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

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

value

[static]

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)
operator() (C++14)	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

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

[edit] See also

is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)	checks if a type has a assignment operator for a specific argument (class template)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)	checks if a type has a copy assignment operator (class template)
std::experimental::is_move_assignable_v (library fundamentals TS)	variable template alias of `std::is_move_assignable::value` (variable template)
std::experimental::is_trivially_move_assignable_v (library fundamentals TS)	variable template alias of `std::is_trivially_move_assignable::value` (variable template)
std::experimental::is_nothrow_move_assignable_v (library fundamentals TS)	variable template alias of `std::is_nothrow_move_assignable::value` (variable template)

Language
Standard library headers
Concepts
Utilities library
Strings library
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Numerics library
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Regular expressions library (C++11)
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Technical Specifications

Type support (basic types, RTTI, type traits)
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rel_ops::operator!=rel_ops::operator>rel_ops::operator<=rel_ops::operator>=
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swap
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forward (C++11)
move (C++11)
move_if_noexcept (C++11)
declval (C++11)