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) |
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.std::is_trivially_assignable<T&, T&&>.std::is_nothrow_assignable<T&, T&&>.If T is not a complete type, (possibly cv-qualified) void, or an array of unknown bound, 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.
If the program adds specializations for any of the templates described on this page, 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
#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 trivially move-assignable? "
<< std::is_trivially_move_assignable<Foo>::value << '\n'
<< "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
Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2196 | C++11 | the behavior was unclear if T&& cannot be formed
|
the value produced is false in this case
|