Obj-C: Difference between “Fairfield” and @“Fairfield” (with at string)?

Question

I just had a ridonkulous typo in my iPhone app, answered here .

Now I'm wondering about the @"..." notation.

why this works:

NSArray *someArray = [NSArray arrayWithObjects: @"Fairfield", nil];

and this does not (even though it compiles, it will throw an EXC_BAD_ACCESS):

NSArray *someArray = [NSArray arrayWithObjects: "@Fairfield", nil];

Edit:

Ok, so you guys have pointed out that I can't add a C string to an NSArray, because it's obviously not an object.

Now another question: Isn't this somewhat of an oversight? I mean, why does the "...WithObjects:" message specify a list of (id) instead of (NSObject *)?

Answer 1

"@Fairfield" is a normal C string with an '@' character in it. @"Fairfield" is an Objective-C string ( NSString on OS X) with no literal '@' in it.

You cannot add C strings to Cocoa collections.

Answer 2

It accepts id rather than NSObject because all initialisers return id . All initialisers return id because subclasses would otherwise override the return type of their ancestors' initialisers.

For example, -[NSMutableString init] can't return NSMutableString * because it subclasses -[NSString init] , which can't return NSString * because it overrides -[NSObject init] .

Unfortunately, implicit type-casting between const char * and id is perfectly legit, so the compiler won't throw a warning, however a static analyser may be able to pick this sort of mishap up fairly easily.

Answer 3

"Fairfield" is a C string, @"Fairfield" is an Objective-C string.

@"Fairfield" is an object (NSString), so you can send it methods ( [@"Fairfield" uppercaseString] ) and add it to Objective-C arrays ( [NSArray arrayWithObjects:@"Fairfield",nil] ). You can only add objects to NSArrays.

On the other hand, "Fairfield" is a C string, and is generally not used in Cocoa. For the most part, you can get by with only using @"Fairfield"

Answer 4

The other reason that a number of things in Cocoa deal with id rather than NSObject* is because, unlike some other languages (say, Java and C#), where all objects in the language must inherit from some global base class, it's entirely possible to have objects that do not descend from NSObject ( NSProxy being one example). It's not something you'd do often, but it is possible. The id type means "pointer to any Objective C instance".