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

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.

The behavior of a program that adds specializations for any of the templates described on this page 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

#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

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

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