How to get a Scala runtime class with generic parameters?

Question

I wonder if it is possible to do something like the following:

import scala.reflect.runtime.universe._
class Bar[T]
def foo[T]()(implicit ctag: reflect.ClassTag[T]) { 
  val clazz = classOf[Bar[ctag.runtimeClass.asInstanceOf[Class[T]]]] 
}

Here the Scala compiler complains:

error: stable identifier required, but ctag.runtimeClass found.

Is there a way to get the class type with type parameters inserted from the runtime type information available in the function?

No, I need the type (classOf[Bar[T]]), not an instance of the class. — Philosophus42, Dec 11 '15 at 15:59
@Mr.V. Thanks, that actually does the trick. Would not have thought it to be this easy... — Philosophus42, Dec 11 '15 at 16:10
I still wonder why this works, because getting `classOf[T]` requires the ClassTag magic to work. — Philosophus42, Dec 11 '15 at 16:16
`def foo[T]() { val clazz = classOf[T] }`will throw `error: class type required but T found` — Philosophus42, Dec 11 '15 at 16:22

Alexey Romanov · Accepted Answer · 2015-12-11T21:22:13.320

Is there a way to get the class type with type parameters inserted from the runtime type information available in the function?

classOf[Bar[T]] works for a very simple reason: it doesn't insert any runtime information! classOf[Bar[T]], classOf[Bar[String]], classOf[Bar[Int]], classOf[Bar[_]] are all the same; that's what type erasure means in JVM context (to avoid misleading, I prefer always using classOf[Bar[_]] where possible). Note that there is actually a single exception: if Bar is Array, because classOf[Array[Int]] and classOf[Array[Object]] (e.g.) are different!

classOf[T] obviously would need runtime information and so it doesn't work.

In addition to this helpful answer, I suggest reading http://stackoverflow.com/a/12232195/409976. — Kevin Meredith, Dec 11 '15 at 20:02

score 0 · Answer 2 · answered Dec 11 '15 at 16:12

0

Thanks to the comment by @Mr. V I realized that its actually easier than I initially thought:

import scala.reflect.runtime.universe._
class Bar[T]
def foo[T]() { 
  val clazz = classOf[Bar[T]] 
}

answered Dec 11 '15 at 16:12

Philosophus42

472
3
11

How to get a Scala runtime class with generic parameters?

2 Answers2