Are objects of type nullptr_t ever needed?

Question

Quoting C++11: (18.2/9)

nullptr_t is defined as follows:

namespace std { typedef decltype(nullptr) nullptr_t; }

The type for which nullptr_t is a synonym has the characteristics described in 3.9.1 and 4.10. [ Note: Although nullptr’s address cannot be taken, the address of another nullptr_t object that is an lvalue can be taken. —end note ]

Do we ever need objects of type nullptr_t (other than nullptr)?

Answer 1

nullptr is a prvalue, so you cannot take its address. (It is not an object.) The note is referring to cases like this:

nullptr_t n;

Then, n is an ordinary object of type nullptr_t so you can take its address. This doesn’t seem to be useful whatsoever, but in generic code this feature may come in somehow.

Answer 2

It's useful in this instance, which is good enough for me:

#include <cstddef>
#include <iostream>
#include <limits>

int bonus(int){return std::numeric_limits<int>::max();}
int bonus(std::nullptr_t){return 0;}

int main()
{
    std::nullptr_t bar;
    std::cout << bonus(0) << "\n";
    std::cout << bonus(nullptr) << "\n";
    std::cout << bonus(bar) << "\n";
}

C++ is a general purpose language and it would be annoying if the type associated with nullptr was not part of the overload resolution system.