[M3devel] REFANY-keyed tables?
Rodney M. Bates
rodney_bates at lcwb.coop
Thu Feb 27 04:28:28 CET 2014
On 02/26/2014 11:08 AM, JC Chu wrote:
> I need a REFANY-to-INTEGER table in my program but RefIntTbl.Default is giving me runtime errors. Apparently Table(Key, Value) uses Key.Equal() and Key.Hash(), but those procedures in Refany, which is use to instantiate RefIntTbl, both raise a fatal exception.
The garbage collector is a (partially) compacting collector, which means it can move
a heap object and adjust all the pointers to it, if it knows where they all are,
which holds for some objects. So the bit contents of a REFANY can change. This
means it is not possible to have a consistent Hash on the pointer value alone.
So Refany.Hash is deliberately coded to raise this exception, to prevent a far
more insidious bug.
> I’m a bit confused since a number of REFANY-keyed tables are shipped with CM3. Is this actually the expected behavior or am I doing anything wrong?
Hmm, can you point out where these are?
I have some pointer-keyed tables, but they use techniques like giving every
object an object sequence number field, explicitly initialized when an object
is allocated, and using that for the Hash function. But this won't work for
REFANY, because it can have no fields.
So the table can only work on a narrower set of allocated object types than REFANY.
If you need the table to hold addresses of a mixture of different types, you can
still do that with a hierarchy of OBJECT types, but they will have to have a common
supertype that contains whatever fields the Hash function needs for consistency.
It would be possible that pointers get passed around in REFANY-typed (statically)
variables, but it is carefully arranged that they all have an allocated type
that has fields a Hash function can work on. Hash would then have to do a
NARROW or TYPECASE to the type it expected.
> — JC Chu
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