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From: "Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>
Subject: Re: To collection (GNAT bug)
Newsgroups: comp.lang.ada
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Date: Sun, 1 Oct 2006 10:00:43 +0200
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On Sun, 01 Oct 2006 01:21:22 GMT, Jeffrey R. Carter wrote:

> Dmitry A. Kazakov wrote:
>> The following does not work in GCC 4.1.1 (20060525), as well as in 3.15p
> 
> Why should it? There's no guarantee that unchecked conversion between 
> access values and System.Address will give meaningful results.

See ARM 13.9 (17), which requires reversibility of Unchecked_Conversion.
That is clearly violated in the example given. Though it is legal not to
provide address to access type conversion through Unchecked_Conversion, it
is still illegal to provide it wrong.

> Use System.Address_To_Access_Conversions.

Address_To_Access_Conversions serves a different purpose. It declares a new
access type, moreover it is a general access type.

BTW, when Address_To_Access_Conversions is composed with an access to
access type conversion, the problem persists (it is a bug after all (:-))
Replace To_Ptr and To_Addr with:

   package Clutter is new Address_To_Access_Conversions (String);
   function To_Addr (X : Ptr) return Address is
   begin
      return Clutter.To_Address (X.all'Unchecked_Access);
   end To_Addr;
   function To_Ptr (X : Address) return Ptr is
      function Cast is
         new Ada.Unchecked_Conversion (Clutter.Object_Pointer, Ptr);
   begin
      return Cast (Clutter.To_Pointer (X));
   end To_Ptr;

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