| Defined in header <type_traits> | ||
|---|---|---|
template< class T > struct is_copy_assignable; | (1) | (since C++11) |
template< class T > struct is_trivially_copy_assignable; | (2) | (since C++11) |
template< class T > struct is_nothrow_copy_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&, const T&>::value. 2) Same as 1), but uses
std::is_trivially_assignable<T&, const T&> 2) Same as 1), but uses
std::is_nothrow_assignable<T&, const 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 > constexpr bool is_copy_assignable_v = is_copy_assignable<T>::value; | (since C++17) | |
template< class T > constexpr bool is_trivially_copy_assignable_v = is_trivially_copy_assignable<T>::value; | (since C++17) | |
template< class T > constexpr bool is_nothrow_copy_assignable_v = is_nothrow_copy_assignable<T>::value; | (since C++17) |
Inherited from std::integral_constant
Member constants
| value [static] | true if T is copy-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_copy_assignable
: std::is_assignable< typename std::add_lvalue_reference<T>::type,
typename std::add_lvalue_reference<const T>::type> {};
template< class T>
struct is_trivially_copy_assignable
: std::is_trivially_assignable< typename std::add_lvalue_reference<T>::type,
typename std::add_lvalue_reference<const T>::type> {};
template< class T>
struct is_nothrow_copy_assignable
: std::is_nothrow_assignable< typename std::add_lvalue_reference<T>::type,
typename std::add_lvalue_reference<const T>::type> {}; |
Notes
The trait std::is_copy_assignable is less strict than CopyAssignable because it does not check the type of the result of the assignment (which, for a CopyAssignable type, must be an lvalue of type T) and does not check the semantic requirement that the argument expression remains unchanged. It also does not check that T satisfies MoveAssignable, which is required of all CopyAssignable types.
Example
#include <iostream>
#include <utility>
#include <type_traits>
struct Foo { int n; };
int main() {
std::cout << std::boolalpha
<< "Foo is trivally copy-assignable? "
<< std::is_trivially_copy_assignable<Foo>::value << '\n'
<< "int[2] is copy-assignable? "
<< std::is_copy_assignable<int[2]>::value << '\n'
<< "int is nothrow copy-assignable? "
<< std::is_nothrow_copy_assignable<int>::value << '\n';
}Output:
Foo is trivally copy-assignable? true int[2] is copy-assignable? false int is nothrow copy-assignable? true
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 move assignment operator (class template) |
| std::experimental::is_copy_assignable_v
(library fundamentals TS) | variable template alias of std::is_copy_assignable::value (variable template) |
| std::experimental::is_trivially_copy_assignable_v
(library fundamentals TS) | variable template alias of std::is_trivially_copy_assignable::value (variable template) |
| std::experimental::is_nothrow_copy_assignable_v
(library fundamentals TS) | variable template alias of std::is_nothrow_copy_assignable::value (variable template) |
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