On Jun 11, 2008, at 12:15 AM, Chris Hanson wrote:
This whole business about '(NSArray *) means NSArray AND any of it's subclasses' is the result of sloppy thinking and confusing 'able to' with 'as per spec'.No, you are wrong. It really is "per spec." That is an intentional capability in both Objective-C and C++: You can return an instance of a class, or any of its subclasses, wherever an instance of that class is specified precisely because classes should be designed to be substitutable in this fashion. This is the essence of polymorphism in object-oriented programming and is summarized by the Liskov substitution principle.
For that matter, it may be worth pointing out John's confusion between "sloppy" and "incorrect". He makes a decent point that returning a mutable object when the method definition specifies an immutable one is sloppy and, indeed, fraught with peril.
But Chris is still correct that doing so is completely allowable, at least under the true-in-Cocoa assumption that the mutable data types are implemented as subclasses of the immutable ones.
-> jp
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