| 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) |
1) If the expression
std::declval<T>() = std::declval<U>() is well-formed in unevaluated context, provides the member constant value equal true. Otherwise, value is false. Access checks are performed as if from a context unrelated to either type. 2) same as (1), but the evaluation of the assignment expression will not call any operation that is not trivial. For the purposes of this check, a call to
std::declval is considered trivial and not considered an odr-use of std::declval. 3) same as (1), but the evaluation of the assignment expression will not call any operation that is not noexcept.
T and U shall each 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, 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 <iostream>
#include <string>
#include <type_traits>
struct Ex1 { int n; };
int main() {
std::cout << std::boolalpha
<< "int is assignable from int? "
<< std::is_assignable<int, int>::value << '\n' // 1 = 1; wouldn't compile
<< "int& is assignable from int? "
<< std::is_assignable<int&, int>::value << '\n' // int a; a = 1; works
<< "int is assignable from double? "
<< std::is_assignable<int, double>::value << '\n'
<< "int& is nothrow assignable from double? "
<< std::is_nothrow_assignable<int&, double>::value << '\n'
<< "string is assignable from double? "
<< std::is_assignable<std::string, double>::value << '\n'
<< "Ex1& is trivially assignable from const Ex1&? "
<< std::is_trivially_assignable<Ex1&, const Ex1&>::value << '\n';
}Output:
int is assignable from int? false int& is assignable from int? true int is assignable from double? false int& is nothrow assignable from double? true string is assignable from double? true Ex1& is trivially assignable from const Ex1&? true
See also
| (C++11)(C++11)(C++11) | checks if a type has a copy assignment operator (class template) |
| (C++11)(C++11)(C++11) | checks if a type has a move assignment operator (class template) |
| std::experimental::is_assignable_v
(library fundamentals TS) | variable template alias of std::is_assignable::value (variable template) |
| std::experimental::is_trivially_assignable_v
(library fundamentals TS) | variable template alias of std::is_trivially_assignable::value (variable template) |
| std::experimental::is_nothrow_assignable_v
(library fundamentals TS) | variable template alias of std::is_nothrow_assignable::value (variable template) |
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