Re: Use of Unchecked_Deallocation and pragma Controlled

[Simon Wright <simon@pushface.org>]

Mark Lorenzen <mark.lorenzen@ofir.dk> writes:

> When Unchecked_Deallocation is used to reclaim the storage claimed
> by an object, the pragma Controlled must be use to prevent automatic
> reclamation (ie. "garbage collection") of storage which would
> otherwise result in "double deallocation" of the object.
> 
> Example:
> 
> type Access_T is access T;
> Pragma Controlled (Access_T);
> 
> procedure Free is new Ada.Unchecked_Deallocation(T, Access_T);
> 
> begin
>   A : Access_T := new T;
>   B : Access_T := A;
> 
>   ...
> 
>   Free(T);    -- OK, reclaim the storage.
> end;          -- OK only if pragma Controlled has been applied.
>               -- Otherwise B could be reclaimed by garbage collection,
>               -- resulting in double deallocation of A.
> 
> Question: Do you guys actually use pragma Controlled even if no
> current Ada compiler generates code with garbage collection?

I don't believe there are any Ada compilers (other than those targeted
to a JVM) that do garbage collection.

But even if there were, it seems to me it would be a pretty crappy
implementation that didn't notice you doing the Free and remember
somehow not to collect the garbage. I rather suspect it wouldn't pass
ACATS either. I wouldn't buy it.

On the other hand, the AARM
<http://www.adaic.org/standards/95aarm/html/AA-13-11-3.html> goes into
a lot of detail about it, so perhaps I'm just confused .. thank
heavens the compiler I'm using doesn't do it! One less thing to worry
about.

-- 
Simon Wright                               100% Ada, no bugs.