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From: "Matthew Heaney" <matthew_heaney@acm.org>
Subject: Re: Help me to chose between ADA 95 and C++
Date: 1999/12/14
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In article <38567806.FD1F4232@averstar.com> , Tucker Taft 
<stt@averstar.com>  wrote:

> You also can't implement an interface in multiple ways with a single type,
> which could be useful for things like the "observer" pattern where a single
> object wants to observe multiple other objects.


There's an example of how to do this in the design patterns archive.

<http://www.acm.org/archives/patterns.html>

Use the search engine to search for "observering multiple subjects" or
something like that.

In my little example, a digital clock observes, simultaneously, both a
battery (to indicate when the power is low) and a timer (its source of
time).  Like this:

package Digital_Clocks is

  type Digital_Clock
    (Battery : access Battery_Type;
     Timer   : access Timer_Type) is limited private;

private

  type Battery_Observer
    (Clock : access Digital_Clock) is
   new Observer with null record;

  type Timer_Observer
    (Clock : access Digital_Clock) is
   new Observer with null record;

  type Digital_Clock
    (Battery : access Battery_Type;
     Timer   : access Timer_Type) is
    new Limited_Controlled with record
      Battery_Obs : Battery_Observer (Digital_Clock'Access);
      Timer_Obs   : Timer_Observer (Digital_Clock'Access);
    end record;
...
end Digital_Clocks;