scala 的 ClassTag 和 TypeTag 的区别 [英] Difference between scala's ClassTag and TypeTag

问题描述

根据scala doc，TypeTag比ClassTag包含更多的信息.在我看来，TypeTag 比 ClassTag 可以做更多的事情，比如把编译时的类型参数信息带到运行时等等.

According to scala doc，TypeTag contains more information than ClassTag. It seems to me that TypeTag can do more things than ClassTag, like bring the type parameter information of compile time to runtime, etc.

但是，下面的示例表明 ClassTag 可以完成这项工作，而 TypeTag 则不然.我想了解原因.

However, the following example shows that ClassTag can do the job, while the TypeTag not. I want to understand why.

import scala.reflect.ClassTag
import scala.reflect.runtime.universe.TypeTag
// def func[T](o: Any): Unit = {
// def func[T : TypeTag](o: Any): Unit = {
def func[T : ClassTag](o: Any): Unit = {
  o match {
    case x: T => println(Some(x))
    case _ => println(None)
  }spark
}
func[Map[Int, Int]](List(1, 2, 3))

只有 ClassTag 会导致模式匹配为 None(这是预期的行为)，前两行注释会出现 Some 分支.

Only ClassTag will lead the pattern matching to None (which is the expected behavior), the first two commented lines will come up with Some branch.

似乎ClassTag 可以在运行时反映对象的类型，而TypeTag 不能.但是TypeTag不是ClassTag的超集吗?我想知道详细的解释.谢谢.

It seems that ClassTag can reflect on object's type on runtime, while TypeTag can't. But isn't TypeTag a superset of ClassTag？ I would like to know the explanation as detailed as possible. Thank you.

推荐答案

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https://medium.com/@sinisalouc/overcoming-scala-8f2422070d20 中的类型擦除

添加更多细节，因为只有链接答案不合适...

ClassTag:关于值的运行时信息，但不适用于通用样式
TypeTag :关于类型的运行时信息

ClassTag : runtime information about value, but not good with generic style
TypeTag : runtime information about type

例如

object Test extends App {

  import scala.reflect.ClassTag
  def func[T : ClassTag](o: Any): Unit = {
    o match {
      case x: T => println(x)
      case _ => println(None)
    }
  }
  func[List[String]](List(1, 2, 3)) // List(1, 2, 3), not None!!! with List[String] type parameter... ClassTag only recognize List scale, not List[T]

  import scala.reflect.runtime.universe._
  def func2[T](o: T)(implicit tag: TypeTag[T]): Unit = {
    tag.tpe match {
      case TypeRef(utype, usymbol, args) => println(args.toString)
      case _ => println(None)
    }
  }
  func2(List(1, 2, 3)) // List(Int)
}

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