In C++, you can't unset a reference. However, you can use pointers, which are actual addresses of the memory location containing the data, and by convention, setting a pointer to 0 means it's unset. Universally, there is a label defined which is translated to 0 called NULL which makes code a lot easier to read.
Now, I'm perfectly used to NULL. I've worked with it every day for nearly 3 years now, and it's fine. However, I've just started work on a project at home in C#.
Initially, I was typing NULL in C#, which kind of made sense. After all, I haven't used Java for a long time. However, I soon got used to using null instead, probably because of the 5+ years of using it before I really got into C++.
Today at work I've been typing null all day!
As I type this out, I realise how incredibly dull it is. Too bad! I'm posting it anyway :P