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From: mjv@objects.mv.com (Michael J. Vilot)
Newsgroups: comp.lang.c++
Subject: Re: Nonvirtual base classes in MI
Message-ID: <1174@zinn.MV.COM>
Date: 8 Feb 91 02:40:22 GMT
Sender: nuucp@zinn.MV.COM
Lines: 36

Keith Gorlen commented on my design decisions in the Booch Components library:
>It seems to me that there is little, if any, advantage to using private
>inheritance as opposed to simply including an instance of the class as a member
>variable,

I agree with Keith, and with Tom Cargill, that private base classes can be
transformed into class members.  There may be some minor performance
differences between the two, but I suspect on average it's a wash.  

> Are there more compelling reasons for using private inheritance than this?

Compelling?  Probably not -- certainly not, in any absolute sense.  Are there
_compelling_ reasons for deriving all classes from a root class Object?  Again,
probably not.  On the other hand, such decisions may make sense within a given
set of design goals and constraints. 

Each decision represents a particular design choice.  In my case, I used
private derivation to express the difference between the abstract class and the
``concrete'' classes derived from it -- the derived classes mix in their
representation.  Making the representation class a private base separates it
from the other members of the class, and highlights the design decision. 

|> The second use I made of MI was to mix in specific behavior for Node classes in
|> the unbounded representations.

>Is this also a case of private inheritance?

No, in this latter case the derivation has to be public:  Node classes mixing
in Managed behavior (similarly for Shared) make the added members publicly
available.  The simplicity of the Managed and Controlled forms hinges on the
transparent substitution of ``new'' and ``delete'' that occurs as a result.

I'm looking forward to a lively discussion in Washington !
--
Mike Vilot,  ObjectWare Inc, Nashua NH
mjv@objects.mv.com  (UUCP:  ...!decvax!zinn!objects!mjv)