Why use iterators instead of array indices?

Solution 1:

The first form is efficient only if vector.size() is a fast operation. This is true for vectors, but not for lists, for example. Also, what are you planning to do within the body of the loop? If you plan on accessing the elements as in

T elem = some_vector[i];

then you're making the assumption that the container has operator[](std::size_t) defined. Again, this is true for vector but not for other containers.

The use of iterators bring you closer to container independence. You're not making assumptions about random-access ability or fast size() operation, only that the container has iterator capabilities.

You could enhance your code further by using standard algorithms. Depending on what it is you're trying to achieve, you may elect to use std::for_each(), std::transform() and so on. By using a standard algorithm rather than an explicit loop you're avoiding re-inventing the wheel. Your code is likely to be more efficient (given the right algorithm is chosen), correct and reusable.

Solution 2:

It's part of the modern C++ indoctrination process. Iterators are the only way to iterate most containers, so you use it even with vectors just to get yourself into the proper mindset. Seriously, that's the only reason I do it - I don't think I've ever replaced a vector with a different kind of container.

I think the array index is more readable. It matches the syntax used in other languages, and the syntax used for old-fashioned C arrays. It's also less verbose. Efficiency should be a wash if your compiler is any good, and there are hardly any cases where it matters anyway.

Even so, I still find myself using iterators frequently with vectors. I believe the iterator is an important concept, so I promote it whenever I can.