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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Operation can be dispatching in only one type
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Date: Fri, 13 Nov 2009 22:14:09 +0100
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Date: 2009-11-13T22:14:10+01:00
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On Fri, 13 Nov 2009 12:43:42 -0800 (PST), xorque wrote:

> The only real requirement is that:
> 
> Archivers use a common protocol: Code just sees operations
> on Archiver_t'Class and doesn't know if it's reading from a Zip, RAR,
> directory, etc.
> 
> Presumably, then, when I open a file using an operation from an
> archiver, I can't know about the implementation of the actual file
> as it's private to the archiver.
> 
> Hence, I've ended up with two private tagged types. Is there a better
> way (I've not ignored your other suggestions, I'm just exploring other
> possibilities first)?

MD is customary replaced my mix-in:

1. Standard mix-in:

  type File_Type is
     abstract new Ada.Finalization.Limited_Controlled with null record;
  procedure Open
            (  File : in out File_Type;
               Path : String              
            )  is abstract;
  procedure Close (File : in out File_Type) is abstract;
  procedure Read ...;
  procedure Write ...;

Files know nothing about archives. Archive has a file as a mixin:

  type Archiver_Type (Transport : not null access File_Type'Class) is
     new Ada.Finalization.Limited_Controlled with null record;
  procedure Open (Archiver : in out Archiver_Type; Path : String);

Open of Archive_Type calls to Open of Archive.Transport. Same with other
operations. They all are defined in terms of the operations of File_Type.
Class through Archive.Transport are dispatching, because it is class-wide.

2. Generic mix-in.

You can put Archive in the hierarchy of File_Type. The implementation of
Archive_Type can be generic taking a descendant of File_Type and then
extending it in the generic body.

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