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From: Jonas Nygren <ehsjony@ehs.ericsson.se>
Subject: Re: Free'ing extended types
Date: 1996/04/29
Message-ID: <3184803D.1208@ehs.ericsson.se>#1/1
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references: <3183AC75.335C@ehs.ericsson.se> <dewar.830716817@schonberg>
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organization: Ericsson Hewlett-Packard Telecommunications AB
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newsgroups: comp.lang.ada
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Date: 1996-04-29T00:00:00+00:00
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Robert Dewar wrote:
> 
> Jonas said
> 
> "   So there's nothing that could lead  to an erroneous execution
> (note
> the X.all'access, the libc then does what's needed)."

I think the above quote is from Laurent Guerby.

> 
> You seem to have the wrong idea of what erroneous is about. An erroneous
> execution is one whose semantics is not specified by the reference
> manual. You seem to think this means that it wlil blow up or do
> something wrong. Not at all! It *may* blow up but it does not have to!

Murphy's law: If it can blow up - it will blow up.

I personally take a very simplistic approach to never use a construct
that I know can blow up.

> 
> You can look at the execution of a particular impementation and conclude
> that a particular erroneous execution will have no ill effect on that
> implementation, but that does not mean that the exeution is not erroneous!

Well, I am aiming for portable code and free'ing memory can have great
effect on the program structure. I don't want to have to rewrite my code
between different language implementations. 

> 
> This is one of these places where no amount of testing can help, only
> a detailed knowledge of the formal semantics of the refrence manual
> can tell you if a given execution is indeed erroneous -- it is not something
> you can (necessarily) observe.

That is what my original question was all about. A pity you did not have
an anwer to that - I guess you have the "detailed knowledge of the formal
semantics" of Ada to respond to my question.

I repeat my original question here:

with Ada.Unchecked_Deallocation;
procedure St is
   type a is tagged record X : Integer; end record;
   type ap is access all a'class;
   procedure free is new Ada.unchecked_deallocation(A'class, ap);
   type b is new a with record
     Y : Integer; end record; -- a is extended

   p : ap := new b;
begin
   free(p); -- erroneous ??
end St;

Could the call on free result in erroneous execution (in any conceivable
law abiding implementation of Ada).

/jonas