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From: nospam@thanks.com.au (Don Harrison)
Subject: Re: Design By Contract
Date: 1997/08/29
Message-ID: <EFnoJv.1vx@syd.csa.com.au>#1/1
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References: <JSA.97Aug28172900@alexandria.organon.com>
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Organization: CSC Australia, Sydney
Newsgroups: comp.lang.ada,comp.lang.eiffel
Date: 1997-08-29T00:00:00+00:00
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Jon S Anthony wrote:

:In article <EFM140.Fy9@syd.csa.com.au> nospam@thanks.com.au (Don Harrison) writes:
:
:>   - Strict enforcement of encapsulation while maximising visibility. Eiffel 
:>     offers a look-but-don't-touch view of object attributes. In Ada83,
:>     an attribute and its type has to be declared in the visible interface in 
:>     order for clients to see it's structure but then they can also update it 
:>     directly. If you declare its type as private, clients can't update it 
:>     directly but neither can they see its structure.
:
: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:

  a.SOME_TYPE
  ...
  if a.y > 0 then
    x := a.y
  end

but prevent you from updating them directly (which would break encapsulation):

  a.y := x        -- Illegal

[You can update them *indirectly* by calling an operation in the object:

  a.set_y (x)]


To get similar read-only semantics in Ada, you have to declare the attribute 
in the package body (hiding it) and return it via an exported function (which
can't have the same name):

  the_a: SOME_TYPE
  ...
  function a return SOME_TYPE is
  begin
    return the_a;
  end;

I prefer the Eiffel model because:

  - It's simple and direct.
  - It doesn't clutter the spec and body with functions.
  - It doesn't force you to invent multiple identifiers (one of which has 
    a contrived name: "the_a" above).


IMO, the other possibilities for attributes in Ada are unattractive because
they are either too permissive (look-and-touch - attributes declared in the 
visible part of the spec) or too restrictive (don't-look-don't touch - 
private attributes).

BTW, someone with a sense of humour may enjoy BM's discussion of Ada private 
types (OOSC-2, 33.4 - Hiding the Representation: The Private Story. P.1085-1087).

[...]


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