ActionScript operator as

does anybody have a good explanation of the as operator?

On one hand it seems to me, that it is often better to use as instead of instantiating a new object.

But then there are situations, when this operator let's me down. For example when loading a text file in XML format through an URLLoader:

private function completeHandler(event:Event):void {
    var loader:URLLoader = URL开发者_开发百科Loader(event.target);
    trace("completeHandler: " + loader.data);

    var x:XML = new XML(loader.data);
    trace("x=" + x);
}

Why do I have to use a constructor here? Why can't I say var x:XML = loader.data as XML; and save some memory?

Thank you for any insights! Alex

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as evaluates whether a variable's type is a super class or subclass of another class. It does not create a new object. The difference to is being that while is returns a Boolean value, as returns either an object of the desired type, or null. It is used for type casts.

See the ActionScript documentation.

A typical use case would be using a MovieClip on the stage, which is retrieved by instance name:

// This will not compile:
var d:DisplayObject = stage.getChildByName ("instance1"); 
d.gotoAndPlay (2);

// This will play the MovieClip from frame 2
var m : MovieClip = stage.getChildByName ("instance1") as MovieClip;
m.gotoAndPlay (2); 

stage.getChildByName() always returns a DisplayObject, regardless if it's really a MovieClip, Sprite, Button, etc. So if you want to use any of class MovieClip's methods, you need to type cast to MovieClip first. This does not, however, create a new MovieClip but simply ensures that you are using the correct type.