Scala generic method - No ClassTag available for T

Question

I'm relatively new to Scala and am trying to define a generic object method. However, when I refer to the parameterized type within the method I am getting "No ClassTag available for T". Here is a contrived example that illustrates the problem.

scala> def foo[T](count: Int, value: T): Array[T] = Array.fill[T](count)(value)
<console>:7: error: No ClassTag available for T
       def foo[T](count: Int, value: T): Array[T] = Array.fill[T](count)(value)
                                                                        ^

Thanks in advance for help in understanding what is wrong here and how to make this contrived example work.

Solution

To instantiate an array in a generic context (instantiating an array of T where T is a type parameter), Scala needs to have information at runtime about T, in the form of an implicit value of type ClassTag[T]. Concretely, you need the caller of your method to (implicitly) pass this ClassTag value, which can conveniently be done using a context bound:

def foo[T:ClassTag](count: Int, value: T): Array[T] = Array.fill[T](count)(value)

For a (thorough) description of this situation, see this document:

https://docs.scala-lang.org/sips/scala-2-8-arrays.html

(To put it shortly, ClassTags are the reworked implementation of ClassManifests, so the rationale remains)

Comments

[Chuck]

Fascinating. With 'import scala.reflect.ClassTag' this works. Thanks.

[Chuck]

People may find this instructive as well - docs.scala-lang.org/overviews/reflection/… - since ClassManifests are going away.

[AlvaPan]

If we do any comparison of two values of type T in the function body, we need an implicit orderer parameter in additional to the ClassTag annotation.

[bbarker]

I found this Java-oriented question helpful as well to understand why we don't need ClassTags for e.g. Lists, but Martin's SIP also hinted at this: stackoverflow.com/questions/1817524/generic-arrays-in-java