How to define our own nullptr in c++98?

Question

Actually I am writing my own version of all library classes, and I don't want to include the STL files into my class file. So, for example, I want to check whether the node is equal to null. If I write something like

#define nullptr 0

Then it is not working with some other node pointer (i.e. Node *root = nullptr)

Answer 1

How to do that is mentioned in the book: Effective C++, 2nd edition by Scott Meyers (newer edition is available) in chapter: "Item 25: Avoid overloading on a pointer and a numerical type.".

It is needed if your compiler doesn't know the nullptr keyword introduced by C++11.

const                         /* this is a const object...     */
class nullptr_t
{
public:
   template<class T>          /* convertible to any type       */
   operator T*() const        /* of null non-member            */
      { return 0; }           /* pointer...                    */

   template<class C, class T> /* or any type of null           */
      operator T C::*() const /* member pointer...             */
      { return 0; }   

private:
   void operator&() const;    /* Can't take address of nullptr */

} nullptr = {};               /* and whose name is nullptr     */

That book is definitely worth reading.

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The advantage of nullptr over NULL is, that nullptr acts like a real pointer type thus it adds type safety whereas NULL acts like an integer just set to 0 in pre C++11.