How to write asInstanceOfOpt[T] where T <: Any

Question

There\'s a handy implementation of asInstanceOfOpt, a safe version of asInstanceOf, given in the answer to How to write "asInstanceOfOption"

Answer 1

Here's working code for 2.9.x. It will give deprecation warnings for 2.10.x, but using ClassTag instead of Manifest and runtimeClass instead of erasure will fix them.

//Precondition: classS must have been produced through primitiveToBoxed, because v will be boxed.
def ifInstanceOfBody[T, S](v: T, classS: Class[_]): Option[S] = {
  if (v == null || !classS.isInstance(v))
    None
  else
    Some(v.asInstanceOf[S])
}
object ClassUtil {
  import java.{lang => jl}

  private val primitiveToBoxedMap = Map[Class[_], Class[_]](
    classOf[Byte] -> classOf[jl.Byte],
    classOf[Short] -> classOf[jl.Short],
    classOf[Char] -> classOf[jl.Character],
    classOf[Int] -> classOf[jl.Integer],
    classOf[Long] -> classOf[jl.Long],
    classOf[Float] -> classOf[jl.Float],
    classOf[Double] -> classOf[jl.Double],
    classOf[Boolean] -> classOf[jl.Boolean],
    classOf[Unit] -> classOf[jl.Void]
  )
  def primitiveToBoxed(classS: Class[_]) =
    primitiveToBoxedMap.getOrElse(classS, classS)
}
class IfInstanceOfAble[T](v: T) {
  def asInstanceOfOpt[S](implicit cS: Manifest[S]): Option[S] =
    ifInstanceOfBody[T, S](v, ClassUtil.primitiveToBoxed(cS.erasure))
}
implicit def pimpInstanceOf[T](t: T) = new IfInstanceOfAble(t)

Testing results:

scala> 1.asInstanceOfOpt[Int]
res9: Option[Int] = Some(1)

scala> "".asInstanceOfOpt[String]
res10: Option[String] = Some()

scala> "foo".asInstanceOfOpt[String]
res11: Option[String] = Some(foo)

scala> 1.asInstanceOfOpt[String]
res12: Option[String] = None

scala> "".asInstanceOfOpt[Int]
res13: Option[Int] = None

The code is slightly more verbose than needed here, mostly because I took it from an existing codebase of mine where I reuse ifInstanceOfBody elsewhere. Inlining into asInstanceOfOpt would fix that and shorten the code somewhat, but most of it is for primitiveToBoxedMap, and trust me that I could not find something like that available in the Scala standard library.