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From: Bart Samwel <bsamwel@wi.leidenuniv.nl>
Subject: Re: OO, C++, and something much better!
Date: 1997/01/10
Message-ID: <32D66934.794B@wi.leidenuniv.nl>#1/1
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references: <JSA.97Jan8123334@alexandria> <E3q536.31D@syd.csa.com.au>
 <dewar.852851538@merv>
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organization: Dept. of Math. and Comp.Sci.; Leiden Univ.; Leiden;
 the Netherlands
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Date: 1997-01-10T00:00:00+00:00
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Robert Dewar wrote:

> values of one type to values of another type. It is that these
> operations can generate abnormal values.

I just thought of a way to implement safe type (read: class) casting
in Eiffel:

Use the Attempted Assignment operator ?= to try to assign, as in:

   local
      object1 : CLASS1
      object2 : CLASS2

   do
      !!object1.make (whatever)
      object2 ?= object1         -- this is what I mean
      if object2 /= Void then
        ...
   end

This operation makes object2 a reference to object1, but belonging to
class CLASS2, but only if, after conversion, object2 satisfies the
class invariant of CLASS2. Otherwise, object2 will be Void.

Oh, by the way, this is meant to be interpreted as humour, not as
a serious proposal. ;-)