Issue #6309 has been updated by mame (Yusuke Endoh).


Ah, I knew what you are proposing by seeing Javadoc:

http://docs.oracle.com/javase/1.4.2/docs/api/java/lang/ref/ReferenceQueue.html
http://docs.oracle.com/javase/6/docs/api/java/lang/ref/WeakReference.html#WeakReference(T, java.lang.ref.ReferenceQueue)

I don't know the (real-world) use case of the feature, though.


Anyway, I mean I'd like you to create a patch written *in C*.
If there is a patch that we can review and import "as is",
I will be happy to assign this ticket to some core committers,
such as ko1 and kosaki.

-- 
Yusuke Endoh <mame / tsg.ne.jp>
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Feature #6309: Add a reference queue for weak references
https://bugs.ruby-lang.org/issues/6309#change-26401

Author: headius (Charles Nutter)
Status: Feedback
Priority: Normal
Assignee: 
Category: 
Target version: 


Most interesting uses of WeakRef are much harder to do efficiently without a reference queue.

A reference queue, as implemented by the JVM, is basically a queue into which weak references are placed some time after the object they refer to has been collected. The queue can be polled cheaply to look for collected references.

A simple example of usage can be seen in the weakling gem, with an efficient implementation of an ID hash: https://github.com/headius/weakling/blob/master/lib/weakling/collections.rb

Notice the _cleanup method is called for every operation, to keep the hash clear of dead references. Failure to have a _cleanup method would mean the hash grows without bounds.

_cleanup cannot be implemented efficiently on MRI at present because there's no reference queue implementation. On MRI, _cleanup would have to perform a linear scan of all stored values periodically to search for dead references. For a heavily used hash with many live values, this becomes a very expensive operation.

It's probably possible to implement reference queues efficiently atop the new ObjectSpace::WeakMap internals, since it already keeps track of weak references and can run code when a weak reference no longer refers to a live object.


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http://bugs.ruby-lang.org/