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From: xorque <xorquewasp@googlemail.com>
Newsgroups: comp.lang.ada
Subject: Operation can be dispatching in only one type
Date: Fri, 13 Nov 2009 12:12:18 -0800 (PST)
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Date: 2009-11-13T12:12:18-08:00
List-Id: <comp.lang.ada>

Hello.

I'm trying to write an archive/directory abstraction in a similar vein
to PhysicsFS
but am at a bit of a loss as to how to design the archiver interface:

with Path;

package Archiver is

  type Archiver_t              is abstract tagged limited private;
  type Archiver_Class_Access_t is access Archiver_t'Class;

  procedure Init
    (Archiver : out Archiver_t) is abstract;

  function Can_Mount
    (Archiver : in Archiver_t;
     Path     : in Path.Real_t) return Boolean is abstract;

  type File_t              is abstract tagged limited private;
  type File_Class_Access_t is access File_t'Class;

  procedure Open
    (Archiver : in     Archiver_t;
     Path     : in     Path.Virtual_t;
     File     :    out File_t) is abstract;

  procedure Close
    (File : in out File_t) is abstract;

private

  type Archiver_t is abstract tagged limited null record;

  type File_t is abstract tagged limited null record;

end Archiver;

The idea of the above is that the main part of the library only deals
with
archivers and "files" (which might only really be pointers to entries
in Zip
files, for example) by 'Class.

The problem with the above is that:

archiver.ads:18:13: operation can be dispatching in only one type

Hopefully someone here knows a better way to handle this.