Using a constant NSString as the key for NSUserDefaults

Question

I\'m using NSUSerDefaults to store user preferences. I remember reading somewhere that setting the keys as constants is a good idea - and I agree. The following code is what I c

Answer 1

You should use:

NSString * const kPolygonNumberOfSides = @"..."; // const pointer

instead of:

NSString const * kPolygonNumberOfSides = @"..."; // pointer to const

The first is a constant pointer to an NSString object, while the second is a pointer to a constant NSString object.

It is a subtle difference. The compiler warning happens because setObject:forKey: is declared as follows:

- (void)setObject:(id)value forKey:(NSString *)defaultName;

It is expecting the defaultName argument to be of type NSString *. When you instead pass in a pointer to a constant, you've given it something different.

Update: I want to point out that these constants should be defined as static if they are only going to be used from within a single file. I say this because I have run across this problem myself: if you do not declare them as static, then they will exist in the global namespace, and you will not be able to use a variable with the same the same name in another file. see Constants in Objective-C for more information. To explain by example, this is what I currently use for keys that I only need to use in one .m file:

static NSString * const kSomeLabel = @"...";