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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: Controlled types and exception safety
Newsgroups: comp.lang.ada
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Date: Tue, 6 Dec 2005 10:50:22 +0100
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Date: 2005-12-06T10:50:26+01:00
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On Tue, 06 Dec 2005 10:00:39 +0100, Maciej Sobczak wrote:

> This brings me to the next problem. Let's say that I provide a separate 
> procedure Duplicate or Copy or Assign or whatever with the 
> commit-or-rollback guarantees for some type (like Stack). Now, some of 
> the types in my program will have ":=" for assignment, and some others 
> will have the Copy procedure, but not ":=".
> I want to create a generic container or some other component that will 
> copy things around internally. It has to use ":=" for some types (like 
> Integer) and Copy for others (like Stack).
> In C++ I solve this problem (aside the fact that there is no problem in 
> the first place) with template type traits or some other application of 
> template specializations.
> What about Ada?

generic
   type Object is limited private;
   with procedure Deep_Copy (Left : in out Object; Right : Object) is <>;
package Container is
   ...
end Container;
-------------------------------
with Container;
generic
   type Object is private;
package Specialized_Container is
   procedure Deep_Copy (Left : in out Object; Right : Object);
   pragma Inline (Deep_Copy);
   package Copying_By_Assignment is new Container (Object);   
end Specialized_Container;
-------------------------------
package body Specialized_Container is
   procedure Deep_Copy (Left : in out Object; Right Object) is
   begin
      Left := Right;
   end Deep_Copy;
end Specialized_Container;

Note also that your example is not much realistic. Transaction model is
expensive. One usually does not compose transactions. This means that
components of a container will be copied destructively, *after* necessary
memory allocation. Only the upper level will take care of a possibility to
roll things back. Thus it makes much sense to distinguish light-weight ":="
(which can't fail) and heavy-weight "Copy".

The container itself could be a red-black tree, which supports roll-backs
after mutations.

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