Simply create associated property with one line of code using a reusable preproc macro

APMacros.h

#import <objc/runtime.h>

#define ASSOCIATIVE_PROPERTY(GET_NAME, SET_NAME, TYPE, ASSOCIATION_POLICY) \
static char GET_NAME##Key; \
- (TYPE) GET_NAME \
{ \
    return objc_getAssociatedObject(self, & GET_NAME##Key); \
} \
\
- (void) SET_NAME:(TYPE)value\
{\
    objc_setAssociatedObject(self, & GET_NAME##Key, value, ASSOCIATION_POLICY);\
}


// Usage

// In category @interface
@property (nonatomic, retain) NSString* oldString;

// In category @implementation
ASSOCIATIVE_PROPERTY(oldString, setOldString, NSString*, OBJC_ASSOCIATION_RETAIN)