Weak reference: Difference between revisions

Revision as of 11:21, 19 December 2019

Revision as of 19:00, 11 May 2020

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If a weak reference is created, and then elsewhere in the code <code>get()</code> is used to get the actual object, the weak reference isn't strong enough to prevent garbage collection, so it may be (if there are no strong references to the object) that <code>get()</code> suddenly starts returning null.<ref>https://web.archive.org/web/20110303225354/http://weblogs.java.net/blog/2006/05/04/understanding-weak-references Java Examples</ref>

<sourcesyntaxhighlight lang=java>

import java.lang.ref.WeakReference;

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}

}

</syntaxhighlight>

</source>

Another use of weak references is in writing a [[cache (computing)|cache]]. Using, for example, a weak [[hash map]], one can store in the cache the various referred objects via a weak reference. When the garbage collector runs &mdash; when for example the application's memory usage gets sufficiently high &mdash; those cached objects which are no longer directly referenced by other objects are removed from the cache.

===Smalltalk===

<sourcesyntaxhighlight lang=Smalltalk>

|a s1 s2|

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ObjectMemory collectGarbage.

a printOn: Transcript. "second element gone"

</syntaxhighlight>

</source>

===Lua===

<sourcesyntaxhighlight lang="lua">

weak_table = setmetatable({}, {__mode="v"})

weak_table.item = {}

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collectgarbage()

print(weak_table.item)

</syntaxhighlight>

</source>

===Objective-C 2.0===

In [[Objective-C]] 2.0, not only garbage collection, but also [[Reference counting|automatic reference counting]] will be affected by weak references. All variables and properties in the following example are weak.

<sourcesyntaxhighlight lang=objc>

@interface WeakRef : NSObject

{

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@end

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The difference between <code>weak</code> (<code>__weak</code>) and <code>unsafe_unretained</code> (<code>__unsafe_unretained</code>) is that when the object the variable pointed to is being deallocated, whether the value of the variable is going to be changed or not. <code>weak</code> ones will be updated to [[Null pointer|<code>nil</code>]] and the <code>unsafe_unretained</code> one will be left unchanged, as a [[dangling pointer]]. The <code>weak</code> references is added to Objective-C since [[Mac OS X Lion|Mac&nbsp;OS&nbsp;X&nbsp;10.7&nbsp;"Lion"]] and [[iOS 5]], together with [[Xcode]] 4.1 (4.2 for iOS), and only when using ARC. Older versions of Mac OS X, iOS, and GNUstep support only <code>unsafe_unretained</code> references as weak ones.

===[[Vala (programming language)|Vala]]===

<sourcesyntaxhighlight lang="vala">

class Node {

public weak Node prev; // a weak reference is used to avoid circular references between nodes of a doubly-linked list

public Node next;

}

</syntaxhighlight>

</source>

===Python===

<sourcesyntaxhighlight lang="pycon">

>>> import weakref

>>> import gc

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File "<stdin>", line 1, in <module>

AttributeError: 'NoneType' object has no attribute 'spam'

</syntaxhighlight>

</source>

== See also ==