Question

So what is actually the difference between these two versions:

@interface Foo : NSObject
// A guy walks into a bar.
@property(nonatomic, copy) NSString *bar;
@end

// Implementation file
@interface Foo ()
@property(nonatomic, retain) NSArray *baz;
@end

and

@interface Foo : NSObject
// A guy walks into a bar.
@public
@property(nonatomic, copy) NSString *bar;

@private
@property(nonatomic, retain) NSArray *baz;
@end

As far as my understanding goes, putting the @property in the .m basically means that it is private. Correct me if I am wrong? Also which is the best implementation then? Is it just a coding style/practice?

Answer 1

The compiler can warn you about things that it knows about.

When I import your header the compiler can see that Foo has a method called bar and setBar:. This means I can use them both

[instanceOfFoo setBar:@"some string"];

NSLog(@"%@", [instanceOfFoo bar]);

whereas because I only imported the header – the compiler can only see the header it is not aware that there are also methods baz and setBaz: available, so doing the following will cause the compiler to barf

[instanceOfFoo setBaz:@"some string"];

NSLog(@"%@", [instanceOfFoo baz]);

I can however still access these properties if I know they exist by using KVC like this without the compiler barfing

[instanceOfFoo setValue:@"some string" forKey:@"baz"];

NSLog(@"%@", [instanceOfFoo valueForKey:@"baz"]);