Does Scala allow for the arbitrary instantiation of generic objects? new B {}?

Question

I was wondering if something like this would be possible to implement in Scala:

def f[B <: AnyRef]()(implicit ct: ClassTag[B]): B = {
  new B {
    override def equals(o: Any) = ...
    override def hashcode(o: Any) = ...
  }
}

My intuition is that even with the ClassTag it should not be possible to instantiate an arbitrary B, as we don't know if it even has a no-args parameter.

But the actual error I'm getting is:

class type required but B found

My current use case is: I want to be able to redefine the equals/hashcode of arbitrary objects that are given to me (I can't really get away from that as I'll have then to deliver the objects to some faulty framework that will use those objects' equals / hashcodes, so I have no control over that). The obvious way to do that would be through inheritance. Is there some way of doing this?

Answer 1

It's definitely impossible in the general case.

Supposed I passed a final type for B, such as String. There would be no way to subclass that. So, you can't do it for arbitrary B.

Depending on how this other framework works, you might be able to pass it an AnyRef instead. Otherwise, you might be able to pull something off with macros, but more information is needed to determine this.

Answer 2

Brian answers the question in the title (you can also consider that B can have abstract methods you wouldn't know to define). But

I want to be able to redefine the equals/hashcode of arbitrary objects that are given to me

suggests a different signature:

def f[B <: AnyRef](x: B): B = ...

and this one is doable. With limitations, of course. If B is an interface, you can use dynamic proxies from Java standard library, and for classes you need a library like CGLIB. Approximately (untested):

def f[B <: AnyRef](x: B): B = {
  val enhancer = new Enhancer()
  enhancer.setSuperclass(x.getClass)
  val interceptor: MethodInterceptor = (obj, method, args, proxy) =>
    method.getName match {
      case "equals" => // your equals impl 
      case "hashCode" => // your hashCode impl 
      case _ => proxy.invokeSuper(obj, args)
    }
  enhancer.setCallback(interceptor)
  enhancer.create() as B
}