What are the uses of the type `std::nullptr_t`?

Question

I learned that nullptr , in addition to being convertible to any pointer type (but not to any integral type) also has its own type std::nullptr_t . So it is possible to have a method overload that accepts std::nullptr_t .

Exactly why is such an overload required?

Answer 1

If more than one overload accepts a pointer type, an overload for std::nullptr_t is necessary to accept a nullptr argument. Without the std::nullptr_t overload, it would be ambiguous which pointer overload should be selected when passed nullptr .

Example:

void f(int *intp)
{
    // Passed an int pointer
}

void f(char *charp)
{
    // Passed a char pointer
}

void f(std::nullptr_t nullp)
{
    // Passed a null pointer
}

Answer 2

There are some special cases that comparison with a nullptr_t type is useful to indicate whether an object is valid.

For example, the operator== and operator!= overloads of std::function could only take nullptr_t as the parameter to tell if the function object is empty. For more details you could read this question .

Answer 3

Also, what other type would you give it, that doesn't simply re-introduce the problems we had with NULL ? The whole point is to get rid of the nasty implicit conversions, but we can't actually change behaviour of old programs so here we are.