Why use iterators instead of array indices?

Question

Take the following two lines of code:

for (int i = 0; i < some_vector.size(); i++)
{
    //do stuff
}

And this:

for (som         


        

Answer 1

Several good points already. I have a few additional comments:

1. Assuming we are talking about the C++ standard library, "vector" implies a random access container that has the guarantees of C-array (random access, contiguos memory layout etc). If you had said 'some_container', many of the above answers would have been more accurate (container independence etc).

2. To eliminate any dependencies on compiler optimization, you could move some_vector.size() out of the loop in the indexed code, like so:

   const size_t numElems = some_vector.size();
   for (size_t i = 0; i 

3. Always pre-increment iterators and treat post-increments as exceptional cases.

for (some_iterator = some_vector.begin(); some_iterator != some_vector.end(); ++some_iterator){ //do stuff }

So assuming and indexable std::vector<> like container, there is no good reason to prefer one over other, sequentially going through the container. If you have to refer to older or newer elemnent indexes frequently, then the indexed version is more appropropriate.

In general, using the iterators is preferred because algorithms make use of them and behavior can be controlled (and implicitly documented) by changing the type of the iterator. Array locations can be used in place of iterators, but the syntactical difference will stick out.