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From: smize@Starbase.NeoSoft.COM (Samuel Mize)
Newsgroups: comp.lang.ada
Subject: Re: HELP-Information hiding/Abstraction (explain difference please)
Date: 12 Mar 1995 00:50:10 -0600
Organization: NeoSoft Internet Services   +1 713 968 5800
Message-ID: <3ju5j2$73q@Starbase.NeoSoft.COM>
References: <j_green-070395115500@merlin.clh.icnet.uk>
NNTP-Posting-Host: starbase.neosoft.com
Date: 1995-03-12T00:50:10-06:00
List-Id: <comp.lang.ada>

In article <j_green-070395115500@merlin.clh.icnet.uk>,
Jacky <j_green@icrf.icnet.uk> wrote:
>Could someone please explain the difference between
>information hiding and data abstraction - Is there one?
>
>I would be grateful for any simple small pieces of code that
>help show the difference.

-----------------------------------------------------------
Data abstraction:

package Abstractions is

  type Stoplight_Color is (Red, Yellow, Green);

  type Person is
    record
      Age: integer;
      -- Lots more fields full of data -- but you can handle
      -- a person as a single, higher-level item.
      --
      -- The concept "person" is abstracted.
      --
      -- The information is NOT hidden -- a 
    end record;

end Abstractions;
-----------------------------------------------------------
Information hiding:

package Hidden_Info is

  type Person is private;

  -- specs for procedures and functions on Person given here

private
  type Person is
    record
      Age: integer;
      -- Lots more fields full of data -- but you can handle
      -- a person as a single, higher-level item.
      --
      -- The concept "person" is abstracted.
      --
      -- The information is HIDDEN -- someone using this package
      -- CANNOT access the record fields.  The user MUST user
      -- the provided procedures and functions.
    end record;

end Abstractions;
-----------------------------------------------------------
As a rule of thumb, information hiding is used to enforce
data abstractions.

Any time you create a user-defined type, you are defining a
data abstraction.

Any time you avoid putting implementation details into the
visible specification of a package, you are using information
hiding.

Samuel Mize - smize@starbase.neosoft.com