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) |
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
.std::is_trivially_assignable<T&, const T&>
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.
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.
template< class T > inline constexpr bool is_copy_assignable_v = is_copy_assignable<T>::value; | (since C++17) | |
template< class T > inline constexpr bool is_trivially_copy_assignable_v = is_trivially_copy_assignable<T>::value; | (since C++17) | |
template< class T > inline constexpr bool is_nothrow_copy_assignable_v = is_nothrow_copy_assignable<T>::value; | (since C++17) |
value
[static] | true if T is copy-assignable, false otherwise (public static member constant) |
operator bool | converts the object to bool , returns value (public member function) |
operator()
(C++14) | returns value (public member function) |
Type | Definition |
---|---|
value_type | bool |
type | std::integral_constant<bool, value> |
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> {}; |
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.
#include <iostream> #include <utility> #include <type_traits> struct Foo { int n; }; int main() { std::cout << std::boolalpha << "Foo is trivially 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 trivially copy-assignable? true int[2] is copy-assignable? false int is nothrow copy-assignable? true
(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) |
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