Re: Question about record rep spec placement

["Norman H. Cohen" <ncohen@watson.ibm.com>]

Ken Garlington wrote:
> 
> We have some Ada83 code that looks like the following:
> 
> package Some_Package is
> 
>   type Some_Record is record
>    -- components here
>   end record;
> 
>   -- some arbitrary declarations here
> 
> private
> 
>   for Some_Record use record
>    -- component rep spec here
>   end record;
> 
> end;
> 
> Are there cases in Ada where a record representation specification
> cannot be deferred until the private part of the package? I'm told that
> when this code was compiled with an Ada 95 compiler, that the record
> rep spec had to be moved to immediately after the record declaration
> for the code to compile correctly. I'm wondering if this is
> 
>   (a) illegal Ada83 code that our Ada83 compiler accepted anyway
>       (much like the use of unchecked_conversion on the left-hand side
>       of an assignment statement, as described in another thread),
> 
>   (b) legal Ada code that the other Ada compiler rejected anyway, or
> 
>   (c) OUCH! Ada83 code that is no longer valid Ada.

The answer lies in those "arbitrary declarations".  If those
declarations contain what is called a "forcing occurrence" in Ada 83,
such as a declaration of an object of type Some_Record, the record
representation clause is illegal.  Essentially the same rules hold in
Ada 95, but with different terminology:  The Ada-95 RM describes the
object declaration or whatever as "freezing" the representation of the
record type.

There are a few cases where the freezing rules make a legal Ada-83
program into an illegal Ada-95 program, but these cases are obscure and
pathological.  For example, because of poor wording in the Ada-83
definition of "forcing occurrence", 

   subtype S is Integer range 0 .. Some_Record'Size;

was not a forcing occurrence in Ada 83, but this declaration does freeze
the representation of Some_Record in Ada 95.

-- 
Norman H. Cohen
mailto:ncohen@watson.ibm.com
http://www.research.ibm.com/people/n/ncohen