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From: jans@tekgvs.LABS.TEK.COM (Jan Steinman)
Newsgroups: comp.object
Subject: Re: Inheritance is NOT Delagation (sic)
Message-ID: <6148@tekgvs.LABS.TEK.COM>
Date: 13 Oct 89 23:08:20 GMT
Distribution: comp
Organization: Tektronix Inc., Beaverton, Or.
Lines: 28

<david@jpl-devvax.JPL.NASA.GOV (David E. Smyth)>
<The average depth of inheritance in most systems is about 3.>

This is somewhat ambiguous.  How is "average depth" defined?

1) Do you mean the number of subclasses divided by the sum of their depths?  
That would be somewhat misleading, as the class chain "Object, Set, Dictionary, 
IdentityDictionary, MethodDictionary" would have an average depth of 2.5, but 
this does not address the usefulness and instance counts of each class.

2) If you mean the number of objects in the system divided by the sum of their 
depths, than I would suspect that Smalltalk-80, at least, is *much* deeper than 
3, since instances tend to be of leafs, rather than of more abstract classes 
somewhere higher up.

3) If you mean the average height that an instance must go up the superclass 
chain at run time in order to locate a desired method, then 3 is too high, at 
least in Smalltalk.  Most methods sent to a Smalltalk object are in it's own 
method dictionary.

No, I don't have numbers right now.  Case #1 and #2 numbers would be easy to 
obtain in a workspace for a given Smalltalk image.  I think the Green Book 
discussion on the Smalltalk interpreter  method cache has some numbers for #3.

							   Jan Steinman - N7JDB
						  Electronic Systems Laboratory
					Box 500, MS 50-370, Beaverton, OR 97077
						(w)503/627-5881 (h)503/657-7703