Warren Harris <warren@metaweb.com> wrote:
> I'd like to understand better how ocaml's weak pointers operate. 

You will be interested in the following important article:
http://portal.acm.org/citation.cfm?id=1411308
:)
 
>First, although it doesn't seem to be specified in the documentation,  
>I assume that weak pointers will *not* be reclaimed (e.g. from a weak  
>hash table) if the program retains some other reference to the object.  

Exactly.

>My second question relates specifically to my application. I would  
>like to have a primary cache of objects, and a secondary index into  
>sub-objects referenced from the primary cache. I.e. CacheA references  
>objects of type A; objects of type A reference objects of type B;  
>CacheB references objects of type B. I would like to guarantee that  
>weak references in CacheB are not flushed unless the corresponding  
>reference from CacheA is first flushed. I assume will be the case if a  
>non-weak reference from A to B is maintained.

The non-weak reference from A to B prevents B being unreachable
without A being unreachable, so yes, a reference from CacheB can
not disappear without the reference from CacheA disappearing
earlier or simultaneously.
This said, if what you want is really a cache, you would probably be
better off fixing its size and renewal strategy yourself than letting
the GC do it for you (by using weak pointers). What it will do has almost
no chance of being the best compromise between memory use and
speed for your application, and it may change without notice from
one version to the other. In short: don't use weak pointers to make caches.

>Can anyone verify?

If you want to experiment to confirm your impressions, the function
Gc.full_major is your friend.

Pascal