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

Question

I'm iterating over a vector and need the index the iterator is currently pointing at. AFAIK this can be done in two ways:

• it - vec.begin()
• std::distance(vec.begin(), it)

What are the pros and cons of these methods?

Answer 1

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;.