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

< cpp‎ | types
Type support
Basic types
Fundamental types
Fixed width integer types (C++11)
Numeric limits
C numeric limits interface
Runtime type information
Type traits
Primary type categories
Type properties
Supported operations
Relationships and property queries
Type modifications
Type transformations
Type trait constants
Defined in header <type_traits>
template< class T >
struct is_move_assignable;
(1) (since C++11)
template< class T >
struct is_trivially_move_assignable;
(2) (since C++11)
template< class T >
struct is_nothrow_move_assignable;
(3) (since C++11)
1) Checks whether a type is MoveAssignable.
Formally, provides the member constant value equal to true if the expression std::declval<T&>() = std::declval<T&&>() is well-formed in unevaluated context, and value equal to false otherwise. T must be a complete type, possibly cv-qualified void or array of unknown bound.
2) same as (1), but evaluation of the move-assignment expression will not call any operation that is not trivial.
3) same as (1), but the evaluation of the move-assignment expression will not call any operation that is not noexcept.


Inherited from std::integral_constant

Member constants

true if T is move-assignable, false otherwise
(public static member constant)

Member functions

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

Member types

Type Definition
value_type bool
type std::integral_constant<bool, value>

[edit] 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> {};

[edit] 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, in for a MoveAssignable type, must be T&).

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

[edit] Example

#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';


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

[edit] See also

checks if a type has a assignment operator for a specific argument
(class template) [edit]
checks if a type has a copy assignment operator
(class template) [edit]