From: Jim Weirich <jim@...>
Date: 2004-06-26T01:43:42+09:00
Subject: Re: Is it considered Harmful?


Eric Hodel said:
> Smalltalk's classes are better factored to handle such occurances.
> Ruby's are not.

I found the following paragraph about Squeak Smalltalk interesting.  Most
Smalltalks use an object table to aid in garbage collection.  An added
benifit of the object table is that swapping two objects is just swapping
the pointers in the object table.  Squeak doesn't use an object table, so
they had to work extra hard to make become: work.  Here is the discussion
(from http://users.ipa.net/~dwighth/squeak/oopsla_squeak.html):

BEGIN QUOTE ===================

     One more issue remained to be dealt with, and that was support of the
become operation without an object table. (The Smalltalk become
primitive atomically exchanges the identity of two objects; to
Smalltalk code, each object appears to turn into, or "become," the
other.) With an object table, the become primitive simply exchanges
the contents of two object table entries. Without an object table, it
requires a full scan of memory to replace every pointer to one object
with a pointer to the other. Since full memory scans are relatively
costly, we made two changes. First, we eliminated most uses of become
in the Squeak image by changing certain collection classes to store
their elements in separate Array objects instead of indexed fields.
However, become operations are essential when adding an instance
variable to a class with extant instances, as each instance must
mutate into a larger object to accommodate the new variable. So, our
second change was to restructure the primitive to one that exchanges
the identity of many objects at once. This allows all the instances
of a class to be mutated in a single pass through memory. The code
for this operation uses the same technique and, in fact, the very
same code, as that used to rectify pointers after compaction.

END QUOTE =====================

-- 
-- Jim Weirich     jim@weirichhouse.org    http://onestepback.org
-----------------------------------------------------------------
"Beware of bugs in the above code; I have only proved it correct,
not tried it." -- Donald Knuth (in a memo to Peter van Emde Boas)