foreach vs. iterators

[Walter Bright]

Sean Kelly wrote:
> Forgive me for resurrecting a horribly battered equine, but I was 
> thinking about this a bit tonight and I've decided that foreach, 
> foreach_reverse, and array operations are not a sufficient general 
> substitute for C++ iterators.
> 
> For example, let's say I want to compute the set union of two sorted 
> ranges, one of size M, the other of size N.  By iterating across the two 
> ranges simultaneously is is easy to see that the complexity for the 
> operation will be max(M,N).  In C++ this is easily accomplished using 
> forward iterators, and the ranges can be anything from arrays to SQL 
> result sets.
> 
> In D however, there is no way to iterate across multiple ranges 
> simultaneously using foreach, so opIndex must be used instead.  With 
> containers that have a constant-time lookup cost this isn't a big deal, 
> but what if these containers are binary trees?  The complexity for such 
> an operation would be M(log(M)) + N(log(N)).
> 
>  From this it seems clear that foreach is not sufficiently adaptable to 
> meet the needs of all sequential algorithms and opIndex is not a 
> reasonable substitute for all cases where foreach may not be used.  What 
> alternatives do we have?  So far, iterator variants are all I've been 
> able to come up with.

You are correct in regards to looping across multiple aggregates 
simultaneously.

On the other hand, C++ iterators aren't a general substitute for 
opApply. For example, it is not at all easy to turn a recursive data 
structure (i.e. binary tree) into a linearized iterator.