Unused variable warning with static NSInteger, but not with NSString

Question

After updating Xcode to version 5.1, I had a warning that told me I had defined a constant that I wasn't using. Its definition looked like this:

static NSInteger const ABCMyInteger = 3;

I was happy to see that it got marked, because I thought this meant that the compiler was now able to check for unused constants in addition local to variables.

I refactored some more, making three NSString constants obsolete. All three were defined very similarly to the NSInteger from above:

static NSString *const ABCMyString = @"ABCMyString";

To my surprise, however, these do not get marked as "unused", though I know for sure that they aren't used anymore.

Can someone explain why an NSInteger does get noticed by the compiler as unused, but an NSString does not?

Answer 1

A primitive variable is just a memory block allocated in a static memory part and initialized by the compiler. The string object, however, is a variable initialized at runtime (in startup, probably), so the compiler adds an implicit call to the constructor and uses the variable as a parameter for that call. So the variable is being used.

The _unused item of the structure is IMHO not a directive, but just a member variable, probably it is added for better alignment (fills the object size to a round size).

Answer 2

The definition of an NSString literal at compile time rely on the use of the NSSimpleCString meta class. This class looks something like this:

@interface NSSimpleCString : NSString {
@package
    char *bytes;
    int numBytes;
#if __LP64__
    int _unused;
#endif
}
@end

@interface NSConstantString : NSSimpleCString
@end

The addition of the _unused flag make me believe that further down the implementation of NSSimpleCString the code will instruct the compiler to silence those warnings with __unused.
You can try yourself by prepending your integer or float constant with __unused like:

__unused static const NSInteger ABCMyInteger = 3;

For a more in depth explanation read the article on literals by Mike Ash