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Subject: Re: strings and multidimensional arrays
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From: Mark Lorenzen <mark.lorenzen@ofir.dk>
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Date: Mon, 13 Oct 2003 23:47:52 +0200
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Date: 2003-10-13T23:47:52+02:00
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"Martin Dowie" <martin.dowie@btopenworld.com> writes:

> "Robert I. Eachus" <rieachus@comcast.net> wrote in message
> news:3F8ADB3D.5010000@comcast.net...
>>      type Unconstrained_Record (D : Disc := One) is record
>
> Isn't this the key line here? The default ("One") allows you to change it
> late (as often

Yes it is. If a record contains a variant_part and the discriminant
governing the variant_part has a default value, then it is allowed to
perform an assignment that changes the value of the discriminant and
therefore also the variant_part.

So the following is allowed:

procedure Option is
   type Optional_Integer (Defined : Boolean := False) is
      record
         case (Defined) is
            when True  => Value : Integer;
            when False => null;
         end case;
      end record;

   Integer_Value : Optional_Integer;

begin
   Integer_Value := Optional_Integer'(Defined => True, Value => 5);
   Integer_Value := Optional_Integer'(Defined => True, Value => 3);
   Integer_Value := Optional_Integer'(Defined => False);
end Option;

> as you like). If you didn't have this, then you would have to provide a
> value at
> declaration and it would be 'fixed' for the life of the object.

Also correct. The following is *not* legal:

procedure Option is
   type Optional_Integer (Defined : Boolean) is
      record
         case (Defined) is
            when True  => Value : Integer;
            when False => null;
         end case;
      end record;

   Integer_Value : Optional_Integer (Defined => True);

begin
   Integer_Value := Optional_Integer'(Defined => True, Value => 5);
   Integer_Value := Optional_Integer'(Defined => True, Value => 3);
   Integer_Value := Optional_Integer'(Defined => False);
end Option;

The last assignment will raise Constraint_Error.

I can't find the paragraph(s) in the ARM, but it is burried there
somewhere.

- Mark Lorenzen