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) Checks whether the type T is CopyAssignable.

Formally, provides the member constant value equal to true if the expression std::declval<T&>() = std::declval<const T&>() is well-formed in unevaluated context, and value equal to false otherwise. T must be a complete type, cv void or array of unknown bound.

2) Same as 1), but evaluation of the copy-assignment expression will not call any operation that is not trivial.

3) Same as 1), but the evaluation of the copy-assignment expression will not call any operation that is not noexcept.

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>

[edit] 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> {};

[edit] 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, in for a CopyAssignable type, must be an lvalue of type T) and does not check the semantic requirement that the argument expression remains unchanged.

[edit] Example

Run this code

#include <iostream>
#include <utility>
#include <type_traits>
struct Foo { int n; };
int main() {
    std::cout << std::boolalpha
              << "Foo is trivally 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 trivally copy-assignable? true
int[2] is copy-assignable? false
int is nothrow copy-assignable? true

[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)
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)
std::experimental::is_copy_assignable_v (library fundamentals TS)	variable template alias of `std::is_copy_assignable::value` (variable template)
std::experimental::is_trivially_copy_assignable_v (library fundamentals TS)	variable template alias of `std::is_trivially_copy_assignable::value` (variable template)
std::experimental::is_nothrow_copy_assignable_v (library fundamentals TS)	variable template alias of `std::is_nothrow_copy_assignable::value` (variable template)

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