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

Properties aren't magical, they're just shorthand. Declaring a @property on your object tells the compiler to create a backing instance variable and accessor methods for it. The actual generated code depends on the attributes you set on your property.

It's important to remember that setting a property using dot syntax is also shorthand. When you call…

self.words = mutWords;

…you're actually invoking the generated accessor method behind the scenes, like this:

[self setWords:mutWords];

Since you specified the copy attribute on your property, you've told the compiler to generate that -setWords: accessor method with code that looks something like this:

- (void)setWords:(NSMutableArray *)words
{
    _words = [words copy];
}

Knowing all that, you can see what's happening: the generated setter method will call -copy on the input argument, and assign the result to the backing instance variable. Because the -copy method is always implemented to return a non-mutable object (performing [aMutableString copy] will return an NSString, and so on) setting that property will always store a non-mutable copy.