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From: nospam@thanks.com.au (Don Harrison)
Subject: Re: Design By Contract
Date: 1997/09/02
Message-ID: <EFuuJK.GKu@syd.csa.com.au>#1/1
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Sender: news@syd.csa.com.au
References: <JSA.97Aug29150908@alexandria.organon.com>
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Organization: CSC Australia, Sydney
Newsgroups: comp.lang.ada,comp.lang.eiffel
Date: 1997-09-02T00:00:00+00:00
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Jon S Anthony wrote:

:In article <EFnoJv.1vx@syd.csa.com.au> nospam@thanks.com.au (Don Harrison) writes:
:
:> :Isn't that exactly the point?  Why should a client be able to see the
:> :actual structure, aka implementation????
:> 
:> No, the point is that there is no need to hide the structure of an attribute
:> so long as it's read-only (look-but-don't-touch). These semantics allow you 
:> to query components of visible objects directly:
:
:I'm not convinced that is sufficient.  Knowing the structure can lead
:one into writing things that still depend on it (efficiency hacks come
:quickly to mind).

The thing is, though, that the designer of the supplier class *intends* the
client to see the structure of attributes in just the same way as the author
of an Ada interface *intends* clients to see the structure of a value returned
by a function.

Exporting happens on a need-to-know basis. If clients need attributes for 
any purpose (including efficiency tuning), then the supplier exports them. 
If they don't, then the supplier keeps them hidden. Note that in the extreme 
case of an attribute whose underlying class exports none of its attributes, 
you have the equivalent to an Ada private attribute. That is, the Ada

package Some_Module is
  type My_type is tagged private;
  ...                     -- exported operations
  No_structure : My_type;
private
  type My_type is tagged record
    ...                   -- hidden attributes
  end record;
end;

is roughly equivalent to the Eiffel

class MY_CLASS
  feature
    ...                   -- exported operations
  feature {NONE}
    ...                   -- hidden attributes
end

class SOME_CLASS
  feature
    no_structure : MY_CLASS
end

So, if a supplier really needs to restrict what clients can see, for whatever 
reason, they're able to.

However, the good thing about Eiffel is that it gives designers the flexibility
of exposing as much or as little of an object's structure, in contrast with 
with Ada's you-see-it-all or you-see-nothing export policy.


Don.                     (Reverse to reply)
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Don Harrison             au.com.csa.syd@donh