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

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

NAME

std::is_move_assignable,std::is_trivially_move_assignable,std::is_nothrow_move_assignable - std::is_move_assignable,std::is_trivially_move_assignable,std::is_nothrow_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 > (since C++17)
inline constexpr bool is_move_assignable_v = is_move_assignable<T>::value;
template< class T > (since C++17)
inline constexpr bool is_trivially_move_assignable_v = is_trivially_move_assignable<T>::value;
template< class T > (since C++17)
inline constexpr bool is_nothrow_move_assignable_v = is_nothrow_move_assignable<T>::value;

Inherited from std::integral_constant

Member constants

value true if T is move-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_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