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From: Jeffrey Carter <spam.jrcarter.not@spam.not.acm.org>
Newsgroups: comp.lang.ada
Subject: Re: Seeking for papers about tagged types vs access to subprograms
Date: Sun, 12 May 2013 11:56:50 -0700
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Date: 2013-05-12T11:56:50-07:00
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On 05/12/2013 01:10 AM, Niklas Holsti wrote:
>
> You mean like this:
>
>     function Open  (File : in Closed_File) return Open_File;
>     function Close (File : in Open_File  ) return Closed_File;
>
> I agree that this is a possible approach. This would work, but at the
> cost of writing the parameter/object name twice in each call:
>
>     File : File_Object;
>     ...
>     File := Open (File);
>     File := Close (File);

I'm not sure I see the point in having an object for a closed file, other than 
the requirements of low-level languages in which such things were 1st 
implemented. Why not something like

type File_Info (<>) is tagged limited private;

function Open   (Name : ...; ...) return File_Info;
function Create (Name : ...; ...) return File_Info;

function Read   (File : in out File_Info) return ...;
procedure Write (File : in out File_Info; Item : in ...);

declare
    File : File_Info := Open ("junk", ...);
begin
    Data := Read (File);
    ...
end;

A File_Info must be opened or created when declared, and is closed when it's 
finalized.

-- 
Jeff Carter
"C++ is like giving an AK-47 to a monk, shooting him
full of crack and letting him loose in a mall and
expecting him to balance your checking account
'when he has the time.'"
Drew Olbrich
52