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From: "Grein, Christoph" <christoph.grein@eurocopter.com>
Newsgroups: comp.lang.ada
Subject: Re: Discriminated record question
Date: Tue, 7 May 2002 14:45:34 +0200 (MET DST)
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Date: 2002-05-07T14:45:34+02:00


> From: Mark.Doherty@uk.thalesgroup.com (Mark Doherty)
> 
> Why does the following raise a constraint error on the declaration of
> 'B'.
> 
> procedure Test is
>     type A_Type (Text_Size : Natural := 0) is
>        record
>             Text : String (1 .. 0);
>         end record;
>     A : A_Type;

Text component is always empty here, whichever Text_Size you use, so all objects 
have the same (small) size.

> 
>     type B_Type (Text_Size : Natural := 0) is
>         record
>             Text : String (1 .. Text_Size);
>         end record;
> 
>     B : B_Type;  

I presume, Storage_Error is raised, not Constraint_Error as you claim.
Here Text component depends on Text_Size. You define an unconstrained object, so 
the discriminant can vary, therefore the compiler has to allocate the maximum 
size, which is Natural'Last - quite big I'd say.
(A different model could be implemented by implicitly allocating on the heap as 
much storage as is currently needed. But this model is rarely used - is it 
currently used at all by some compiler?)

So the difference is not so subtle...