Re: Representation Clauses And Freezing

[Tucker Taft <stt@averstar.com>]

"Marin D. Condic" wrote:
> 
> I am trying to find a reasonable workaround to a problem involving
> representation clauses and tagged records. I believe I am running into
> problems with freezing rules. Below are two compiler listings which are
> very similar. When the record type is visible, the compiler is happy.
> When the record type is private, it complains. The question in my mind
> is *WHY* is there some sort of restriction on this? What compiler
> mission cannot be achieved unless the record is visible? Is there some
> sort of workaround that would enable representation clauses on private
> tagged records? (Inheritance from a rep-claused-tagged-record is its own
> problem!)

The behavior is mysterious, and probably just a bug.  I would report
it to the compiler vendor, and see what they say.

-Tuck

> 
> GNAT 3.12p  (19990629) Copyright 1992-1999 Free Software Foundation,
> Inc.
> 
> Compiling: rep_clause_01.ads (source file time stamp: 2000-07-17
> 14:20:14)
> 
>      1. with System ;
>      2.
>      3. package Rep_Clause_01 is
>      4.
>      5.     type UInteger_16 is mod 2**16 ;
>      6.     for UInteger_16'Size use 16 ;
>      7.
>      8.     type Rec_Type is tagged record
>      9.         Field_01        : Uinteger_16           := 0 ;
>     10.         Field_02        : Uinteger_16           := 0 ;
>     11.         Field_03        : Uinteger_16           := 0 ;
>     12.     end record ;
>     13.
>     14. private
>     15.
>     16.     for Rec_Type use record
>     17.         Field_01        at  4 range 0..15 ;
>     18.         Field_02        at  6 range 0..15 ;
>     19.         Field_03        at  8 range 0..15 ;
>     20.     end record ;
>     21.
>     22.     for Rec_Type'Size use 10 * System.Storage_Unit ;
>     23.     for Rec_Type'Alignment use 2 ;
>     24.
>     25. end Rep_Clause_01 ;
> 
>  25 lines: No errors
> 
> GNAT 3.12p  (19990629) Copyright 1992-1999 Free Software Foundation,
> Inc.
> 
> Compiling: rep_clause_02.ads (source file time stamp: 2000-07-17
> 14:20:14)
> 
>      1.
>      2. with System ;
>      3.
>      4. package Rep_Clause_02 is
>      5.
>      6.     type UInteger_16 is mod 2**16 ;
>      7.     for UInteger_16'Size use 16 ;
>      8.
>      9.     type Rec_Type is tagged private ;
>     10.
>     11. private
>     12.
>     13.     type Rec_Type is tagged record
>     14.         Field_01        : Uinteger_16           := 0 ;
>     15.         Field_02        : Uinteger_16           := 0 ;
>     16.         Field_03        : Uinteger_16           := 0 ;
>     17.     end record ;
>     18.
>     19.     for Rec_Type use record
>     20.         Field_01        at  4 range 0..15 ;
>     21.         Field_02        at  6 range 0..15 ;
>     22.         Field_03        at  8 range 0..15 ;
>     23.     end record ;
>     24.
>     25.     for Rec_Type'Size use 10 * System.Storage_Unit ;
>                                      |
>         >>> size for "Rec_Type" too small, minimum allowed is 128
> 
>     26.     for Rec_Type'Alignment use 2 ;
>                                        |
>         >>> alignment for "Rec_Type" must be at least 4
> 
>     27.
>     28. end Rep_Clause_02 ;
> 
> MDC
> --
> ======================================================================
> Marin David Condic - Quadrus Corporation - http://www.quadruscorp.com/
> Send Replies To: m c o n d i c @ q u a d r u s c o r p . c o m
> Visit my web site at:  http://www.mcondic.com/
> 
> "Nothing in the world is more dangerous than sincere ignorance and
> conscientious stupidity."
> 
>     -- Martin Luther King, Jr
> ======================================================================

-- 
-Tucker Taft   stt@averstar.com   http://www.averstar.com/~stt/
Technical Director, Commercial Division, AverStar (formerly Intermetrics)
(http://www.averstar.com/services/IT_consulting.html)  Burlington, MA  USA