Feeds

To iterate is human

Iterator versus the Enumeration Method

Build a business case: developing custom apps

In the previous article, I made the following observation:

A collection that holds its elements but doesn't allow you to traverse them is unlikely to prove popular. There are many ways to offer traversal, but if the caller needs to be able to know the position of elements in some way there are essentially only three general designs that keep the collection's internal representation hidden from the caller.

And a reader made the following comment:

A fourth option, which is in the general case superior to all three of your options, is the internal iterator. It delegates management of the iteration to the collection, rather than adding repetitive boilerplate to your functional code.

In this particular case, the design pattern mentioned is also known as the Enumeration Method pattern. The use of this pattern was mentioned in the article, along with a link to a corresponding article, although it appears that it was overlooked.

However, the posted comment touches on a whole topic area that deserves more attention than there was either space or topic focus for in the previous article. In fact, it deserves at least a whole article!

In essence, where Iterator is normally characterised by the introduction of an additional kind of object that performs iteration over some kind of collection, Enumeration Method introduces a method on the collection that calls back on a piece of supplied code for each element in the collection. The key benefit here is that you are only dealing with a single unit of design responsibility. All the mechanics of iteration are contained within the collection. Very compact, very cohesive. Nice — just what we want.

However, there are some other considerations at play: some are related to matters of perception; others are related to questions of practice; the rest are related to requirements and design goals; all are related to understanding the context that frame, and the forces that drive, a particular pattern's application.

The Gang of Four tried to shoehorn this additional iteration pattern, in the guise of Internal Iterator, into their write up of Iterator, but it must be said with limited success.

Its presence is fairly low key, with most of the detail tucked away towards the end of the pattern write up, the last item in the Sample Code section. The unification of these two different design approaches was something of a compromise to try to accommodate a single iteration pattern for all OO-related languages. Specifically, an attempt to square a C++ view of the world with a Smalltalk view of the world... which is always a challenge.

However, the fundamentally different design structures, philosophies, and trade-offs of the two approaches undermine any claim that they can be considered the same pattern. A pattern represents a recurring design solution, with an associated set of consequences, to a recurring problem, whose forces are understood and arise within a specific context.

It turns out that although both can be characterised in the general sense as iteration patterns the similarity ends there: the two approaches have almost nothing in common; the consequences of applying each one have almost nothing in common; indeed, even the problem forces that they resolve differ in the detail.

Gartner critical capabilities for enterprise endpoint backup

More from The Register

next story
Why has the web gone to hell? Market chaos and HUMAN NATURE
Tim Berners-Lee isn't happy, but we should be
Microsoft boots 1,500 dodgy apps from the Windows Store
DEVELOPERS! DEVELOPERS! DEVELOPERS! Naughty, misleading developers!
'Stop dissing Google or quit': OK, I quit, says Code Club co-founder
And now a message from our sponsors: 'STFU or else'
Apple promises to lift Curse of the Drained iPhone 5 Battery
Have you tried turning it off and...? Never mind, here's a replacement
Linux turns 23 and Linus Torvalds celebrates as only he can
No, not with swearing, but by controlling the release cycle
Scratched PC-dispatch patch patched, hatched in batch rematch
Windows security update fixed after triggering blue screens (and screams) of death
This is how I set about making a fortune with my own startup
Would you leave your well-paid job to chase your dream?
prev story

Whitepapers

Top 10 endpoint backup mistakes
Avoid the ten endpoint backup mistakes to ensure that your critical corporate data is protected and end user productivity is improved.
Implementing global e-invoicing with guaranteed legal certainty
Explaining the role local tax compliance plays in successful supply chain management and e-business and how leading global brands are addressing this.
Backing up distributed data
Eliminating the redundant use of bandwidth and storage capacity and application consolidation in the modern data center.
The essential guide to IT transformation
ServiceNow discusses three IT transformations that can help CIOs automate IT services to transform IT and the enterprise
Next gen security for virtualised datacentres
Legacy security solutions are inefficient due to the architectural differences between physical and virtual environments.