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From: Bryan <brobinson.eng@gmail.com>
Newsgroups: comp.lang.ada
Subject: Encapsulating Ada.Direct_IO
Date: Tue, 16 Nov 2010 20:44:49 -0800 (PST)
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Date: 2010-11-16T20:44:49-08:00
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I'm trying to port some code to Ada for dealing with Big5-encoded
files. I realize that I might be able to use Ada.Wide_Text_IO, but I'm
trying to learn Ada and understand the language better.  I'm still
working on wrapping my head around types and packages.  My original
code in C++ opens a file as binary and parses it byte by byte and
breaking it into Big5 characters depending on the byte codes.  I
thought I'd try to do something similar by encapsulating Ada.Direct_IO
into a package.  I'm not having much luck, however.

Spec file:
======================
with Ada.Direct_IO;
package Big5_Text_IO is
  type File_Type is limited private;
  procedure Close( File : in out File_Type );
private
  package Byte_IO is new Ada.Direct_IO(Character);
  type File_Type is new Byte_IO.File_Type;
end Big5_Text_IO;
======================

Body file
======================
package body Big5_Text_IO is
  procedure Close( File : in out File_Type ) is
  begin
  	Byte_IO.Close(File);
  end Close;
end Big5_Text_IO;
======================

Test driver:
======================
with Big5_Text_IO;
with Ada.Text_IO;
procedure Big5_Test is
  Input_File : Big5_Text_IO.File_Type;
begin
  Ada.Text_IO.Put_Line("OK?");
end Big5_Test;
======================

If I leave out the Close method and remove the body file, I can build
the test driver with no issues.  Otherwise, I get the following from
GNAT:

======================
gcc -c big5_text_io.adb
big5_text_io.adb:6:23: expected private type "Ada.Direct_Io.File_Type"
from instance at big5_text_io.ads:11
big5_text_io.adb:6:23: found private type "Big5_Text_IO.File_Type"
defined at big5_text_io.ads:12
gnatmake: "big5_text_io.adb" compilation error
======================

I would *greatly* appreciate any tips in how I can better design my
package so that it can encapsulate Ada.Direct_IO or some other method
of binary I/O.  I looked at the GNAT source and I'm hoping I won't
have to emulate what they have done...its a bit over my head at this
point.