哪些Ruby类支持.clone？ [英] Which Ruby classes support .clone?

问题描述

Ruby在对象中定义了 #clone 。
让我惊讶的是，有些类在调用它时会引发异常。
我发现 NilClass ， TrueClass ， FalseClass ， Fixnum 具有这种行为。

Ruby defines #clone in Object. To my suprise, some classes raise Exceptions when calling it. I found NilClass, TrueClass, FalseClass, Fixnum having this behaviour.

1）是否存在完整的类列表（至少是核心类），不允许使用 #clone ？
还是有办法检测特定的类是否支持 #clone ？

1) Does a complete list of classes (at least core-classes) exist, which do not allow #clone ? Or is there a way to detect if a specific class supports #clone ?

2）什么 42.clone 是错误的吗？

2) What is wrong with 42.clone ?

推荐答案

我不认为有一个正式的清单，至少除非您希望阅读其中的内容。 2）无效的原因是由于对Fixnums进行了优化。它们在内部存储/传递为其实际值（即true，false和nil），而不是作为指针。天真的解决方案是让 42.clone 返回相同的 42 ，但是不变的 obj.clone.object_id！= obj.object_id 将不再成立， 42.clone 实际上不会被克隆。

I don't think there is a formal list, at least unless you count reading the source. The reason 2) doesn't work is because of an optimization applied to Fixnums. They are stored/passed internally as their actual values (so are true, false and nil), and not as pointers. The naive solution is to just have 42.clone return the same 42, but then the invariant obj.clone.object_id != obj.object_id would no longer hold, 42.clone wouldn't actually be cloning.

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