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std:: is_null_pointer

From cppreference.net
C++
Metaprogramming library
Type traits
Type categories
(C++11)
is_null_pointer
(C++11) ( DR* )
(C++11)
(C++11)
(C++11)
(C++11)
(C++11)
Type properties
(C++11)
(C++11)
(C++11)
(C++11)
(C++14)
(C++11)
(C++17)
(C++23)
(C++11) (deprecated in C++26)
(C++11)
(C++11)
(C++11) ( until C++20* )
(C++11) (deprecated in C++20)
(C++11)
(C++11)
(C++20)
(C++20)
(C++23)
(C++17)
Type trait constants
(C++11) (C++17) (C++11) (C++11)
Metafunctions
(C++17)
(C++17)
(C++17)
Supported operations
Relationships and property queries
Type modifications
Type transformations
(C++11) (deprecated in C++23)
(C++11) (deprecated in C++23)
(C++11)
(C++20)
(C++11) ( until C++20* ) (C++17)

Compile-time rational arithmetic
Compile-time integer sequences
(C++14)
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 , const std:: nullptr_t , volatile std:: nullptr_t , or const volatile std:: 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

Template parameters

T - a type to check

Helper variable template

template < class T >
constexpr bool 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 >

Possible implementation 

template<class T>
struct is_null_pointer : std::is_same<std::nullptr_t, std::remove_cv_t<T>> {};

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

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()
{
}

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

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)