Re: Unchecked_Deallocation subtleties

[Nick Roberts <nickroberts@blueyonder.co.uk>]

On Wed, 09 Apr 2003 23:39:31 +0200, Samuel Tardieu <sam@rfc1149.net> wrote:

>>>>>> "Nick" == Nick Roberts <nickroberts@blueyonder.co.uk> writes:
>
> Nick> Consider the following example skeleton piece of code:
>
> Nick>    declare
> Nick>       X: Some_Access_Type;
> Nick>    begin
> Nick>       ...         -- [1]
> Nick>       Y := X.all; -- [2] last use of X
> Nick>       ...         -- [3]
> Nick>       Free(X);    -- [4]
> Nick>    end;
>
> Nick> Although it may be that no compiler would (or even could) do so
> Nick> in practice, other rules in the ARM made it theoretically
> Nick> possible for the implementation to automatically reclaim the
> Nick> storage occupied by whatever X pointed to (assuming no other
> Nick> access value pointed to it) at any point in the elided code
> Nick> section [3].
>
> I would say that Free(X) is certainly a use of X, so the last use of X
> is not in [2], it is in [4].

Hmmmm. Obviously you are right, Sam, on the face of it. Theoretically 
(there's that word again), a compiler could apply a special rule to an 
instance of Unchecked_Deallocation (that, for each call, the object pased 
to it is not considered to have been used), on the basis that this would 
never do any harm and might provide opportunities for earlier automatic 
storage reclamation. In practice I doubt any compiler has ever actually 
done this.

Furthermore (on going back to the manuals), I note that Ada allows the 
Controlled pragma to be applied to an access type to instruct the compiler 
not to perform automatic storage reclamation, although neither the Ada 83 
nor the Ada 95 standards made it requisite to apply this pragma to an 
access type for which Unchecked_Deallocation is also used.

So, I'm trying to point out my guess as to the thought processes of the Ada 
83 (Green) design team, despite the fact that it all turned out to be a bit 
theoretical.

-- 
Nick Roberts
Jabber: debater@charente.de [ICQ: 159718630]