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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Discriminated types with default discriminants
Newsgroups: comp.lang.ada
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Date: Thu, 3 Nov 2005 18:28:18 +0100
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Date: 2005-11-03T18:28:06+01:00
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On Thu, 03 Nov 2005 16:50:12 +0100, Maciej Sobczak wrote:

> Consider this:
> 
> procedure Hello is
> 
>     type Discriminated(Size : Integer := 10) is
>        record
>           Value : String (1..Size);
>        end record;
> 
>     S : Discriminated;
> 
> begin
>     null;
> end Hello;
> 
> 
> Compiler (GNAT) gives me two warnings:
> 
>       5.          Value : String (1..Size);
>                                      |
>          >>> warning: creation of object of this type may raise 
> Storage_Error
> 
>       8.    S : Discriminated;
>             |
>          >>> warning: Storage_Error will be raised at run-time
> 
> 
> Moreover, it keeps a promise and indeed the program raises STORAGE_ERROR 
> at run-time.
> 
> What's happening here? Why the object S is not created with 10 as the 
> default discriminant?
> 
> I can get rid of the second warning by declaring S as:
> 
> S : Discriminated(10);
> 
> Indeed - program runs fine (i.e. it does not raise any exception). 
> What's the difference?
> 
> I can get rid of the first warning with this:
> 
>     subtype MyInt is Integer;
>     type Discriminated(Size : MyInt := 10) is
>     -- ...
> 
> What's the difference?

No difference, just another GNAT's "feature", GNAT will raise Storage_Error 
at run-time.

What will indeed make difference is this:

   subtype Expected_Sizes is Natural range 0..1000;
   type Discriminated (Size : Expected_Sizes := 10) is record
      Value : String (1..Size);
    end record;

But better is to remove the default value.

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de