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

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
}

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.