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From: johnson@p.cs.uiuc.edu
Newsgroups: comp.object
Subject: Re: Inheritence and Delegation
Message-ID: <135300005@p.cs.uiuc.edu>
Date: 13 Oct 89 11:45:00 GMT
References: <126109@sun.Eng.Sun.COM>
Lines: 18
Nf-ID: #R:sun.Eng.Sun.COM:126109:p.cs.uiuc.edu:135300005:000:896
Nf-From: p.cs.uiuc.edu!johnson    Oct 13 06:45:00 1989


>Lynn Stein, a graduate student here at Brown, proved that delegation and
>inheritance are equivalent.  Hence, inheritance is not a "special case" of
>delegation.  For the proof, see her paper in the OOPSLA '87 Proceedings:
>"Delegation Is Inheritance."

The proof consisted of reducing all objects to metaclasses.  This is
not very convincing unless you have Smalltalk style metaclasses in
your language.  Thus, the proof does not apply to C++, Eiffel, etc.

I think that it is better to reduce delegation to regular message
passing.  It is easy to write a stub that implements a message by forwarding
it to a component.  If you also pass along the receiver as an argument to
the message then the delegatee can send messages to the original receiver.
This is the essence of delegation.  In my experience, the overhead of
having to do it yourself is so small as to be negligible.

Ralph Johnson