What is the most effective way to get the index of an iterator of an std::vector?

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.

Note: it is a common name for a container iterator,std::container_type::iterator it;.

I would prefer std::distance(vec.begin(), it) as it will allow me to change the container without any code changes. For example, if you decide to use std::list instead of std::vector which doesn't provide a random access iterator your code will still compile. Since std::distance picks up the optimal method depending on iterator traits you'll not have any performance degradation either.