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Scala“ cursor.asInstanceOf [:: [A]]。tl = newElem”是什么意思

[英]Scala “cursor.asInstanceOf[::[A]].tl = newElem” what does it mean

原文 2016-12-07 19:48:59 7 1 scala/ casting/ listbuffer

I am looking at the source code for Scala's mutable ListBuffer here https://github.com/scala/scala/blob/v2.11.8/src/library/scala/collection/mutable/ListBuffer.scala#L158 我在这里查看Scala可变ListBuffer的源代码https://github.com/scala/scala/blob/v2.11.8/src/library/scala/collection/mutable/ListBuffer.scala#L158

and I ran into this cast on line 158. I have been unable to figure out what .asInstanceOf[::[A]] means. 我在第158行遇到了这个问题。我一直无法弄清楚.asInstanceOf[::[A]]是什么意思。 :: is not a bounds notation that I could find in the documentation. ::不是我可以在文档中找到的界限符号。 But, it is a method for a List. 但是，这是用于列表的方法。 Can anyone explain what this cast is doing? 谁能解释这个演员在做什么？

1 个解决方案

:: is a list constructor that is composed of head and tail (as opposed to Nil which has neither). ::是一个由头和尾组成的列表构造函数（与Nil都不一样）。 In other words it's a kind of list that guarantees that it contains at least one element (head). 换句话说，这是一种保证至少包含一个元素（头）的列表。

You can find its definition in List.scala : https://github.com/scala/scala/blob/v2.11.8/src/library/scala/collection/immutable/List.scala#L439 您可以在List.scala找到其定义： https : //github.com/scala/scala/blob/v2.11.8/src/library/scala/collection/immutable/List.scala#L439

So what this cast does is that it turns the cursor typed as List[A] into more specific ::[A] , that allows accessing its tl variable, representing list tail. 因此，此强制转换的作用是将类型为List[A]的cursor转换为更特定的::[A] ，从而允许访问其tl变量，表示列表尾部。 Which is necessary for efficient update operation. 这对于有效的update操作是必需的。