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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Newsgroups: comp.lang.ada
Subject: Re: Your wish list for Ada 202X
Date: Wed, 23 Apr 2014 09:45:27 +0200
Organization: cbb software GmbH
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Xref: news.eternal-september.org comp.lang.ada:19513
Date: 2014-04-23T09:45:27+02:00
List-Id: <comp.lang.ada>

On Tue, 22 Apr 2014 18:23:19 -0500, Randy Brukardt wrote:

> "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
> news:10o6qo98dwzjs$.rdpzvq7w1lep.dlg@40tude.net...
> ...
>> Which is exactly the problem - this introduces erroneous states, where
>> there should be none. From the SW design POV there shall be no way to
>> create a closed file, an uninitialized widget, a float with the FPU turned
>> off etc.
> 
> I suspect that in the case of files, that would be too limiting to be of 
> much use. One could not have a container (or array) of files, for instance, 
> because there would be no way to provide the file names to each element 
> before it existed.

Firstly, this is another language problem, e.g. lack of user-defined
aggregates and user-defined array constructors.

Secondly, there is a pattern for this. I am using a stock object to
initialize such things. Another example where this is needed is a handle
wrapping a non null pointer:

   type Safe_Handle (<>) is new Ada.Finalization.Controlled with private;
   ...
private
   type Safe_Handle is new Ada.Finalization.Controlled with
       Object : non null access Object_Type'Class := Stock_Object'Access;
   end record;

In the case of files there already is such stick object: /dev/null.

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