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From: Martin Krischik <krischik@users.sourceforge.net>
Newsgroups: comp.lang.ada
Subject: Re: Type safety on wikipedia
Date: Fri, 27 Jan 2006 19:54:33 +0100
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Date: 2006-01-27T19:54:33+01:00
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jimmaureenrogers@worldnet.att.net wrote:

> 
> Florian Weimer wrote:
>> Now, suppose that X is a pool-specific access value for some type T,
>> and Free is a corresponding instance of Ada.Unchecked_Deallocation.
>> Suppose that
>>
>>
>>    Free (X);
>>
>> has just been exected.  Suppose the next thing to be evaluated is
>>
>>    declare
>>       Y : T := X.all;
>>    begin
>>       ...
> 
> 
> Let's look at an actual program doing an equivalent action:

> with Ada.Text_Io;
> 
> procedure Access_Test is
>    type Int_Access is access Integer;
>    P : Int_Access;
> begin
>    P := null;
>    Ada.Text_Io.Put_Line(Integer'Image(P.All));
> end Access_Test;

Make that

with Ada.Text_Io;

procedure Access_Test is
    type Int_Access is access Integer;
    procedure Deallocate is new Unchecked_Deallocation ....

   P : Int_Access := new Integer;
   Q : Int_Access := P;
begin
    Deallocate (P);
    Ada.Text_Io.Put_Line(Integer'Image(Q.All));
 end Access_Test;

That is what the garbage collection chapter of the article is all about. 
For a type save language behavior of this code snippet must be well
defined. But in Ada it is not. 

Martin

-- 
mailto://krischik@users.sourceforge.net
Ada programming at: http://ada.krischik.com