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Date: Sun, 27 May 2018 10:15:00 -0700 (PDT)
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Subject: Re: why "unconstrained subtype in component declaration" while said
 component given default value ?
From: Shark8 <onewingedshark@gmail.com>
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Xref: reader02.eternal-september.org comp.lang.ada:52733
Date: 2018-05-27T10:15:00-07:00
List-Id: <comp.lang.ada>

On Saturday, May 26, 2018 at 6:29:11 PM UTC-6, Mehdi Saada wrote:
> > Record types need to support Representation Clauses, where as normal 
> > variables do not.
> That's quite an argument. But still...
> >    type A (N: Positive) is record
> >       II : Integer;
> >       Chaine : String(1..N) := Give_String(N);
> >    end record;
> how can one define bit by bit the representation of A since N is not statically defined?

It's not hard; in fact it's the standard way to deal with something like SSL's message-format. I don't remember the details of the actual records, but here's the concept:

Message Fields: Type, Length, Encryption, Text.

With Encryption;
Package Message is
  Type Message_Type is ( Internal, External, Loop, Heartbeat )
    with Size => 2; -- Whatever.
  
  Type Message( <> ) is private;
  -- Operations.
Private
    Package Enc renames Encryption;
    Type Message( Length : Positive; Style : Message_Type ) is record
     Text       : String(1..Length);
     Encryption : Enc.Method; -- Encryption method; say 6 bits.
    End record;
    
   For Message use record
    Style      at 0 range 0..2;
    Encryption at 0 range 3..8;
    Length     at 0 range 9..40;
   End Record;
End Message;

Or something like that; like I said, that's the general idea, not the specifics.
To read up on record representation clauses read this:
http://archive.adaic.com/standards/83lrm/html/lrm-13-04.html