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From: "Randy Brukardt" <randy@rrsoftware.com>
Newsgroups: comp.lang.ada
Subject: Re: Gem 39 - compiler specific?
Date: Thu, 3 Sep 2009 17:06:48 -0500
Organization: Jacob Sparre Andersen
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Date: 2009-09-03T17:06:48-05:00
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"Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de> wrote in message 
news:6o3frhrv0n0p$.8wj0gszs5h07$.dlg@40tude.net...
...
> Why do we have Access_To_Address_Conversions? Why not to require
> Unchecked_Conversion to do all the checks Access_To_Address_Conversions
> does in the corresponding cases? I mean, there is no obvious reason why 
> the
> implementation of Access_To_Address_Conversions should be safer than one 
> of
> Unchecked_Conversion. The information the compiler has at the 
> instantiation
> point is just same.

Because the intent is that Unchecked_Conversion never actually does anything 
other than change the type. In particular, it should not change the 
representation. But it is not uncommon for System.Address and an access type 
to have a different representation. For instance, the Janus/Ada 16-bit 
MS-DOS compilers uses a 32-bit segmented value for System.Address, but 
access types were just 16-bit pointers into the heap. (Our early 32-bit 386 
compilers used 48-bit addresses; that was quite useful on early DOS 
extenders, for instance.)

The intent leads to the rule that Unchecked_Conversion is only well-defined 
when the sizes of the two types are the same. As noted, that doesn't need to 
be the case for System.Address.

You seem to want to muddle up the semantics to include some real 
conversions. That would be a problem inside of generics (what if the actual 
parameter is System.Address?? Do you get the special semantics or not? 
Either answer is uncomfortable and requiring the special semantics would 
have bad consequences on code generation and legality rules), and in any 
case is not very consistent.

                                 Randy.