Re: Self-referential types

["Matthew Heaney" <matthew_heaney@acm.org>]

In article <7u25pu$p4k$1@nnrp1.deja.com> , Ted Dennison 
<dennison@telepath.com>  wrote:

>>   type Digital_Clock (...) is
>>     limited record
>>       Master_Obs : Master_Obs_Type (Digital_Clock'Access);
>>       Slave_Obs  : Slave_Obs_Type (Digital_Clock'Access);
>>       Control    : Control_Type (Digital_Clock'Access);
>>     end record;
>
> Ok...What does this buy you over using non-anonomous access fields and
> an initilization? And how does this constitute MI?

Because the observer types have a primitive operation (Update) called by
their subject.  The Master_Timer subject calls back the Master_Obs
component of the Digital_Clock, and the Slave_Timer subject calls back
the Slave_Obs component.

See the article about multiple subjects in the design patterns archive,
from whence this was paraphrased.

Perhaps it would have been more obvious to have done it this way:

  type Digital_Clock (...) is
    new Limited_Controlled with record
      Master_Obs : Master_Obs_Type (Digital_Clock'Access);
      Slave_Obs  : Slave_Obs_Type (Digital_Clock'Access);
    end record;

Syntactically, Digital_Clock has only a single parent,
Limited_Controlled.  But logically, it has three parents
(Limited_Controlled once, and Observer_Type twice).


Matt

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