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From: "Martin Dowie" <martin.dowie@nospam.baesystems.com>
Newsgroups: comp.lang.ada
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Subject: Re: Restrictions on compilers for Microsoft's .NET project
Date: Mon, 11 Feb 2002 19:20:50 -0000
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> Certainly I am not expecting to be able to change variants on the fly,
> but I have a lot of code that receives an access value denoting a
> record and I have to check the value of a discriminant to determine
> which fields of the record are available to me.  If I get it wrong
> Compaq Ada will detect my error just as surely as it will detect an
> out-of-range value of the discriminant.  Those possibly-present
> fields from various variants are located at the same offsets as
> each other.
>
> The Microsoft publication I quoted described this as "unsafe".  Is
> this prohibited by .NET ?  Is it allowed in Ada95 as in Ada83 ?

Well, I can see how this would be "unsafe" in C-style languages but
with all checks in place Ada programs will prevent you from assigning
to variant fields if the discriminant doesn't allow it.

So in:

   type A_Variant (Value : Foo := Foo'First) is record
      case Value is
         when Bar1 =>
            Int : Integer;
         when Bar2 =>
            Bool : Boolean;
      end case;
   end record;

if you wanted to have a data item with Value = Bar2 but an integer
value assigned to the memory area that Bool occupies you can, but
only by using a big ugly thing like an unchecked conversion.

otherwise, if you want to have 'Bool' assigned a value, you have
to change the discriminant to Bar2 at the same time.