On Thu, Apr 8, 2010 at 1:21 PM, Patrick M. Rutkowski <rutsk...@gmail.com> wrote: > Agreed, but there's always the danger the +array method, which might > actually be implemented in NSArray.m, will not properly initialize the > more specific NSMutableArray object. > > Of course, in this specific case that's the case, but it might be the > case with other class hierarchies. > > But, nonetheless I'm troubled that nobody in this thread has > acknowledged that yet :-o
Take a look at the declaration for this method: + (id)array; This is subtle... but it actually tells you everything you need to know for this question. The key is the "id" return type. Although at the language level this just means that it returns some kind of object, it has a deeper meaning when it comes to Cocoa conventions. More specifically, an "id" type on a factory method like this means that the method will return an instance of the class that the message is sent to, even if it's a subclass. (It could return an instance of a subclass of that class, but that's perfectly legal.) The "id' means that subclasses will work correctly, and that the implementation uses [self alloc]. Virtually all factory methods are declared like this, and thus that's how they work. As a counterexample, look at +[NSParagraphStyle defaultParagraphStyle]: + (NSParagraphStyle *)defaultParagraphStyle; It's not declared to return 'id'. This means that [NSMutableParagraphStyle defaultParagraphStyle] is *not* guaranteed to return an instance of NSMutableParagraphStyle. It could, but you shouldn't count on it. You have to assume that the object you get is always a straight NSParagraphStyle, and code accordingly. (In this particular case, you'd make a mutable copy of the returned object, then use that.) As a bonus, this convention means that the compiler will yell at you if you try to write code like: NSMutableParagraphStyle *style = [NSMutableParagraphStyle defaultParagraphStyle]; Of course you can't count on warnings to keep you safe all the time, but it helps. So check for the 'id' return value, and if it's there, you can code in confidence. Mike _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: http://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
