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 (copy, nonatomic) NSMutableArray *words;  

I initialised the array "correctly": (note this is the third attempt, so please ignore the fact that I'm not using mutableCopy and other better ways of doing this)

NSArray *fixedWords = @[@"Eeny", @"Meeny", @"Miny", @"Moe", @"Catch", @"A", @"Tiger", @"By", @"His", @"Toe"]; NSMutableArray *mutWords = [[NSMutableArray alloc] initWithArray:fixedWords]; self.words = mutWords; 

However when I later came to reorder the array, it crashed on the removeObjectAtIndex line:

id object = [self.words objectAtIndex:fromIndexPath.row]; NSUInteger from = fromIndexPath.row; NSUInteger to = toIndexPath.row; [self.words removeObjectAtIndex:from]; 

With the error message

unrecognized selector sent to instance 

Took a lot of digging to figure out that this is because the copy means that assigning the NSMutableArray results in creation of a standard (nonmutable) NSArray. Can anyone explain why this is the correct behaviour?

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).