blocks|key|2074808|text|根据我的《编程Perl》第三版，在退出时，Perl+5会执行一次“代价高昂的标记和清除”来回收循环引用。您将希望尽可能地避免循环引用，否则直到程序退出时它们才会被回收。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|2074809|Perl5确实通过Scalar::Utils模块提供了弱引用。|offset|length|2074810|Perl6将转向可插拔的垃圾收集方案(好吧，underlying+VM+will+have+multiple+garbage+collection+options和这些选项的行为可能会对Perl产生影响)。也就是说，您可以在各种垃圾收集器之间进行选择，也可以实现自己的垃圾收集器。想要一个复制收集器吗？好的。想要一个颜色收集器吗？你说对了。标记/清除、压缩等？为什么不行？|2074811|entityMap|0|LINK|mutability|MUTABLE|url|http://search.cpan.org/~gbarr/Scalar-List-Utils-1.23/lib/Scalar/Util.pm#weaken|1|http://docs.parrot.org/parrot/2.2.0/html/docs/pdds/pdd09_gc.pod.html^0|0|9|D|0|0|M|1N|1|0^^$0|@$1|2|3|4|5|6|7|R|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|S|8|@]|9|@$D|T|E|U|1|V]]|A|$]]|$1|F|3|G|5|6|7|W|8|@]|9|@$D|X|E|Y|1|Z]]|A|$]]|$1|H|3|-4|5|6|7|10|8|@]|9|@]|A|$]]]|I|$J|$5|K|L|M|A|$N|O]]|P|$5|K|L|M|A|$N|Q]]]]

According to my copy of Programming Perl 3rd ed., on exit Perl 5 does an "expensive mark and sweep" to reclaim circular references. You'll want to avoid circular references as much as possible because otherwise they won't be reclaimed until the program exits.

Perl 5 does offer weak references through the <a href="http://search.cpan.org/~gbarr/Scalar-List-Utils-1.23/lib/Scalar/Util.pm#weaken" rel="nofollow noreferrer">Scalar::Utils</a> module.

Perl 6 will move to a pluggable garbage collected scheme (well, the <a href="http://docs.parrot.org/parrot/2.2.0/html/docs/pdds/pdd09_gc.pod.html" rel="nofollow noreferrer">underlying VM will have multiple garbage collection options</a> and the behavior of those options can have an effect on Perl). That is, you'll be able to choose between various garbage collectors, or implement your own. Want a copying collector? Sure. Want a coloring collector? You got it. Mark/sweep, compacting, etc? Why not?

blocks|key|554787|text|看看Proxy+Objects吧。|type|unstyled|depth|inlineStyleRanges|entityRanges|offset|length|data|554788|entityMap|0|LINK|mutability|MUTABLE|url|http://www.perl.com/pub/a/2002/08/07/proxyobject.html?^0|2|D|0|0^^$0|@$1|2|3|4|5|6|7|L|8|@]|9|@$A|M|B|N|1|O]]|C|$]]|$1|D|3|-4|5|6|7|P|8|@]|9|@]|C|$]]]|E|$F|$5|G|H|I|C|$J|K]]]]

Have a look at <a href="http://www.perl.com/pub/a/2002/08/07/proxyobject.html?" rel="nofollow noreferrer">Proxy Objects</a>.

blocks|key|554831|text|为了回收循环引用，Perl在某些情况下(我认为是在线程死的时候)应用了标记和清除替代GC。请注意，“每个值都是一个字符串”Perl节使得创建真正的循环引用变得困难；这是可行的，但“普通”Perl代码不可行，这就是引用计数在Perl中工作得很好的原因。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|554832|entityMap^0|0^^$0|@$1|2|3|4|5|6|7|D|8|@]|9|@]|A|$]]|$1|B|3|-4|5|6|7|E|8|@]|9|@]|A|$]]]|C|$]]

