Re: Variant record assignment fails discriminant check

[Adam Beneschan <adam@irvine.com>]

On Sep 2, 7:25 am, vldmr <vldm...@gmail.com> wrote:
> For the folllowing variant record:
> ------------------------------------------
> type t_X520CommonName_Choice_Id is (
>         X520CommonName_teletexString,
>         X520CommonName_printableString
> -- more choices
>     );
> type t_X520CommonName (choice_Id : t_X520CommonName_Choice_Id :=
> X520CommonName_teletexString) is record -- CHOICE
>     case choice_Id is
>         when X520CommonName_teletexString =>
>             teletexString : t_TeletexString_Acc; -- TeletexString
> (SIZE (1..ub-common-name))
>         when X520CommonName_printableString =>
>             printableString : t_PrintableString_Acc; --
> PrintableString (SIZE (1..ub-common-name))
> -- ... more choices
>      end case;
> end record;
> ----------------------------------
> the following code:
> ---------------------------------
>     procedure BDec_X520CommonNameContent(tagId0 : t_Asn_Tag;v: out
> t_X520CommonName;) is
>     begin
>         case tagId0 is
>             when
>                 16#14000000# | -- UNIV PRIM TELETEXSTRING_TAG_CODE
>                 16#34000000#   -- UNIV CONS TELETEXSTRING_TAG_CODE
>                 =>
>                 declare
>                     teletexString : t_TeletexString_Acc;
>                 begin
>                     teletexString := Adr_2_t_TeletexString_Acc
> ( Asn1_Alloc ( t_TeletexString'Size / 8));
>                     Check_Asn1_Alloc (teletexString.all'Address,
> exceptProc);
>                     BDec_TeletexStringContent (tagId0, elmtLen0,
> teletexString.all, totalElmtsLen1, exceptProc);
>                     v := (choice_Id => X520CommonName_teletexString,
> teletexString => teletexString); -- CONSTRAINT_ERROR
>                 end;
>         -- ... more choices
>        end case;
>     end BDec_X520CommonNameContent;
> --------------------------------------
> when executed produces 'discriminant check failed' error on assignment
> to v.
>
> Why is that? As far as I understand the variable v shall be mutable.

Only if the actual parameter is mutable when you call the procedure.

Say you declare

   XYZ : t_X520CommonName (X520CommonName_printableString);

...
   BDec_X520CommonNameContent (Something, XYZ);

XYZ is a constrained record.  You wouldn't want
BDec_X520CommonNameContent violating the constranit of XYZ by changing
the discriminant, would you?

On the other hand, if you declare

   ZYX : t_X520CommonName;
...
   BDec_X520CommonNameContent (Something, ZYX);

ZYX is unconstrained, so it's OK for BDec_X520CommonNameContent to
change the discriminant.  This means that the compiler has to generate
a little extra code to pass something to the procedure to tell it
whether it's OK to change the discriminant.

OK, so I'll bet your next question is: So how do I declare an object
to be unconstrained (mutable), but give its discriminant an initial
value other than the default (X520CommonName_teletexString)?

   WXY : t_X520CommonName := (choice_Id =>
X520CommonName_printableString,
                              others => <>);

                                          -- Adam