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

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

NAME

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

Inherited from std::integral_constant

Member constants

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

Member functions

              converts the object to bool, returns value
operator bool (public member function)

operator() returns value
              (public member function)
(C++14)

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