std::is_copy_assignable,std::is_trivially_copy_assignable, (3) - Linux Manuals

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

NAME

std::is_copy_assignable,std::is_trivially_copy_assignable, - std::is_copy_assignable,std::is_trivially_copy_assignable,

Synopsis

Defined in header <type_traits>
template< class T >                  (1) (since C++11)
struct is_copy_assignable;
template< class T >                  (2) (since C++11)
struct is_trivially_copy_assignable;
template< class T >                  (3) (since C++11)
struct is_nothrow_copy_assignable;

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 =                           (since C++17)
is_copy_assignable<T>::value;
template< class T >
inline constexpr bool is_trivially_copy_assignable_v =                 (since C++17)
is_trivially_copy_assignable<T>::value;
template< class T >
inline constexpr bool is_nothrow_copy_assignable_v =                   (since C++17)
is_nothrow_copy_assignable<T>::value;

Inherited from std::integral_constant

Member constants

value    true if T is copy-assignable, false otherwise
[static] (public static member constant)

Member functions

operator bool converts the object to bool, returns value
        (public member function)
operator()    returns value
(C++14)       (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 <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