root property
root:DisplayObject [read-only]…… The root property is set to null for any display object that has not been added to the display list, unless it has been added to a display object container that is off the display list but that is a child of the top-most display object in a loaded SWF file.
quote from ActionScript 3.0 Language and Components Reference
package{
import flash.display.MovieClip;
import flash.events.Event;// added on 2011.03.24
public class SomeClass extends MovieClip{
private var _root:MovieClip;
private var _parent:MovieClip;
public function SomeClass(){
addEventListener(Event.ADDED_TO_STAGE, initialize);
}// end SomeClass constructor
/**
* Initializes the SomeClass, when an instance is added to the display list
*/
private function initialize(evt:Event):void{
removeEventListener(Event.ADDED_TO_STAGE, initialize);
_root = root as MovieClip;
_parent = parent as MovieClip;
}// end initialize
}// end SomeClass definition
}// end package