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Why are weak pointers useful?

Asked 2009-09-28T03:32:41.593
15

I've been reading up on garbage collection looking for features to include in my programming language and I came across "weak pointers". From here:

Weak pointers are like pointers, except that references from weak pointers do not prevent garbage collection, and weak pointers must have their validity checked before they are used.

Weak pointers interact with the garbage collector because the memory to which they refer may in fact still be valid, but containing a different object than it did when the weak pointer was created. Thus, whenever a garbage collector recycles memory, it must check to see if there are any weak pointers referring to it, and mark them as invalid (this need not be implemented in such a naive way).

I've never heard of weak pointers before. I would like to support many features in my language, but in this case I cannot for the life of me think of a case where this would be useful. For what would one use weak pointer?

pointers garbage-collection weak-references
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3 Answers

5

If your language's garbage collector is incapable of collecting circular data structures, then you can use weak references to enable it to do so. Normally, if you have two objects which have references to each other, but no other outside object has a reference to those two, they would be candidates for garbage collection. But, a naïve garbage collector wouldn't collect them, since they contain references to each other.

To fix this, you make it so one object has a strong reference to the second, but the second has a weak reference to the first. Then, when the last outside reference to the first object goes away, the first object becomes a candidate for garbage collection, followed shortly thereafter by the second, since now its only reference is weak.

answered 2009-09-28T03:54:24.437
1

Use them when you wanted to keep a cached list of objects but not prevent those objects from getting garbage collected if the "real" owner of the object is done with it.

A web browser might have a history object that keeps references to image objects that the browser loaded elsewhere and saved in the history/disk cache. The web browser might expire one of those images (user cleared the cache, the cache timeout elapsed, etc) but the page would still have the reference/pointer. If the page used a weak reference/pointer the object would go away as expected and the memory would be garbage collected.

answered 2009-09-28T03:43:15.473
-1

Weak references can for example be used in caching scenarios - you can access data through weak references, but if you don't access the data for a long time or there is high memory pressure, the GC can free it.

answered 2009-09-28T03:42:29.857

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