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From: kgorlen@alw.nih.gov (Keith Gorlen)
Newsgroups: comp.lang.c++
Subject: Re: Nonvirtual base classes in MI
Message-ID: <959@nih-csl.nih.gov>
Date: 9 Feb 91 13:37:59 GMT
References: <1174@zinn.MV.COM> <64173@brunix.UUCP>
Sender: news@nih-csl.nih.gov
Reply-To: kgorlen@alw.nih.gov (Keith Gorlen)
Organization: National Inst. of Health
Lines: 29

In article <64173@brunix.UUCP>, sdm@cs.brown.edu (Scott Meyers) writes:
|> In article <1174@zinn.MV.COM> mjv@objects.mv.com (Michael J. Vilot) writes:
|> | 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.  
|> 
|> No no no.  You lose virtual functions when you turn inheritance into member
|> containment.  There is a conceptual distinction between the following:
|> 
|>     A has a member B             ==> A "has a" B (or A "contains a" B)
|>     A publicly inherits from B   ==> A "is a" B
|>     A privately inherits from B  ==> A "is implemented in terms of a" B
|> 
|> Private inheritance is a convenient way to share code when there is no
|> other conceptual relationship between two classes.  I usually call this
|> "code stealing."  When stealing the code in another class, you often have
|> to do some class specific stuff; this is where virtual functions come in.
|> The member functions in the (private) base class can call virtual functions
|> to do the customization.  This won't work with member objects, because they
|> have no way of knowing what they're a part of.
|> 

That sounds like a good reason to me!

-- 
	Keith Gorlen			phone: (301) 496-1111
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