Re: Storage Pools and alloca

[Simon Wright <simon@pushface.org>]

Robert A Duff <bobduff@shell01.TheWorld.com> writes:

> Storage_Size is supposed to return the amount of "reserved" space,
> which in virtual memory systems might be zero.  ('Storage_Size is
> kind of useless, actually.  I've never written a program that
> queried 'Storage_Size.  Has anybody?)

We have used Storage_Size clauses where

  we want to use new/unchecked deallocation rather than managing
  storage reclamation by hand

  we know the maximum number of instances

  we don't want to have lots of malloc/free churn because of rumours
  about its performance

  we don't want to have lots of malloc/free churn because of heap
  fragmentation issues (long-running embedded system, no memory
  management)

As an example, for a maximum on 4 instances,

   type Handle is access Instance;
   for Handle'Storage_Size use Instance'Object_Size / 8 * 4;

(I know see that perhaps I should have used the standard
'Max_Size_In_Storage_Elements rather than the GNAT 'Object_Size. Oh
well, easy to change that sort of thing with a code generator!)


As for querying it, well, I think you may be right ..