Jesse Jones <jesjones@mindspring.com> writes:
I've been working on a multimethod library for C++ and, in my opinion, it's turned out rather well. It's modeled closely after the multimethods in CLOS and Dylan so it's more flexible than the other (pure C++) multimethod libraries I have seen. In particular, new methods can be added at runtime, polymorphic dispatch doesn't require exact matches, and methods can call the next most specific method (sort of how a virtual member can call the inherited member).
The nicest feature of multimethods is that they allow classes to be extended with virtual members without editing the classes. It's completely transparent: the class doesn't need to be coded to allow for multimethods and new multimethods can be added without touching any existing code. It's a far nicer model than using the Visitor pattern.
The other advantage is that it makes multiple dispatch trivial. This is useful for things like intersection tests, drag and drop, event + view message handling, etc. Without multiple dispatch these sorts of things are rather painful to write and normally require a lot of switching on types.
The design is quite simple: there's a generic function object to which you dynamically add compatible function objects (using operator<<), an operator() that is invoked which will then call the most specialized function object (or method) that applies to the actual arguments, and a next_method member that can be used within a method to call the next most specialized method.
The source can be found at <http://sourceforge.net/projects/multimethods/>. Currently it works with CodeWarrior 9.2. There are a bunch of test files, but no examples or docs as of yet.
So, does this sound like something boost would be interested in?
How does this stack up against http://tinyurl.com/5qxky? -- Dave Abrahams Boost Consulting http://www.boost-consulting.com