There are different types of magic (this is how tie is implemented amongst other things.), I am using the '~' user extension type. Multiple variables can be attached to a reference as well and as long as they specify a different 'namespace', they won't clobber each other.

I choose to let my implementation use any ref blessed or not. The table portion refers to additional methods that can be implemented.

    Function pointer    Action taken
    ----------------    ------------
    svt_get             Do something after the value of the SV is retr
+ieved.
    svt_set             Do something after the SV is assigned a value.
    svt_len             Report on the SV's length.
    svt_clear           Clear something the SV represents.
    svt_free            Free any extra storage associated with the SV.
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To get the value bound to the variable, I'm simply returning it's pointer stored in mg_ptr. See the Magic Virtual Tables section in perldoc:perlguts for a better explanation (It's late and I'm bug-eyed tired). I'm only implementing free though it could be extended to open up the other functions. I probably will do that. If I did, let's say you were trying to make a cache of the output of some larger process, you would be able to add a callback to the data structure via set && clear magic that would invalidate that part of the cache when updated and it wouldn't require any direct action by the user.

The implementation as it stands is exposed through a single routine but this might change as the function attach is almost too DWIM.
Here's a run down of what it looks like.

 attach ( $somepointer, { seen=>1 }, 'opt_name_to_avoid_collision') or
+ die "Couldn't attach;
 # Later on...
 my $attached = attach($somepointer);
 $attached->{bar}= uc $attached->{bar};
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Not a compelling example above but I can think of some better ones which I'll post when it's cleaned up. If you think about how matching works and how it tracks where it is in a string, well you can do the same thing with any sub now without trying to compute unique keys from caller() + data and storing it in a hash. Lazily processing lists non-destructively is another good example.

-Lee

"To be civilized is to deny one's nature."