NAME¶

std::nullptr_t - std::nullptr_t

Synopsis¶

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. Its values are
null pointer constants (see NULL), 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 checks if a type is std::nullptr_t
(C++14) (class template)
C documentation for
nullptr_t