Re: Tagged Rep Spec

[Matthew Heaney <matthew_heaney@acm.org>]

On 15 Jun 1999 14:08, jsanchor@cs5.dasd.honeywell.com wrote:

> I was wondering. Can a person Rep Spec a tagged record?

The correct term is rep "clause."  

Although the RM allows you to specify a rep clause for a tagged type, I
don't see the point, since you don't know 

  1) how big the tag is

  2) if the compiler needs the tag to be placed at a specific offset

  3) if there even is a tag


If you need to transmit this data across an external interface, then use
the streams facility.  That way you won't have to go mucking around with
non-portable, ill-defined rep clauses.

If you don't need type extension for this abstraction, then don't bother
making the type tagged.  Not every type needs to be a tagged type.

 
> Type  Interrupt_Record_Type is tagged
>    Record
>      watchdog_warning   : boolean;
>      utility_clock               : boolean;
>      timer1                        :boolean;
>      rs232                         :boolean;
>      mailbox                      :boolean;
>    end record;
> 
> for Interrupt_Record_Type use
>    record
>      watchdog_warning   at 0 range 0..0;
>      utility_clock               at 0 range 1..1;
>      timer1                        at 0 range 2..2;
>      rs232                         at 0 range 3..3;
>      mailbox                      at 0 range 4..4;
>    end record;


This looks like it doesn't need to be tagged, as it seems low-level.