[Caml-list] Re: Constants immediatelly disappear from the Weak array. (PR#1925)

[Damien Doligez <damien.doligez@inria.fr>]

On Wednesday, November 12, 2003, at 10:13 AM, Aleksey Nogin wrote:

> When trying to switch from 3.06 to 3.07+2 I've noticed the following 
> difference in how the Weak module works:

[
   In 3.06, an empty list [] does not disappear from a weak array;
   in 3.07+2 it disappears immediately
]

> Basically, in 3.07+2, the empty list disappears from the weak array as 
> soon as it is added into it. Is this a bug or a feature?

Feature.


> The old behavior is useful because it allows to assume that while a 
> value is referenced somewhere, Weak.get will always return Some. This 
> allowed using Weak.get as an indicator of whether it was OK to discard 
> some "helper" data (which should be only discarded after the primary 
> data is no longer in use).

Yes, but.  In 3.06, [] will never be removed from the weak array, and
you will keep your helper data forever.

The source your the problem is that [] is not allocated in the heap.
It is represented by an integer value.  Hence, there is no sharing
between various occurrences of [] in your data and the notion of a
weak pointer to [] does not make sense.

If you really want to make sure that you are manipulating pointers,
you should use a record or a list ref.

-- Damien

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