std::is_null_pointer
From cppreference.com
Defined in header <type_traits> | ||
template<class T > struct is_null_pointer; | (since C++11) | |
std::is_null_pointer is a UnaryTypeTrait.
Checks whether T is the type std::nullptr_t.
Provides the member constant value that is equal to true, if T is the type std::nullptr_t, conststd::nullptr_t, volatilestd::nullptr_t, or constvolatilestd::nullptr_t.
Otherwise, value is equal to false.
If the program adds specializations for std::is_null_pointer or std::is_null_pointer_v(since C++17), the behavior is undefined.
Contents |
[edit]Template parameters
| T | - | a type to check |
[edit]Helper variable template
template<class T > constexprbool is_null_pointer_v = is_null_pointer<T>::value; | (since C++17) | |
Inherited from std::integral_constant
Member constants
value [static] | true if T is the type std::nullptr_t (possibly cv-qualified), 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> |
[edit]Possible implementation
template<class T>struct is_null_pointer :std::is_same<std::nullptr_t, std::remove_cv_t<T>>{}; |
[edit]Notes
std::is_pointer is false for std::nullptr_t because it is not a built-in pointer type.
In libc++, std::is_null_pointer is not available in C++11 mode.
| Feature-test macro | Value | Std | Feature |
|---|---|---|---|
__cpp_lib_is_null_pointer | 201309L | (C++14) (DR11) | std::is_null_pointer |
[edit]Example
Run this code
#include <type_traits> static_assert(std::is_null_pointer_v<decltype(nullptr)>); static_assert(!std::is_null_pointer_v<int*>); static_assert(!std::is_pointer_v<decltype(nullptr)>); static_assert(std::is_pointer_v<int*>); int main(){}
[edit]Defect reports
The following behavior-changing defect reports were applied retroactively to previously published C++ standards.
| DR | Applied to | Behavior as published | Correct behavior |
|---|---|---|---|
| LWG 2247 | C++11 | the type trait for detecting std::nullptr_t was missing | added |
[edit]See also
(C++11) | checks if a type is void (class template) |
(C++11) | checks if a type is an array type (class template) |
(C++11) | checks if a type is a pointer type (class template) |
(C++11) | checks if a type is an enumeration type (class template) |
(C++11) | checks if a type is a union type (class template) |
(C++11) | checks if a type is a non-union class type (class template) |
(C++11) | checks if a type is a function type (class template) |
(C++11) | checks if a type is an object type (class template) |
