Why does a (copy, nonatomic) NSMutableArray property create NSArrays?

Question

I made a mistake while creating a TableView class, and accidentally kept my @property as copy when I defined it:

@property         


        

Answer 1

-copy, as implemented by mutable Cocoa classes, always returns their immutable counterparts. Thus, when an NSMutableArray is sent -copy, it returns an NSArray containing the same objects.

Because words has the memory qualifier copy, this line:

NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords];
self.words = mutWords;

Expands out to:

NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords];
self.words = [mutWords copy];

Given that NSMutableArray is a subclass of NSArray, the compiler doesn't complain, and you now have a ticking time bomb on your hands because NSArray does not recognize it's mutable subclass' methods (because it cannot mutate it's contents).