Perl applies a mark-and-sweep alternate GC in some occasions (when a thread dies, I think) in order to reclaim circular references. Note that the "every value is a string" Perl stanza makes it difficult to create true circular references; this is feasible, but "normal" Perl code does not, which is why reference counting works well with Perl.

blocks|key|3482305|text|简单的回答是，Perl5不会，而不是自动处理循环引用。除非您在代码中采取显式措施，否则任何包含循环引用的数据结构都不会被回收，直到创建它们的线程死亡。这被认为是一种可接受的折衷方案，因为它避免了运行时垃圾回收的需要，因为运行时垃圾回收会降低执行速度。|type|unstyled|depth|inlineStyleRanges|offset|length|style|BOLD|entityRanges|data|3482306|如果您的代码创建了具有循环引用的数据结构(即，树的节点包含回根节点的引用)，则需要使用Scalar::Util模块来“削弱”指向根节点的引用。这些弱引用不会添加到它们所指向的引用计数中，因此当最后一个外部引用消失时，整个数据结构将被自动释放。|3482307|示例：|3482308|use+Scalar::Util+qw(weaken);

...

++++my+$new_node+=+{+content+=>+$content,+root+=>+$root_node+};
++++weaken+$new_node->{root};
++++push+@{$root_node->{children}},+$new_node;|code-block|syntax|javascript|3482309|如果您在向数据结构添加新节点时使用这样的代码，那么实际计算的对根的引用只有那些来自结构外部的引用。这正是你想要的。然后，一旦对根的最后一个外部引用消失，根以及它的所有子代都将被回收。|3482310|entityMap^0|E|4|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|S|8|@$9|T|A|U|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|V|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|W|8|@]|D|@]|E|$]]|$1|J|3|K|5|L|7|X|8|@]|D|@]|E|$M|N]]|$1|O|3|P|5|6|7|Y|8|@]|D|@]|E|$]]|$1|Q|3|-4|5|6|7|Z|8|@]|D|@]|E|$]]]|R|$]]

The quick answer is that Perl 5 does not handle circular references automatically. Unless you take explicit measures in your code, any of your data structures which include circular references will not be reclaimed until the thread that created them dies. This is considered to be an acceptable tradeoff in that it avoids the need for runtime garbage collection that would slow down execution.

If your code creates data structures with circular references (i.e. a tree whose nodes contain references back to the root), you will want to use the Scalar::Util module to "weaken" the references that point back toward the root node. These weak references will not add to the reference count of whatever they point to, so the entire data structure will be automatically deallocated when the last external reference vanishes.

Example:

<pre><code>use Scalar::Util qw(weaken);

...

 my $new_node = { content =&gt; $content, root =&gt; $root_node };
 weaken $new_node-&gt;{root};
 push @{$root_node-&gt;{children}}, $new_node;
</code></pre>

If you use code like this whenever you add new nodes to your data structure, then the only references to the root that are actually counted are those from outside of the structure. This is exactly what you want. Then the root, and recursively all of its children, will be reclaimed as soon as the last external reference to it vanishes.

Unlike Java, Perl uses reference count for garbage collection. I have tried searching some previous questions which speak about C++ RAII and smart pointers and Java GC but have not understood how Perl deals with the circular referencing problem. 

Can anyone explain how Perl's garbage collector deals with circular references? Is there any way to reclaim circular referenced memory which are no longer used by the program or does Perl just ignores this problem altogether?

Garbage collection in Perl

翻译质量差，导致语言生硬或混乱。

没有提供实际的解决方法或示例。

解答不清晰，无法理解或解决问题。

页面排版不美观，阅读体验差。

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与Java不同，Perl使用引用计数进行垃圾收集。我曾经尝试搜索过一些关于Perl、智能指针和C++ GC的问题，但是没有理解Perl语言是如何处理循环引用问题的。有人能解释一下Perl的垃圾收集器是如何处理循环引用的吗？有没有办法回收程序不再使用的循环引用内存，或者Perl完全忽略了这个问题？

问Perl中的垃圾回收
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