NSNumber's copy is not allocating new memory

Question

I am implementing a copyWithZone method for a custom A class, in which a NSNumber pointer was declared as (retain) property

@class A <NSCopying>
{
  NSNumber *num;
}
@property (nonatomic, retain) NSNumber *num; // synthesized in .m file

-(id) copyWithZone:(NSZone*) zone {

   A *new = [[A alloc] init];
   new.num = [num copy];
   return new;
}

When I debug, I always find new.num is the same address as the self.num.

Even if I use

new.num = [NSNumber numberWithFloat: [num floatValue]];

I still get the same address. In the end, I have to use

new.num = [[[NSNumber alloc] initWithFloat:[num floatValue]] autorelease]

to achieve the result I want. I am just wondering why NSNumber complies to but does not return a new memory address when copied?

Thanks

Leo

Answer 1

NSNumber is immutable. Making a copy is pointless and, thus, the frameworks just return self when copy is invoked.

If a class implements NSCopying, you should mark the property as copy (not retain). -copy on immutable classes (NSString) will simply return a reference to the object (w/a bumped retain count). If passed a mutable instance, it'll be copied to an immutable instance. This prevents an external party from changing the state behind your object's back.