reflection - Polymorphic instantiation in Scala using TypeTag and ClassTag -

in scala 2.9 1 implement polymorphic instantiation as

def newinstance[t](implicit m: manifest[t]) =     m.erasure.newinstance.asinstanceof[t] 

but of 2.10 manifest being replaced typetag, , not clear me how achieve similar typetag. prefer if typetag version preserved available type information.

i know above works traits/classes not require constructor args, , ven not work, works enough need. if can better new reflection apis great.

typetag not yet replacement manifest because it's part of experimental , unstable scala reflection. shouldn't use production of now.

for use case showed, runtime class needed (not full type information generics etc.), scala 2.10 introduced classtag, can use this:

def newinstance[t: classtag] =   implicitly[classtag[t]].runtimeclass.newinstance.asinstanceof[t] 

or:

def newinstance[t](implicit ct: classtag[t]) =   ct.runtimeclass.newinstance.asinstanceof[t] 

anyway, manifest isn't deprecated yet, guess can still use it.

edit:

using typetag achieve same:

import scala.reflect.runtime.universe._  def newinstance[t: typetag] = {   val clazz = typetag[t].mirror.runtimeclass(typeof[t])   clazz.newinstance.asinstanceof[t] } 

the above solution still uses java reflection. if want puristic , use scala reflection, solution:

def newinstance[t: typetag]: t = {   val tpe = typeof[t]    def fail = throw new illegalargumentexception(s"cannot instantiate $tpe")    val noargconstructor = tpe.member(nme.constructor) match {     case symbol: termsymbol =>       symbol.alternatives.collectfirst {         case constr: methodsymbol if constr.paramss == nil || constr.paramss == list(nil) => constr       } getorelse fail      case nosymbol => fail   }   val classmirror = typetag[t].mirror.reflectclass(tpe.typesymbol.asclass)   classmirror.reflectconstructor(noargconstructor).apply().asinstanceof[t] }