I'd be interested in a concise summary of the features/benefits. Right now I really only see one:
Classes get "inside-out" property storage without needing to define fields and associated data storage hashes/arrays explicitly.
I see three major points going for the Alter proposition. Rewording the one you mention,
In an Alter-class you get to keep Perl's original model, "an object is just a reference...", only each class gets to see its own reference. The ingenious twist of using a hash "inside-out", keyed by the object('s id) is not needed. If you want your object to "be" a hash keyed by field name, that's fine. If it's naturally a scalar, or code, you can have that too.
Second, Alter objects get garbage collection and thread support from Perl, the class has nothing to do with that. Destructors can be freely used for other purposes.
Third, though this hasn't been conclusively demonstraded in code, it is possible to provide generic hooks for Data::Dumper, Storable and the like that support all Alter classes and which can be inherited or even imported. With inside-out classes, this is hard to achieve because only the class knows where its data is stored. An Alter-object carries all its data with it, ready to dump and able to be restored.
As for the cost, you have rightly observed that both techniques require the call to either id() or ego() on each access, with equal possibility of caching. The call to id() is essentially free, except for the call overhead, but ego() has a hash access in the path, which counts. Preliminary benchmarks show that the impact is measurable, but not dramatic. At the moment I am less concerned with efficiency but with getting it right.
On a broader note, inside-out has shown what it takes to make black-box inheritance work in Perl. In particular, some kind of out-of-band storage is needed for which it cleverly uses the existing refaddr() as a primitive. My goal is to design a primitive (ego()) for out-of-band storage expressly for the purpose. In that sense I consider the sketch of the Alter module we're discussing a step beyond inside-out.
This discussion (and another one on clpm) is helping me to spot potential problems and limitations of the approach.
Anno
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