Advance iterator for the std::vector std::advance VS operator +?

Adding will only work with random access iterators. std::advance will work with all sorts of iterators. As long as you're only dealing with iterators into vectors, it makes no real difference, but std::advance keeps your code more generic (e.g. you could substitute a list for the vector, and that part would still work).

For those who care, the standard describes advance and distance as follows (§24.3.4/1):

Since only random access iterators provide + and - operators, the library provides two function templates advance and distance. These function templates use + and - for random access iterators (and are, therefore, constant time for them); for input, forward and bidirectional iterators they use ++ to provide linear time implementations.

Also note that starting with C++11, the standard added a parameter to std::next, so you can advance by a specified amount using it (and std::prev similarly). The difference from std::advance is that it returns the modified iterator (which std::advance doesn't), which can be convenient in some cases.

That depends on what you need:

If you need genericity, use std::advance(it,2). If someone comes along and changes your std::vector into a std::list, the code will still compile, even though advancing now takes linear time instead of constant time.

If you need performance, use it+=2. If someone comes along and changes your std::vector into a std::list, the code will fail to compile, pointing (maybe with a helpful comment) at a serious performance issue.