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From: Dmitry A. Kazakov <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Counter-proposal for variable arrays
Date: Thu, 16 Oct 2003 09:38:03 +0200
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Date: 2003-10-16T09:38:03+02:00
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On Wed, 15 Oct 2003 19:52:22 -0500, "Randy Brukardt"
<randy@rrsoftware.com> wrote:

>"Hyman Rosen" <hyrosen@mail.com> wrote in message
>news:1065709998.992831@master.nyc.kbcfp.com...
>> Dmitry A. Kazakov wrote:
>> > It is already so. Consider:
>> > type Tricky (I, J : Positive) is record
>> >    X : String (1..I);
>> >    Y : String (1..J);
>> > end record;
>> > The offset to Y is variable and depends on the value of I.
>>
>> I Didn't Know You Could Do That In Ada(tm). :-)
>> It really works that way? And you can have procedures
>> which just take unconstrained Tricky parameters, and
>> they'll know how to deal with all instances? Cool!
>
>You can write this, but it often isn't implemented with variable offsets.
>What Janus/Ada does is to use discontinous storage for
>depends-on-discriminant components. The component itself is a descriptor,
>and has fixed size. The array data is allocated separately. That way, we can
>actually change the data memory when the discriminants are changed by
>assignment, and we don't use more memory than necessary.

An interesting approach. But what will happen if I write:

type Tricky_Ptr is access Tricky;
for Tricky_Ptr'Storage_Pool use My_Stack_Pool;

Do you switch between representations?

---
As I see, your compiler is ready for:

type Crazy (T : Tag) is record
   Anything : Root_Type'Class (T);
end record;

(:-))

---
Regards,
Dmitry Kazakov
www.dmitry-kazakov.de