< cpp‎ | types
Utilities library
Language support
Type support (basic types, RTTI)
Library feature-test macros (C++20)
Dynamic memory management
Program utilities
Coroutine support (C++20)
Variadic functions
Debugging support
Three-way comparison
General utilities
Date and time
Function objects
Formatting library (C++20)
Relational operators (deprecated in C++20)
Integer comparison functions
Swap and type operations
Common vocabulary types
Elementary string conversions

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 *).

[edit] 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.

[edit] Example

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

[edit] See also

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