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.
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)