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 *).


nullptr_t is available in the global namespace when <stddef.h> is included, even if it is not a part of C99~C17 (referenced by C++11~C++20).

nullptr_t is also a part of C since C23.


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

#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(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)


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
implementation-defined null pointer constant
(macro constant)
checks if a type is std::nullptr_t
(class template)
C documentation for nullptr_t

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