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?
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).