Flex/AS3 Best way to construct a derived class instance from an existing base class instance?

Question

What is the best way to handle the instantiation of a derived class from an existing base class.
I have a base class which is being created via remote_object [RemoteClass alias] from the server.   I have other specialized classes that are derived from this baseclass, but serialization with the server always happens with the base class.     The base class has meta data that defines what the derived class is, for example
[RemoteClass (alias="com.myco...')]
public Class Base
     public var derivedType:String;
     public function Base()
public Class Derived extends Base
     public "some other data"
     public function Derived()
In my Cairgorm command which retrieves this object from ther server I want to do this:
public function result (event: Object):void
    var baseInstance:Base = event.result;
     if (baseInstance.derivedType = "derived")
          var derivedInstance:Derived = new Derived( baseInstance );
What is the most efficient way of doing this?   It appears to me that doing a deep-copy/clone and instantiation of the derived class is pretty inefficient as far as memory allocation and data movement via the copy.

Answer 1

Thanks for the assistance.  Let me try to clarify.
MY UI requires a number of composite classes.    The individual components of the composite classes are being transfered to/from the server at different times depending upone which component has changed state.    The construction of the composite classes from the base class happens in my clients business logic.
Composition happens in a derived class; but server syncronization happens using the base class.    When I recieve the object from Blazeds through the remote object event, it is in the form of the base class.  I then need to instantiate the derived class and copy the elements of the base class into it (for later composite construction).   And likewise when sending the base class back to the server, I need to upcast the derived class to its base class.   But in this case just a mere upcast does not work.  I actually need to create a new base class and copy the attrbutes into it.  I believe this is limitation of how remoting works on Flex/AS3.
My question is, what is the best way to turn my base class into it's derived class so further composite construction can take place.   The way I am currently doing it is to create a  load method on the base class, that takes the base class as on argument.  The load function, copies all of the instance attribute references from the base class to the target class.
public Class Base
     public function Base()
     public function load(fromClass:Base)
    {  //  copy the references for all of the instance attributes from the fromClass to this class }
Then,  after I recieve the base class from the server.   I create a new derived class and pass the base class into the load function like this:
            for (var i:int=0; i < event.result.length; i++) {
                var derived:Derived = new Derived();
                derived.load(event.result[i]);
The drawbacks of this approach is that it now requires 2 extra instance creations per object serialization.   One on recieving the object from the server and one sending it to the server.    I assume copying references are pretty efficient.  But, there is probably some GC issues.     The worst of it is in code maintenance.   The load function now has to be manually maintained and kept in sync with the server class.
It would be interesting to hear how others have solved this problem.      The server side is an existing application with around 2M LOC, so changing the code on the server is a non-starter.
Thanks for your help.