Question

package {
    import flash.display.Sprite;

public class test1 extends Sprite {

private var tmp:Object;

public function test1() {
  createObj(tmp);
  if(tmp == null) {
    trace("nothing changed");
  }
}

private function createObj(obj:Object):void {
  obj = new Object();
}

}
}

In the above code the output on the console is :
nothing changed

Why?

If the argument to createObj was passed by reference(which is the
default behavior of actionscript), why did it not get modified?

Answer 1

You don’t pass a reference. You are passing null which is assigned to the local variable obj for use within the function.

Passing arguments by value or by reference:

To be passed by reference means that
only a reference to the argument is
passed instead of the actual value. No
copy of the actual argument is made.
Instead, a reference to the variable
passed as an argument is created and
assigned to a local variable for use
within the function.

In createObj you are creating a new reference which you must return:

public function test1() {
  tmp = createObj();
  if(tmp != null) {
    trace("Hello World!");
  }
}

private function createObj():Object {
  return new Object();
}