Negative index for array in cpp

Question

while browsing through a queue examples, I came across

http://login2win.blogspot.in/2008/07/c-queues.html

Its a queue example in which the indexes start from -1, but the program seems to be working fine

Is this valid, shouldn't this be a undefined behavior in some cases?

Answer 1

You're correct. This code is invalid because it is dereferencing memory outside of that array.