One correction... o.doIt() would compile because Object is a dynamic class and you can try to access anything on it.
- Gordon -----Original Message----- From: [email protected] [mailto:[EMAIL PROTECTED] On Behalf Of Jens Halm Sent: Wednesday, April 12, 2006 5:54 PM To: Ted Patrick Subject: Re: [flexcoders] programatically assigning an ArrayCollection as the DataProvider bug... Ted, it seems like you have a wrong perception of what a type cast actually is, not only in ActionScript, but in general. A type cast is NOT about changing the type at runtime. if you write something like this: var o:Object = new ClassA(); var b:ClassB = ClassB(o); this will actually *fail* (throw an error at runtime) unless ClassA would be a subclass of ClassB. You cannot change the type at runtime. There will definitly not be any constructor invocation or anything similar. The purpose of casting is just: At compile time: Tell the compiler what type you actually want to use. Let's say you have ClassA which has a method doIt. If you type the instance of ClassA as Object, the compiler will only allow you to use properties and methods of Object. Because it does not know what the runtime type will be. So if you would do this: var o:Object = new ClassA(); o.doIt(); you'll get a compiler error, because Object does not have a doIt method. If you then cast it back: var a:ClassA = ClassA(o); a.doIt(); the compiler won't complain. Casting is necessary because we have compile time type checking. The purpose at runtime: Just check if the cast you used was legal. So a statement like ClassA(o) just means: pass the instance of o through *unchanged* if it actually *is* already an instance of ClassA, otherwise throw an error. Now to add to the confusion: In ActionScript there are some exceptions, where something that looks like a simple cast actually *does* include invoking a function. This is the case for many top level types like Array. If you write var a:Array = Array(o); it will not be treated like a usual cast, it is a plain function call. Which IMHO is very unfortunate and confusing. Other languages like Java do not have these kind of special cases. So basically with the old syntax you could not cast for these types, thus the new syntax var a:Array = o as Array was introduced. There are only two differences between those two options to cast: The new one (using "as") does not have these special cases with the top level classes, so now you *can* actually do a simple cast to Array. The second difference is how errors are treated if the cast fails. The old syntax throws a runtime error, the new one just returns null, if the cast fails (meaning you tried to trick the compiler). So really, casting is not about changing the type at runtime. Jens -- Flexcoders Mailing List FAQ: http://groups.yahoo.com/group/flexcoders/files/flexcodersFAQ.txt Search Archives: http://www.mail-archive.com/flexcoders%40yahoogroups.com Yahoo! Groups Links -- Flexcoders Mailing List FAQ: http://groups.yahoo.com/group/flexcoders/files/flexcodersFAQ.txt Search Archives: http://www.mail-archive.com/flexcoders%40yahoogroups.com Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/flexcoders/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
