An interesting juxtaposition in the ACCU 2009 schedule put my talk on “adopting MVC in Objective-C and Cocoa” next to Peter Sommerlad’s talk on “Design patterns with modern C++”. So the subject matter in each case was fairly similar, but then the solutions we came up with were entirely different.
One key factor was that Peter’s solutions try to push all of the “smarts” of a design pattern into the compiler, using templates and metaprogramming to separate implementations from interfaces. On the other hand, my solutions use duck typing and dynamic method resolution to push all of the complexity into the runtime. Both solutions work, of course. It’s also fairly obvious that they’re both chosen based on the limitations and capabilities of the language we were each using. Nonetheless, it was interesting that we both had justifications for our chosen (and thus One True) approach.
In the Stroustroup corner, the justification is this: by making the compiler resolve all of the decisions, any problems in the code are resolved before it ever gets run, let alone before it gets into the hands of a user. Whereas the Cox defence argues that my time as a programmer is too expensive to spend sitting around waiting for g++ to generate metaprogramming code, so replace the compilation with comparitively cheap lookups at runtime – which also allows for classes that couldn’t have possibly existed at compiletime, such as those added by the Python or Perl bridge.
This provided concrete evidence of a position that I’ve argued before – namely that Design Patterns are language-dependent. We both implemented Template Method. Peter’s implementation involved a templatized abstract class which took a concrete subclass in the realisation (i.e. as the parameter in the <T>). My implementation is the usual Cocoa delegate pattern – the “abstract” (or more correctly undecorated) class takes any old id as the delegate, then tests whether it implements the delegation sequence points at runtime. Both implement the pattern, and that’s about where the similiarities end.