NAME

std::nullptr_t - std::nullptr_t

Synopsis

Defined in header <cstddef>
typedef decltype(nullptr) nullptr_t; (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

nullptr_t is available in the global namespace when <stddef.h> is included, even if
it is not a part of C.

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)