ARC macros for weak references

dct_weak.h

#import <Availability.h>

#if __IPHONE_OS_VERSION_MIN_REQUIRED < __IPHONE_4_3
    #warning "This library uses ARC which is only available in iOS SDK 4.3 and later."
#endif

#if !defined dct_weak && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_5_0
	#define dct_weak weak
	#define __dct_weak __weak
	#define dct_nil(x)
#elif !defined dct_weak && __IPHONE_OS_VERSION_MIN_REQUIRED >= __IPHONE_4_3
	#define dct_weak unsafe_unretained
	#define __dct_weak __unsafe_unretained
	#define dct_nil(x) x = nil
#endif

These are the macros I use to define weak variables for ARC code where the minimum required OS is unknown, i.e. my common classes. This way, it doesn't matter if the minimum target OS is iOS 4.3 or iOS 5, the code will run as optimally as possible.

The trouble with these macros is that once dct_weak is defined control drops through to the final #else and the compiler starts emitting the warning. I've corrected it with https://gist.github.com/1354106 which explicitly emits a warning if the minimum iOS requirement is not met.