Question about new allocator

Question about new allocator

[jes2358]

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.

Re: Question about new allocator

[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

Re: Question about new allocator

[Keith Allan Shillington]

The convention in Ada is to raise the exception STORAGE_ERROR.  In RM95 it
is alluded to in the note at 4.8(14), and explained more fully in 13.11
(Storage Management).  You will find, in reading 13.11 that the implementor
has some choice in the matter.  In order to accomplish your objective, you
may want to implement a Storage_Pool that allows you to check space
available.

jes2358@imap3.asu.edu wrote in article
<Pine.SOL.3.91.970210095755.3813A-100000@general3.asu.edu>...
> 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.
>