std::nullptr_t

 

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Defined in header <cstddef>
using nullptr_t = decltype(nullptr);		(since C++11)

std::nullptr_t is the type of the null pointer literal nullptr. It is a distinct type that is not itself a pointer type or a pointer to member type. Prvalues of this type are null pointer constants, and may be implicitly converted to any pointer and pointer to member type.

sizeof(std::nullptr_t) is equal to sizeof(void*).

Notes

The C++ standard requires <stddef.h> to place the contents of <cstddef> in the global namespace, and thereby requires nullptr_t to be available in the global namespace when <stddef.h> is included.

nullptr_t is not a part of C until C23.

It is unspecified whether the declaration of std::nullptr_t is available in any other standard library header. An implementation may avoid introducing this name even when the standard requires std::nullptr_t to be used, by e.g. spelling decltype(nullptr) instead.

Example

If two or more overloads accept different pointer types, an overload for std::nullptr_t is necessary to accept a null pointer argument.

Run this code

#include <cstddef>#include <iostream>   void f(int*){std::cout<<"Pointer to integer overload\n";}   void f(double*){std::cout<<"Pointer to double overload\n";}   void f(std::nullptr_t){std::cout<<"null pointer overload\n";}   int main(){int* pi{};double* pd{};   f(pi); f(pd); f(nullptr);// would be ambiguous without void f(nullptr_t)// f(0); // ambiguous call: all three functions are candidates// f(NULL); // ambiguous if NULL is an integral null pointer constant // (as is the case in most implementations)}

Output:

Pointer to integer overload Pointer to double overload null pointer overload

See also

nullptr(C++11)	the pointer literal which specifies a null pointer value
NULL	implementation-defined null pointer constant (macro constant)
is_null_pointer (C++11)(DR*)	checks if a type is std::nullptr_t (class template)
C documentation for nullptr_t