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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Access parameters and accessibility
Date: Wed, 17 Dec 2014 09:28:41 +0100
Organization: cbb software GmbH
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Date: 2014-12-17T09:28:41+01:00
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On Wed, 17 Dec 2014 07:02:56 +0000 (UTC), Natasha Kerensikova wrote:

> Now you got me really worried, because I thouhgt access discriminants
> were safe. It was under that assumption that I actually started to
> really like them.
> 
> More specifically, the pattern I caught myself using a lot looks like
> the following:
> 
>    type Preprocessed_Data (Input_Data : access Input_Data_Type) is record

not null access?

>       Computed_Value_1 : Type_1;
>       Computed_Value_2 : Type_2;
>    end record;
> 
>    procedure Private_Helper (Data : in Preprocessed_Data) is
>    begin
>       --  Do stuff qui Computed_Value_* and maybe also with Input_Data
>    end Private_Helper;
> 
>    procedure Process (Input_Data : in out Input_Data_Type) is
>       Data : Preprocessed_Data (Input_Data'Access);
>    begin
>       --  Initialize Data.Computed_Value_*
>       Private_Helper (Data);
>    end Process;
> 
> For course, it means that Input_Data is actually aliased, so it must be
> of a tagged type (or maybe a limited type works too?).
> 
> That way a strong connection is maintained between the original data and
> whatever is derived from it, and it gets to be handled in a single value
> rather than letting the number of arguments explode when there are
> several preprocessed types or steps. Though I started building such
> types to use generics that expected a single type/value.
> 
> Am I doing something danegerous here?
> Should I try to find another pattern to achieve the same effects?

The language should. Access disciminant is a hack. The pattern should have
been:

   procedure Process (Input_Data : in out Input_Data_Type) is
      Data : Preprocessed_Data (Input_Data);

with a proper constructor "virtualizing" Input_Data_Type "discriminant" of
Preprocessed_Data.

   type Preprocessed_Data (Input_Data : Input_Data_Type) is ...;
private
   type Preprocessed_Data is ... -- No discriminant actually
   overriding
      procedure Initialize (Input_Data : Input_Data_Type);

> From what I understood, here accessibility is statically known to be
> fine, isn't it? Or is there a trap hidden in there?

AFAIK, it is safe.

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