std::is_move_assignable,std::is_trivially_move_assignable, (3) - Linux Manuals

std::is_move_assignable,std::is_trivially_move_assignable,: std::is_move_assignable,std::is_trivially_move_assignable,

NAME

std::is_move_assignable,std::is_trivially_move_assignable, - std::is_move_assignable,std::is_trivially_move_assignable,

Synopsis

Defined in header <type_traits>
template< class T >                  (1) (since C++11)
struct is_move_assignable;
template< class T >                  (2) (since C++11)
struct is_trivially_move_assignable;
template< class T >                  (3) (since C++11)
struct is_nothrow_move_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&, T&&>::value.
2) Same as 1), but uses std::is_trivially_assignable<T&, T&&>
3) Same as 1), but uses std::is_nothrow_assignable<T&, 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_move_assignable_v =                           (since C++17)
is_move_assignable<T>::value;
template< class T >
inline constexpr bool is_trivially_move_assignable_v =                 (since C++17)
is_trivially_move_assignable<T>::value;
template< class T >
inline constexpr bool is_nothrow_move_assignable_v =                   (since C++17)
is_nothrow_move_assignable<T>::value;

Inherited from std::integral_constant

Member constants

value    true if T is move-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_move_assignable
 : std::is_assignable< typename std::add_lvalue_reference<T>::type,
                    typename std::add_rvalue_reference<T>::type> {};

template< class T>
struct is_trivially_move_assignable
 : std::is_trivially_assignable< typename std::add_lvalue_reference<T>::type,
                              typename std::add_rvalue_reference<T>::type> {};

template< class T>
struct is_nothrow_move_assignable
 : std::is_nothrow_assignable< typename std::add_lvalue_reference<T>::type,
                            typename std::add_rvalue_reference<T>::type> {};

Notes

The trait std::is_move_assignable is less strict than MoveAssignable because it does
not check the type of the result of the assignment (which, for a MoveAssignable
type, must be T&), nor the semantic requirement that the target's value after the
assignment is equivalent to the source's value before the assignment.

The type does not have to implement a move assignment operator in order to satisfy
this trait; see MoveAssignable for details.

Example

// Run this code

 #include <iostream>
 #include <string>
 #include <type_traits>
 struct Foo { int n; };
 struct NoMove {
  // prevents implicit declaration of default move assignment operator
  // however, the class is still move-assignable because its
  // copy assignment operator can bind to an rvalue argument
  NoMove& operator=(const NoMove&) { return *this; }
 };
 int main() {
  std::cout << std::boolalpha
      << "std::string is nothrow move-assignable? "
      << std::is_nothrow_move_assignable<std::string>::value << '\n'
      << "int[2] is move-assignable? "
      << std::is_move_assignable<int[2]>::value << '\n'
      << "Foo is trivally move-assignable? "
      << std::is_trivially_move_assignable<Foo>::value << '\n';

  std::cout << std::boolalpha
      << "NoMove is move-assignable? "
      << std::is_move_assignable<NoMove>::value << '\n'
      << "NoMove is nothrow move-assignable? "
      << std::is_nothrow_move_assignable<NoMove>::value << '\n';
 }

Output:

 std::string is nothrow move-assignable? true
 int[2] is move-assignable? false
 Foo is trivially move-assignable? true
 NoMove is move-assignable? true
 NoMove is nothrow move-assignable? false