它亢++11)
 checks if a type has a move assignment operator
(class template)
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