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

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Cpp/docs/latest/types/is copy assignable


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.

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 <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

(C++11)(C++11)(C++11)

checks if a type has a 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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