C++ nullptr_t

from https://en.cppreference.com/w/cpp/types/nullptr_t

Definition

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.

#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)
NULL	implementation-defined null pointer constant(macro constant)
is_null_pointer(C++14)	checks if a type is std::nullptr_t(class template)