std::nullptr_t (3) Linux Manual Page

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.

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 *pi)

{
    std::cout << "Pointer to integer overload\n";
}
void f(double *pd)

{
    std::cout << "Pointer to double overload\n";
}
void f(std::nullptr_t nullp)

{
    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 the pointer literal which specifies a null pointer value (C++11)
                implementation-defined null pointer constant
NULL (macro constant)
is_null_pointer checks if a type is std::nullptr_t
                (class template)
(C++14)