Scala isInstanceOf [T]函数无法使用有限的ClassTag / TypeTag

Question

The follow code:

abstract class Foo[T: ClassTag] {
    def v(a: Any): Any = a match {
        case _ if a.isInstanceOf[T] => Some(a)
        case _ => None
    }
}

yield the following in compilation:

Warning: abstract type T is unchecked since it is eliminated by erasure
    case _ if a.isInstanceOf[T] =>

Strangely, case match will work as intended. Is there a way to make isInstanceOf[T] be aware of the ClassTag context bound as well?

Answer 1

I wouldn't say this is working as expected:

scala> (new Foo[String] {}).v(List(1))
res10: Any = Some(List(1)) // Not a String!

Don't use isInstanceOf , as it doesn't use ClassTag s at all. You can use the ClassTag extractor, instead:

abstract class Foo[T: ClassTag] {
    def v(a: Any) = a match {
        case _: T => Some(a)
        case _ => None
    }
}

scala> (new Foo[String] {}).v(1)
res3: Option[Any] = None

scala> (new Foo[String] {}).v("abc")
res4: Option[Any] = Some(abc)

This is all syntactic sugar for:

abstract class Foo[T](implicit ct: ClassTag[T]) {
    def v(a: Any) = a match {
        case ct(a) => Some(a)
        case _ => None
    }
}