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From: peterli@ucsfmis.ucsf.edu (Peter Li)
Newsgroups: comp.lang.c++
Subject: Re: Inheritance vs. Composition
Message-ID: <2775@ucsfcca.ucsf.edu>
Date: 21 Feb 90 01:16:15 GMT
References: <CIMSHOP!DAVIDM.90Feb11005056@uunet.UU.NET> <GZA.90Feb13025537@mentor.cc.purdue.edu>
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Reply-To: peterli@mis.ucsf.edu (Peter Li)
Organization: Univ. of Calif. San Francisco, Medical Information Sciences
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In article <GZA.90Feb13025537@mentor.cc.purdue.edu> gza@mentor.cc.purdue.edu (William R Burdick) writes:
>The difference between inheritance (or generalization) and composition
>(or aggregation)  is a tough point   to  figure  out without some good
>examples (I hope I  can give some here).
> ...[deleted]
>In OOPLs, you can use instance variables  for the part-of relationship
>and you can use subclassing for the is-a relationship.

Here is a monkey-wrench:

Consider the classes INTEGER and RATIONAL, 
would you say RATIONAL inherit from INTEGER,
or RATIONAL is completely independent but
composed of two INTEGERs, or ...
INTEGER is a subset of RATIONAL (something
that OOP doesn't address) ?

You can extend this into REALs and COMPLEXes.

Figuring out mathematical semantics vs.
representations is sure messy ... :-).

Peter Li