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From: Maciej Sobczak <no.spam@no.spam.com>
Newsgroups: comp.lang.ada
Subject: Discriminated types with default discriminants
Date: Thu, 03 Nov 2005 16:50:12 +0100
Organization: CERN - European Laboratory for Particle Physics
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Date: 2005-11-03T16:50:12+01:00
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Hi,

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?


-- 
Maciej Sobczak : http://www.msobczak.com/
Programming    : http://www.msobczak.com/prog/