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From: sra@ecs.soton.ac.uk (Stephen Adams)
Newsgroups: comp.object
Subject: inheritance and `type loss'
Message-ID: <SRA.90Jul10155606@alonzo.ecs.soton.ac.uk>
Date: 10 Jul 90 14:56:06 GMT
Sender: sra@ecs.soton.ac.uk
Distribution: comp
Organization: Southampton University Computer Science
Lines: 35

This might be a naive question, but it has been puzzling me
for a while.  Consider the class integer.  This class
supports two methods:

	+       which takes another integer and returns the sum
	image   which returns a string denoting the integer
		as a decimal number

so 2+4 -> 6  and  (6).image -> "6"

Suppose I create a subclass called roman.  I provide a new
method called image which returns roman numerals e.g.
roman_6.image -> "VI".  The + operation is inherited from
integers.

I declare the following variable and initialize it

	V:roman := make a roman with the value 5

I have two questions:

(a)  what should   (V+V).image  yield?  "10" or "X"?
		   (V+1).image          "6" or "VI"?
		   (1+V).image          "6" or "VI" or error (in +)?

(b)  what do various object oriented systems actually do?


If V+V is not a roman (but an integer, 10) then some
information has been lost.  This might be reasonable but it
does seem surprising.
--
Stephen Adams                        S.Adams@uk.ac.soton.ecs (JANET)
Computer Science                     S.Adams@ecs.soton.ac.uk (Bitnet)
Southampton S09 5NH, UK              S.Adams@sot-ecs.uucp    (uucp)