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.