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

 

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is_assignableis_trivially_assignableis_nothrow_assignable (C++11)(C++11)(C++11)
is_copy_assignableis_trivially_copy_assignableis_nothrow_copy_assignable (C++11)(C++11)(C++11)
is_move_assignableis_trivially_move_assignableis_nothrow_move_assignable (C++11)(C++11)(C++11)
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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) 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.

Helper variable templates

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

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>

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

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)