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

From Cppreference

Defined in header `<type_traits>`

template< class T > struct is_copy_assignable;	(1)	(C++11 feature)

template< class T > struct is_trivially_copy_assignable;	(2)	(C++11 feature)

template< class T > struct is_nothrow_copy_assignable;	(3)	(C++11 feature)

1) If the copy-assignment expression std::declval<T&>() = std::declval<const T&>() is well-formed in unevaluated context, provides the member constant value equal true. For any other type, value is false.

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`	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)

Member types


Type	Definition

`value_type`	`bool`

`type`	std::integral_constant<bool, value>

[edit] Equivalent definition

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

[edit] Example

#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_assignable is_trivially_assignable is_nothrow_assignable (C++11)
(C++11)
(C++11)

checks if a type has a assignment operator for a specific argument
(class template)

is_move_assignable is_trivially_move_assignable is_nothrow_move_assignable (C++11)
(C++11)
(C++11)

checks if a type has a move assignment operator
(class template)