I would prefer
it - vec.begin() precisely for the opposite reason given by Naveen: so it wouldn’t compile if you change the vector into a list. If you do this during every iteration, you could easily end up turning an O(n) algorithm into an O(n^2) algorithm.
Another option, if you don’t jump around in the container during iteration, would be to keep the index as a second loop counter.
it is a common name for a container iterator,