In the early days of C++ when it was bolted on top of C, you could not use NULL as it was defined as (void*)0. You could not assign NULL to any pointer other than void*, which made it kind of useless. Back in those days, it was accepted that you used 0 (zero) for null pointers.
To this day, I have continued to use zero as a null pointer but those around me insist on using NULL. I personally do not see any benefit to giving a name (NULL) to an existing value – and since I also like to test pointers as truth values:
if (p && !q)
do_something();
then using zero makes more sense (as in if you use NULL, you cannot logically use p && !q – you need to explicitly compare against NULL, unless you assume NULL is zero, in which case why use NULL).
Is there any objective reason to prefer zero over NULL (or vice versa), or is all just personal preference?
Edit: I should add (and meant to originally say) that with RAII and exceptions, I rarely use zero/NULL pointers, but sometimes you do need them still.