Validity of renamed objects?

Validity of renamed objects?

[Jacob Sparre Andersen]

I've been reading section 8.5.1 in the LRM, and I can't figure out when
the compiler is supposed to check the validity of a renamed object.

An example:

procedure Renaming_1 is
   Object  : String (1 .. 9) := "dead beef";
   Index   : Natural := 0;
   Renamed : Character renames Object (Index);
begin
   Index := (Object'First + Object'Last) / 2;
   if Renamed /= ' ' then
      raise Program_Error
        with "Expected Renamed to match the changed value of Index.";
   end if;
end Renaming_1;

1) Should I get a Constraint_Error when Renamed is declared?

2) What will the value of Renamed be after Index has been changed?

Greetings,

Jacob

PS: I know what the version of GNAT I have at hand does, but what it
    does is not completely obvious from my reading of the LRM.
-- 
»But you have to be a bit wary of a ship that collects
 snowflakes.«                                  -- Diziet Sma

Re: Validity of renamed objects?

[J-P. Rosen]

Le 01/04/2014 15:04, Jacob Sparre Andersen a écrit :
> I've been reading section 8.5.1 in the LRM, and I can't figure out when
> the compiler is supposed to check the validity of a renamed object.
> 
> An example:
> 
> procedure Renaming_1 is
>    Object  : String (1 .. 9) := "dead beef";
>    Index   : Natural := 0;
>    Renamed : Character renames Object (Index);
> begin
>    Index := (Object'First + Object'Last) / 2;
>    if Renamed /= ' ' then
>       raise Program_Error
>         with "Expected Renamed to match the changed value of Index.";
>    end if;
> end Renaming_1;
> 
> 1) Should I get a Constraint_Error when Renamed is declared?
Yes
> 2) What will the value of Renamed be after Index has been changed?
> 

Assuming Index is 1 (in order to avoid Constraint_Error), Renamed is
unchanged.

The principle is that when you elaborate the renaming declaration, you
evaluate the renamed object, and from then on the renaming identifier
designates that object. It's not a macro!


-- 
J-P. Rosen
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Tel: +33 1 45 29 21 52, Fax: +33 1 45 29 25 00
http://www.adalog.fr

Re: Validity of renamed objects?

[Markus Schöpflin]

Am 01.04.2014 15:04, schrieb Jacob Sparre Andersen:

> I've been reading section 8.5.1 in the LRM, and I can't figure out when
> the compiler is supposed to check the validity of a renamed object.
>
> An example:
>
> procedure Renaming_1 is
>     Object  : String (1 .. 9) := "dead beef";
>     Index   : Natural := 0;
>     Renamed : Character renames Object (Index);
> begin
>     Index := (Object'First + Object'Last) / 2;
>     if Renamed /= ' ' then
>        raise Program_Error
>          with "Expected Renamed to match the changed value of Index.";
>     end if;
> end Renaming_1;
>
> 1) Should I get a Constraint_Error when Renamed is declared?

Yes, see 8.5(3).

"Object (Index)" is evaluated when the renaming declaration is elaborated.

Markus

Re: Validity of renamed objects?

[Jacob Sparre Andersen]

Markus Schöpflin <no.spam@spam.spam> writes:
> Am 01.04.2014 15:04, schrieb Jacob Sparre Andersen:

>> I've been reading section 8.5.1 in the LRM, and I can't figure out
>> when the compiler is supposed to check the validity of a renamed
>> object.

> Yes, see 8.5(3).
>
> "Object (Index)" is evaluated when the renaming declaration is
> elaborated.

Yes.  Reading that paragraph it is quite clear.  Thanks!  (Also to
Jean-Pierre.)

Greetings,

Jacob
-- 
"If you think Tuck has said something that is completely
 wrong, you are almost certainly missing something :-)"