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Date: 26 Sep 92 21:08:29 GMT
From: gvls1!aviary!dmarshal@louie.udel.edu  (Dave Marshall)
Subject: Re: converting (limited) private types?
Message-ID: <1750@aviary.Stars.Reston.Unisys.COM>
List-Id: <comp.lang.ada>

In article <1a0ijeINNfei@huon.itd.adelaide.edu.au>, andrewd@cs.adelaide.edu.au 
(Andrew Dunstan) writes:

[some stuff deleted]
> Trouble is, file types in Ada are limited private. So you can't assign
> to objects of the type. So you can't make a stack of them.
> 
> While pondering this, an evil thought came into my head. Before succumbing
> to my better feelings, I quickly dialled up the university and tried the
> following:
> 
>   with text_io; use text_io;
>   with unchecked_conversion;
>   procedure file_conv is
> 
Ack! A totally UNJUSTIFIED use of UNCHECKED_CONVERSION!  Flagellate yourself
thrice daily for a week.

[remainder of evil program deleted]
> 
> What I want to know is:
> 
> (1) APART from any righteous feelings about breaking the language's
> privacy mechanism, are there any good reasons for not doing something
> like this?

Yes.  Don't used UNCHECKED_CONVERSION if you don't have to.

> 
> (2) Is there a better way to do it, one which does not rely on this
> sort of hack?

Yes. Access types.  A simple example follows at the end of this response.

> 
> (3) How can I close the damned file? (I've thought of a really disgusting
> way, which oddly enough should be reasonably portable, but I certainly won't
> publish it on the net.)

Maybe that's for the best. :)  [No chops-busting intended.]

What follows is my trivial example of using access types to keep track of
limited private types.  In it, I keep an array of FILE_TYPE while I create
files, write to them, and close them.  Creating a stack package, or even
using some other generic stack package, ought to be equally trivial.

Here it is:


with TEXT_IO;

procedure FILE_TEST is

  type FILE_POINTER is access TEXT_IO.FILE_TYPE;

  type POINTERS is array ( INTEGER range <>) of FILE_POINTER;

  MY_ARRAY   : POINTERS(1..5);

  SOME_NAMES : constant array (INTEGER range 1..5) of STRING(1..8) :=
    ( "myfile.1",
      "myfile.2",
      "myfile.3",
      "myfile.4",
      "myfile.5");

begin

  for I in MY_ARRAY'RANGE loop

    MY_ARRAY(I) := new TEXT_IO.FILE_TYPE;

    TEXT_IO.PUT_LINE( "Creating file" & INTEGER'IMAGE(I));

    TEXT_IO.CREATE ( MY_ARRAY(I).all, NAME => SOME_NAMES(I));

  end loop;

  for J in MY_ARRAY'RANGE loop

    TEXT_IO.PUT_LINE ( "Writing to file" & INTEGER'IMAGE(J));

    TEXT_IO.PUT_LINE ( MY_ARRAY(J).all, "I am file" & INTEGER'IMAGE(J));

  end loop;

  for K in MY_ARRAY'RANGE loop

    TEXT_IO.PUT_LINE ( "Closing file" & INTEGER'IMAGE(K));

    TEXT_IO.CLOSE ( MY_ARRAY(K).all);

  end loop;

end FILE_TEST;

Apologies to purists everywhere who take offense at my sloppiness in the
declarative section.  Have a nice day.
-- 
                                           Dave Marshall
                                           dmarshal@stars.reston.unisys.com