std::is_move_assignable, std::is_trivially_move_assignable, std::is_nothrow_move_assignable
From cppreference.com
| 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
| (C++11)(C++11)(C++11) |
checks if a type has a assignment operator for a specific argument (class template) |
| (C++11)(C++11)(C++11) |
checks if a type has a copy assignment operator (class template) |