# std::nullptr_t

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< cpp‎ | types

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

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