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From: Jesse Jones (jesjones_at_[hidden])
Date: 2004-09-17 19:04:42

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

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
<>. 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?

   -- Jesse

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