How do I use objc_setAssociatedObject/objc_getAssociatedObject inside an object?

If I use objc_setAssociatedObject/objc_getAssociatedObject inside a category implementation to store a simulated instance variable in a setter method, how would I access the key in the getter method since any variables declared in the setter method would be outside the scope of the getter method?

Edit: To clarify, if I were to use the following pattern, where should I declare STRING_KEY so that I could use it in both the setter and the getter method.

@interface NSView (simulateVar)
-(void)setSimualtedString:(NSString *)myString;
-(NSString *)simulatedString;
@end
 
@implementation NSView (simulateVar)
 
-(void)setSimualtedString: (NSString *)myString
{
    objc_setAssociatedObject(self, &STRING_KEY, myString, OBJC_ASSOCIATION_RETAIN);
}
 
-(NSString *)simulatedString
{
    return (NSString *)objc_getAssociatedObject(self, &STRING_KEY);
}
 
@end

Declare a static variable so that you can use its address as the key. The call to objc_setAssociatedObject takes a void* and only the address of your static variable is actually used, not the contents of a NSString... that is only wasting memory.

You just need to add:

static char STRING_KEY; // global 0 initialization is fine here, no 
                        // need to change it since the value of the
                        // variable is not used, just the address

I know that this question is quit old but I think for completeness there is another way how to use associated objects worth mentioning. This solution utilizes @selector and therefore there is no need for any extra variable or constant.

@interface NSObject (CategoryWithProperty)

@property (nonatomic, strong) NSObject *property;

@end

@implementation NSObject (CategoryWithProperty)

- (NSObject *)property {
    return objc_getAssociatedObject(self, @selector(property));
}

- (void)setProperty:(NSObject *)value {
    objc_setAssociatedObject(self, @selector(property), value, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

@end

(inspired by http://www.tuaw.com/2013/04/10/devjuice-better-objective-c-associated-objects/)

For associated storage void * keys, I like this way of doing it:

static void * const kMyAssociatedStorageKey = (void*)&kMyAssociatedStorageKey; 

This avoids having another constant string in the executable, and by setting its value to the address of itself, you get good uniquing, and const behavior (so you'll nominally get a compiler complaint if you do something that would change the value of the key), without any extra unnecessary exported symbols.

Declare a static (compilation unit-scope) variable at the top level of the source file. It may help to make it meaningful, something like this:

static NSString *MYSimulatedString = @"MYSimulatedString";

Quite close. Here is a full example.

.h-file

@interface NSObject (ExampleCategoryWithProperty)

@property (nonatomic, retain) NSArray *laserUnicorns;

@end

.m-file

#import <objc/runtime.h>

static void * LaserUnicornsPropertyKey = &LaserUnicornsPropertyKey;

@implementation NSObject (ExampleCategoryWithProperty)

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, LaserUnicornsPropertyKey);
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, LaserUnicornsPropertyKey, unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

@end

Just like a normal property - accessible with dot-notation

NSObject *myObject = [NSObject new];
myObject.laserUnicorns = @[@"dipwit", @"dipshit"];
NSLog(@"Laser unicorns: %@", myObject.laserUnicorns);

---

Easier syntax

Alternatively you could use @selector(nameOfGetter) instead of creating a static pointer. Why? See https://stackoverflow.com/a/16020927/202451. Example:

- (NSArray *)laserUnicorns {
    return objc_getAssociatedObject(self, @selector(laserUnicorns));
}

- (void)setLaserUnicorns:(NSArray *)unicorns {
    objc_setAssociatedObject(self, @selector(laserUnicorns), unicorns, OBJC_ASSOCIATION_RETAIN_NONATOMIC); 
}

The accepted answer already got it right, but I'd like to add an explanation: the point of the "key" is to get a unique (per process/program) identifier that will not have any accidental collisions.

Imagine the key being declared as NSUInteger: lots of people would use standard values like 0, 1 or 42. Especially if you're using some library or framework, collisions are likely.

So instead, some clever Apple engineer had the idea to declare the key to be a pointer with the intent that you declare a variable and pass the pointer to that variable as a key. The linker will assign that variable an address that is unique within your application and so you are guaranteed to get a unique, collision free value.

In fact, you can pass any value you like, the pointer is not dereferenced (I've tested that). So passing (void *)0 or (void *)42 as the key does work, although it's not a good idea (so please don't do that). For objc_set/getAssociatedObject, all that matters is that the value passed as key is unique within your process/program.