Re: Allocators and exceptions

["Dmitry A. Kazakov" <mailbox@dmitry-kazakov.de>]

On Sun, 09 Sep 2007 13:31:11 -0700, Maciej Sobczak wrote:

> Consider:
> 
> procedure P is
> 
>    type T (Init : Integer) is record
>       C : Positive := Positive (Init);
>    end record;
> 
>    type T_Access is access T;
> 
>    Ptr : T_Access;
> 
> begin
>    Ptr := new T (-5);
> exception
>    when Constraint_Error =>
>       -- is memory leaked or deallocated?
>       null;
> end;
> 
> I want to know whether the memory that was allocated for the new
> object was immediately deallocated due to the exception.
> By "immediately" I mean before the control even goes to the exception
> part.
> 
> I don't find this guarantee anywhere in the AARM and it scares me.

The question is not only about deallocation, it is also about finalization
of the components initialized before the initialization raised an
exception. Components are initialized in an arbitrary order.

I cannot tell for sure, but I guess that your example is a bounded error.
Depending on the compiler the effect should be either Constraint_Error and
everything rolled back including memory allocation, or else it is
Program_Error + some mess.

But you should really ask a language lawyer.

P.S. Exceptions in constructors is a bad idea. Consider this:

   type T (Init : Integer) is record
      B : access Positive := new Positive;
      C : Positive := Positive (Init);
   end record;

   Ptr := new T (-5);

Now you might really want the compiler not to deallocate Ptr, so that you
were able to free Ptr.B before. But where do you know that B was
initialized?

-- 
Regards,
Dmitry A. Kazakov
http://www.dmitry-kazakov.de