C++ / Why Iterators Got It All Wrong

[Mark via Digitalmars-d]

On Tuesday, 29 August 2017 at 12:50:08 UTC, Robert M. Münch wrote:
> Maybe of interest: 
> https://www.think-cell.com/en/career/talks/iterators/#1
>
> I haven't read everything, so not sure if it worth to take a 
> look.

Iterators have many problems. Andrei's talk some years ago, 
titled "Iterators Must Go", points out many of them in a clear 
fashion. I think most of the problems stem from the fact that 
they are inherently a leaky abstraction. Iterators basically 
treat all data structures as a singly/doubly linked list or as 
arrays (if they allow random access). There is no natural or 
intuitive way of describing, say, a tree or an arbitrary graph as 
a list/array. Ranges and cursors try to solve some of the issues 
but they still have this fundamental problem.

When I think of a good iteration interface, the only thing that 
comes to mind is SQL queries (= relational algebra, more or 
less). You have a few basic operators (Select, From, etc.) for 
querying that are intuitive, simple and safe. Furthermore, they 
compose elegantly, allowing you to express fairly complicated 
queries using these few basic operators. It's a true abstraction 
- I don't know and I don't care if the operators are implemented 
underneath using iterators, ranges, cursors or whatever (of 
course, sometimes I do care, when performance is a priority...).

It may be unrealisic to expect such a nice abstraction of 
iteration for other data structures, not to mention a universal 
one which will work for all interesting data structures. Still, 
if and when I can dispense with iteration altogether, I'm happy 
to do so.