Re: Question about new allocator

[brennanw@wanda.vf.pond.com (Bill Brennan)]

In article <Pine.SOL.3.91.970210095755.3813A-100000@general3.asu.edu>,
 <jes2358@imap3.asu.edu> wrote:
>I am wondering if the new allocator will return NULL if the heap is full 
>as it does in C++ or if Ada has some other convention.  I am new to Ada 
>and can't seem to find this discussed anywhere.  The context I am 
>thinking about this is in trying to write a Boolean type IsFull function 
>for a Queue package.  If you can help with a suggestion, please email to
>jsherman@asu.edu.  Thanks in advance.

The method in which Ada communicates that the heap is exahausted is
to raise the Storage_Error exception.  Your IsFull function would look
something like this:

function IsFull( Queue: Queue_Type ) return Boolean is

   Heap_Exhausted: Boolean := True;

   Dummy_Ptr: Item_Ptr_Type;

   procedure Delete is new Unchecked_Deallocation( Item_Type, Item_Ptr_Type );

begin
   begin
      Dummy_Ptr := new Item_Type;
      Delete( Dummy_Ptr );  -- restore the Item to the heap.
   exception 
   when Storage_Error =>
      Heap_Exhausted := True; 
   end;
   return Heap_Exhausted;
end;


(this hasn't been compiled, so beware of small errors!)

In general, I find that a function like "IsFull" is not very useful for
allocating off of the heap.  For one thing, it's quite inefficient to 
allocate-deallocate-reallocate for every addition to the Queue (assuming
you're calling IsFull before each addition).  It's more efficient and
idiomatic to include an exception handler for the dreaded Storage_Error
in the same code which would call IsFull.

--bill