Re: No call for Ada (was Re: Announcing new scripting/prototyping language)

["Robert I. Eachus" <rieachus@comcast.net>]

Martin Krischik wrote:

> Wich reminds me. Does anybody know what - should - happen when:
> 
> Access_X is access X;
> All_X is access all X;
> 
> for Access_X'Pool use Access_Pool;
> for All_X'Pool use All_Pool;
> 
> Some_X : Access_X := new X;
> Another_X : All_X := Some_X;
> 
> function Deallocate is new Unchecked_Deallocation (X, All_X);
> 
> begin
>     Deallocate (Another_X);
> end;

No.  The execution of the call to Deallocate is erroneous.  That means 
that anything can happen: RM 13.11.2(16) "The execution of a call to an 
instance of Unchecked_Deallocation is erroneous if the object was 
created other than by an allocator for an access type whose pool is 
Name'Storage_Pool."

The proper thing to do is to do the necessary check inside your 
Deallocate.  Unfortunately, there is no language defined attribute that 
can be used to determine the storage pool associated with an access 
object.  Of course, if you define a storage pool, you can also provide 
such an operation:

function Storage_Pool_Of(X: Address) return Root_Storage_Pool'Class;

But it would be nice to have 'Storage_Pool defined for both subtypes (as 
at present) and for all access objects.  That way you could do the 
necessary check to see whether access values denote an object in the 
default storage pool.  In fact, it might be better to have both 
'Storage_Pool, and 'Allocated, where Object'Allocated returns a boolean. 
  ('Storage_Pool should raise an exception if called with a parameter 
that does not belong to any storage pool.)  Or perhaps we could add a value:

No_Storage_Pool: constant Root_Storage_Pool;

to System.Storage_Pools.

However, in the meantime, the best policy is probably to only 
instantiate Unchecked_Deallocation for types which are not declared 
access all.

-- 

                                           Robert I. Eachus

"The terrorist enemy holds no territory, defends no population, is 
unconstrained by rules of warfare, and respects no law of morality. Such 
an enemy cannot be deterred, contained, appeased or negotiated with. It 
can only be destroyed--and that, ladies and gentlemen, is the business 
at hand."  -- Dick Cheney