std::is_assignable, std::is_trivially_assignable, std::is_nothrow_assignable
| Defined in header <type_traits>
|
||
template< class T, class U >
struct is_assignable;
|
(1) | (since C++11) |
template< class T, class U >
struct is_trivially_assignable;
|
(2) | (since C++11) |
template< class T, class U >
struct is_nothrow_assignable;
|
(3) | (since C++11) |
std::declval<T>() = std::declval<U>() is well-formed in unevaluated context, provides the member constant value equal to true. Otherwise, value is false. Access checks are performed as if from a context unrelated to either type.If T or U 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, class U >
constexpr bool is_assignable_v = is_assignable<T, U>::value;
|
(since C++17) | |
template< class T, class U >
constexpr bool is_trivially_assignable_v = is_trivially_assignable<T, U>::value;
|
(since C++17) | |
template< class T, class U >
constexpr bool is_nothrow_assignable_v = is_nothrow_assignable<T, U>::value;
|
(since C++17) | |
Inherited from std::integral_constant
Member constants
value [static] |
true if T is assignable from U, 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>
|
Notes
This trait does not check anything outside the immediate context of the assignment expression: if the use of T or U would trigger template specializations, generation of implicitly-defined special member functions etc, and those have errors, the actual assignment may not compile even if std::is_assignable<T,U>::value compiles and evaluates to true.
Example
#include <string>
#include <type_traits>
struct C { int n; };
static_assert(
std::is_assignable<int, int>::value == false && // Error, 1 = 1 won't compile
std::is_assignable<int&, int>::value == true && // OK, int a; a = 1;
std::is_assignable<int, double>::value == false && // Error, 1 = 1.0;
std::is_nothrow_assignable_v<int&, double> == true && // OK, int a; a = 1.0;
// uses implicit conversion double -> char, then string::operator=(char)
std::is_assignable_v<std::string, double> == true && // OK, s = 69.6; s == 'E'
std::is_trivially_assignable_v<C&, const C&> == true
);
int main() {}