Blog Archive for / 2006 / 04 /

Message Handling Without Dependencies

Wednesday, 19 April 2006

My article, Message Handling Without Dependencies has been published in the May 2006 issue of Dr Dobb's Journal.

In the article, I describe a technique using templates and virtual functions to reduce dependencies when passing messages in C++. I have used this technique to great effect in production code.

Posted by Anthony Williams
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Review of Refactoring to Patterns by Joshua Kerievsky

Wednesday, 12 April 2006

Refactoring To Patterns brings together the Patterns movement, and the practice of Refactoring commonplace in the Agile community. Whereas the original Gang of Four book told us what patterns were, what sort of problems they solved, and how the code might be structured, Refactoring To Patterns illustrates how, why and when to introduce patterns into an existing codebase.

The opening chapters cover the background, introducing both refactoring and design patterns, and the context in which the book was written. This gives the reader a clear overview of what is involved in Refactoring to Patterns, and paves the way for the refactoring catalogue which makes up the bulk of the book.

The catalogue is divided into chapters based on the type of change required — is this a refactoring to simplify code, generalize code, or increase encapsulation and protection? Each chapter has an introduction which gives an overview of the refactorings contained within that chapter, followed by the refactorings themselves. These introductions clearly illustrate the principles and choices which would lead one to follow the refactorings that follow.

Each refactoring starts with a brief one sentence summary, and before and after structure diagrams with reference to the structure diagrams for the relevant pattern in the Design Patterns book. The sections that follow then cover the Motivation for using this refactoring, step-by-step Mechanics, and a worked Example, relating back to the steps given for the Mechanics. Finally, some of the refactorings finish with Variations on the same theme. The examples are all pulled from a small sample of projects, which are introduced at the beginning of the catalogue section, and help illuminate the instructions given in the Mechanics section. The mechanics themselves are generally clear, and broken down into small steps — sometimes smaller steps than I might take in practice, but I think this is probably wise, as large steps can easily confuse. Finally, the Motivation sections do a good job of explaining why one would choose to do a particular refactoring, and any pitfalls to doing so — the "Benefits and Liabilities" tables provide a useful summary.

This book is well written, easy to read, and genuinely useful. It has helped me put some of the refactorings I do into a larger context, and given me insight into how I can integrate patterns with existing code, rather than designing them in up front. As John Brant and Don Roberts highlight in their Afterword, this is a book to study, the real benefit comes not from knowing the mechanics, but by understanding the motivation, and the process, so that one may apply the same thinking to other scenarios not covered by this book. If you are serious about software development, buy this book, inwardly digest it, and keep it by your side.

Highly Recommended.

Buy this book

Refactoring to Patterns
Joshua Kerievsky
Published by Addison-Wesley
ISBN 0-321-21335-1

Buy from Amazon.co.uk
Buy from Amazon.com

Posted by Anthony Williams
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Review of Working Effectively With Legacy Code by Michael C. Feathers

Wednesday, 12 April 2006

Michael puts a new spin on the term "legacy code", identifying it as code without automated tests. For those of us used to the more traditional meaning of the term, referring to old codebases full of cruft and quick fixes on top of hasty modifications on top of quick fixes, this is a somewhat unusual attitude — we like to think that our new code is clean and not "legacy code", even if it doesn't have automated tests. However, it doesn't take long for the former to turn into the latter — leave it 6 months whilst you work on another project, so you've forgotten the details, then rush through a bug fix because it needs to be in production tomorrow, and you're well on the way.

A lot of the book is spent covering different techniques for getting nasty convoluted code under test. These techniques often focus on breaking dependencies, so you can instantiate an object in a test harness, or call a function without it sending an email or talking to the database. In many cases, these are sensible recommendations for improvements to the codebase, but in some cases, Michael recommends techniques you wouldn't expect to see in production code, such as defining preprocessor macros with the same name as functions to avoid calling them, or writing dummy implementations of API functions. This is not to say that the book recommends such techniques wholesale — Michael is keen to point out that these techniques should only be used to get the code under test, so that it can be refactored safely.

Chapters are helpfully named, with titles like "This class is too big, and I don't want it to get any bigger", and the book gives good advice on how to deal with the nastiest codebases. The key recommendation underlying it all is "get the code under test, so you can refactor safely", and this is borne in mind with those techniques that require changing the production code in order to get it under test — these techniques provide step by step instructions to help you make the required changes without breaking anything.

The content of this book is excellent, and the writing clear, so it is unfortunate that it is marred by numerous minor errors, such as spelling mistakes, or using the wrong identifier when referring to a code example. However, this minor niggle is not enough to stop me recommending it — every software developer should have a copy.

Highly Recommended. This book is a must-have for anyone who has to maintain code — which is pretty much every software developer on the planet.

Buy this book

Working Effectively With Legacy Code
Michael C. Feathers
Published by Prentice Hall PTR
ISBN 0-13-117705-2

Buy from Amazon.co.uk
Buy from Amazon.com

Posted by Anthony Williams
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