Do Objective-C Category names do anything?

Question

A class can be extended in Objective C using a category such as:

@interface NSString (CategoryName)
-(NSString *)myFabulousAddition;  // a fabulous additional method
@end

/////////////////////////////
@implementation NSString (CategoryName)

-(NSString *)myFabulousAddition {
    // do something fabulous...

}
@end

In this small example, I would be adding the method myFabulousAddition to NSString. I could then call it by [anNSString myFabulousAddition] just as if it were part of the NSString set of methods. Great and useful.

In the Apple documents regarding Categories, the docs state:

There’s no limit to the number of categories that you can add to a class, but each category name must be different, and each should declare and define a different set of methods.

What if you have something like this:

@interface NSString (CategoryName)
-(NSString *)myFabulousAddition;  // a fabulous additional method
@end

@interface NSString (ANOTHERCategoryName)
-(NSString *)myFabulousAddition;  // a DIFFERENT fabulous additional method 
                                  // BUT with same name as the other category
@end


/////////////////////////////

@implementation NSString (CategoryName)

-(NSString *)myFabulousAddition {
    // do something fabulous...

}
@end
@implementation NSString (ANOTHERCategoryName)

-(NSString *)myFabulousAddition {
    // do something equally fabulous, but DIFFERENT...

}
@end

The lack of a name in the parenthesis indicates that the form is an extension to the class, like so:

@interface MyObject ()   // No name -- an extension vs category to MyObject
- (void)setNumber:(NSNumber *)newNumber;
@end

Does the category name have any meaning to the compiler or linker? Is the category name part of the method signature in anyway or is it part of a primitive namespace? If the category name is meaningless, how do you know if you are about to stomp on another method and get undefined behavior?

Unknown Unknown · Accepted Answer · 2011-04-16T19:47:49

The way to avoid stomping on methods is to prefix your category method names, like this:

@interface NSString (MyCompanyCategoryName)

- (NSString *)MYCO_fabulousAddition;

@end

If you get a collision of method names from different categories, then which one 'wins' at run time is completely undefined.

The name of a category is almost entirely useless, with the exception being that the nameless category (i.e. ()) is reserved for class extensions. Methods from class extensions are supposed to be implemented in the class' main @implementation.

Do Objective-C Category names do anything?

5 Answers