A hash-table.
Sets the weakness state of hash-table. Value may be:
:value or t -- An entry is kept if there is a pointer to the value from another object.
:key -- An entry is kept if there is a pointer to the key from another object.
:both -- An entry is kept if there are pointers to both the key and the value.
:one or :either -- An entry is kept if there is a pointer to either the key or the value.
nil -- Make the hash-table non-weak. All entries are kept.
By default, hash-tables are not weak, which means that they keep alive all the keys and the values in the table.
A weak hash-table allows entries to be removed if there are no other pointers to them. The weakness-state tells the system which entries may be removed like this.
Entries that can be removed are removed after a garbage collector operation which identifies that they can be removed. This means that if the relevant object(s) (the key or the value) have been promoted to a higher generation, a garbage colletion (mark and sweep) of the higher generation is required to remove them from the table. Note that by default the system does not automatically call mark-and-sweep on generation 2 or higher.
The weakness-state of a hash-table can be changed repeatedly, at any time, at any point using any of the weak values listed above.