I ran into a bit of a problem yesterday. You know when you define a class name in the Class field of the properties of a symbol present in flash library, whose Base class is your custom class and not native MovieClip (or any other native type). Say you have MySymbol_1, MySymbol_2 .. MySymbol_n present in your flash library. For which the class names are like SymbolClass_1, SymbolClass_2 .. SymbolClass_n which extend a Base class of SymbolBase. And if now you want to add those symbols on stage, you would ideally do:
this.addChild ( new SymbolClass_1 ());
var mySymbol :SymbolBase = new SymbolClass_1 ();
this.addChild ( mySymbol );
Now, here comes a situation (at least it was for me :)). What if you wanted to add a child whose class name is unknown up to a point when a user interacts. Say the user clicked on a button named as Button1, based on which SymbolClass_1 is supposed to be added. This is the solution I learnt a few days back. There is a getDefinitionByName public method available in the flash.utils package which allows you to get a class reference by the name parameter and then you can use that class reference and instantiate it in another variable of data type of your base class.
var myClassReference :Class = getDefinitionByName ( 'SymbolClass' + buttonNumber ) as Class;
var mySymbol :SymbolBase = new myClassReference;
This way, you can use dynamic variables in your class names and would instantiate the right class on demand. Btw, that is the solution I got, if anyone has any other solution/recommendation, please tell me.