How to use the same category in multiple classes?

Question

I have stack category for NSMutableArray that I use in Class A

@implementation NSMutableArray (StackUtil)

- (void)push:(id)stackObject {

 [self addObject:stackObject];
}
- (id)pop {

 id retVal = [[self lastObject]retain];
 [self removeLastObject];
 return [retVal autorelease];
}

@end

How do I properly include this category for for Class B which is unrelated to Class A? Also I noticed that when I #import Class A into the header for Class C the category methods work, but I get the " object may not respond to push message" warning. Could someone clear up how categories are "reused" and why they have names (StackUtil in this case), and how they are used.

Answer 1

You should have a corresponding @interface NSMutableArray (StackUtil) in a header file that declares the category. Importing that header should be enough to confer use of your new methods onto any NSMutableArray in the scope of the import.

@interface NSMutableArray (StackUtil)
- (void) push:(id)stackObject;
- (id) pop;
@end

Certainly the @interface, and usually the @implementation, should be in files of their own, independent of your classes A, B and C, since they are general purpose additions, not something belonging to one of those client classes.