找不到元素类型 T 的类清单 [英] cannot find class manifest for element type T

问题描述

试图从 编译一些代码这个SO问题并遇到此错误消息cannot find class manifest for element type T.这是显示行为的另一个片段:

Was trying to compile some code from this SO question and run into this error message cannot find class manifest for element type T. Here is another snippet that shows the behavior:

scala> def f[T](a:T, b:T):Array[T] = { new Array[T](2) }
<console>:4: error: cannot find class manifest for element type T
       def f[T](a:T, b:T):Array[T] = { new Array[T](2) }

我可以看到 new collection.mutable.GenericArray[T](2) 解决了这个问题.显然提供清单是另一种选择……但是提供清单意味着什么"?

I can see that new collection.mutable.GenericArray[T](2) fixes the issue. Apparently providing a manifest is the other option... But what does "providing a manifest mean"?

推荐答案

要提供类型信息，您可以使用上下文绑定

To provide type information you can use a context bound

def f[T : Manifest](a:T, b:T):Array[T] = { new Array[T](2) }

或作为隐式参数的清单:

or the manifest as an implicit argument:

def f[T](a:T, b:T)(implicit manifest : Manifest[T]) : Array[T] = { new Array[T](2) }

前者是后者的语法糖.需要 manifest 因为关于 T 的类型信息是由于 JVM 的通用类型错误而丢失.

The former is syntactic sugar for the later. The manifest is needed because the type information about T is missing due to generic type errasure of the JVM.

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