What are the differences between using array offsets vs pointer incrementation?

Question

Given 2 functions, which should be faster, if there is any difference at all? Assume that the input data is very large

void iterate1(const char* pIn, int Size)
{         


        

Answer 1

With modern compiler there shouldn't be any difference in performance between the two, especially in such simplistic easily recognizable examples. Moreover, even if the compiler does not recognize their equivalence, i.e. translates each code "literally", there still shouldn't be any noticeable performance difference on a typical modern hardware platform. (Of course, there might be more specialized platforms out there where the difference might be noticeable.)

As for other considerations... Conceptually, when you implement an algorithm using the index access you impose a random-access requirement on the underlying data structure. When you use a pointer ("iterator") access, you only impose a sequential-access requirement on the underlying data structure. Random-access is a stronger requirement than sequential-access. For this reason I, for one, in my code prefer to stick to pointer access whenever possible, and use index access only when necessary.

More generally, if an algorithm can be implemented efficiently through sequential access, it is better to do it that way, without involving the unnecessary stronger requirement of random-access. This might prove useful in the future, should a need arise to refactor the code or to change the algorithm.