Given an object, how can I instantiate a new object whose class is the same plus one additional trait

Question

If I have some object x, and I want to create a new instance with the same class as x, I can say:

x.getClass.newInstance

If x was of some (unknown) class T, the new instance is also of class T.

But I actually want to create a new instance which is T with A. In other words, I want to do something like:

(x.getClass with A).newInstance

But that doesn't work. Is it possible to do this?

I'm curious - how would you know if x already extended the A trait, obviating the need to add the inheritance? — swartzrock
– swartzrock, Commented Jan 21, 2014 at 13:36
Well, in my actual use case it's not entirely unknown. I have an abstract class which could be extended by any number of classes. The class is something like a configuration, and I want it to function like a factory and spit out a new class which includes the configuration and some additional functionality (which I'm getting from the trait). — Tim Goodman
– Tim Goodman, Commented Jan 21, 2014 at 16:41
Normally I would make the additional functionality its own class, and supply the configuration as an argument to the constructor of that class. But in this case I wanted to keep a bunch of members of the configuration class protected, which is how I got to the point of wanting my additional functionality to actually extend the configuration class. — Tim Goodman
– Tim Goodman, Commented Jan 21, 2014 at 16:41
In retrospect, I suspect that there are better ways to architect this. — Tim Goodman
– Tim Goodman, Commented Jan 21, 2014 at 19:45

Vladimir Matveev · Accepted Answer · 2014-01-21 07:29:47Z

2

First of all, you cannot just add a trait or interface to existing class because JVM classes are static - you have to create a subclass of both. But you cannot dynamically create classes without manually fiddling with byte code, not in Scala at least. To create new object of a class you have to define this class first, and it is only possible when you know in advance which classes you're extending. So no, it is impossible (at least, extremely inconvenient).

Depending on your concrete use case, however, it is possible to emulate this, for example, with composition plus implicit conversion:

implicit class SomeClassAView(x: SomeClass) extends A {
    // Implement A methods using x object
}

def expectingA(x: A) { ... }

val x: SomeClass = ...
expectingA(x)  // x will be implicitly converted to SomeClassAView which extends A

answered Jan 21, 2014 at 7:29

Vladimir Matveev

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som-snytt · Accepted Answer · 2014-01-21 08:39:13Z

If the T is unknown, then compilation on the fly is handled handily:

apm@mara:~/goof$ scalam -cp /tmp
Welcome to Scala version 2.11.0-M7 (OpenJDK 64-Bit Server VM, Java 1.7.0_25).
Type in expressions to have them evaluated.
Type :help for more information.

scala> new ts.X {}
res0: ts.X = $anon$1@13ba518f

scala> import tools.reflect.ToolBox
import tools.reflect.ToolBox

scala> import reflect.runtime._
import reflect.runtime._

scala> import universe._
import universe._

scala> val tb = currentMirror.mkToolBox()
tb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] = scala.tools.reflect.ToolBoxFactory$ToolBoxImpl@62920919

scala> tb.eval(tb.parse("new ts.X with ts.Y {}"))
res2: Any = __wrapper$1$88859f071ea8497cb9bf4b87aa027c85.__wrapper$1$88859f071ea8497cb9bf4b87aa027c85$$anon$1@5fa8881b

scala> res2.asInstanceOf[ts.X with ts.Y]
res3: ts.X with ts.Y = __wrapper$1$88859f071ea8497cb9bf4b87aa027c85.__wrapper$1$88859f071ea8497cb9bf4b87aa027c85$$anon$1@5fa8881b

There is an issue to make REPL-compiled classes visible to the toolbox.

For now, roll your own:

scala> val rtm = runtimeMirror($intp.classLoader)
rtm: reflect.runtime.universe.Mirror = JavaMirror with scala.tools.nsc.interpreter.IMain$TranslatingClassLoader@5349c9cb of type class scala.tools.nsc.interpreter.IMain$TranslatingClassLoader with classpath [(memory)] and parent being scala.reflect.internal.util.ScalaClassLoader$URLClassLoader@115f5925 of type class scala.reflect.internal.util.ScalaClassLoader$URLClassLoader with classpath [file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/resources.jar,file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/rt.jar,file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/jsse.jar,file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/jce.jar,file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/charsets.jar,file:/usr/lib/jvm/java-7-openjdk-amd64/jre/lib/rhino.jar,file:/home/apm/scala-2.11.0-M7/lib/akka-actors.jar,file:/hom...
scala> val rtb = rtm.mkToolBox()
rtb: scala.tools.reflect.ToolBox[reflect.runtime.universe.type] = scala.tools.reflect.ToolBoxFactory$ToolBoxImpl@58f59add

scala> :pa -raw
// Entering paste mode (ctrl-D to finish)

package foo
class Foo

// Exiting paste mode, now interpreting.


scala> rtb.eval(rtb.parse("new foo.Foo"))
res6: Any = foo.Foo@7c20db7f

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