NSNumber weirdo .... ?

Question

Hi,

for a day now I stare at the following routine and can't get my grips around it. I have a class such as:

MyClass.h

...
@interface MyClass : NSObject {
    NSNumber *myNumber1;
    NSNumber *myNumber2;
}

@property (nonatomic, retain) NSNumber *myNumber1;
@property (nonatomic, retain) NSNumber *myNumber2;

@end

and the m-file

#import "MyClass.h"
@synthesize myNumber1, myNumber2; 
@implementation MyClass

-(id) init {
    self = [super init];

    NSLog(@"Retain Counts myNumber1: %d, myNumber2: %d", [myNumber1 retainCount], [myNumber2 retainCount]);

    myNumber1 = [NSNumber inbitWithint:10];
    myNumber2 = [NSNumber inbitWithint:2];

    NSLog(@"Retain Counts myNumber1: %d, myNumber2: %d", [myNumber1 retainCount], [myNumber2 retainCount]);

    return self;
}
...

I use this class as a globals container and instantiate it from every other class in my app using a

MyClass *myGlobals = [[MyClass alloc] init];

===>>> the weirdo >>>

Running the routine I have the following facinating console output:

Retain Counts (before) - myNumber1: 0, myNumber2: 0
Retain Counts (after)  - myNumber1: 1, myNumber2: 26

How can that be?

Answer 1

If this is your actual code, it shouldn't work at all, it should simply crash. If not, you should cut and paste your actual code.

However, a couple of things.

First, if you use an NSNumber as an ivar, like anything else, you must take ownership of it. If you plan to use a convenience constructor, you must either assign it using the property on self, or send it an explicit retain message.

Second, you typed something like initWithInt: here. If you are actually using that message, then you have never actually allocated the number in the first place. You must replace this with one of the following:

myNumber1 = [[NSNumber alloc] initWithInt:10]; // explicitly create the number
myNumber1 = [[NSNumber numberWithInt:10] retain]; // take ownership through retain
[self setMyNumber1:[NSNumber numberWithInt:10]]; // use the property accessors to deal with ownership and the convenience constructor to create the number

However you choose to do it, you must at some point take ownership of the object.

Answer 2

You should never even look at retainCount as it carries little significance anyway. The reason you see the count of 26 is that probably somewhere in the frameworks (or even in your own app) other instances of NSNumber exist that are created with the same int 2. An NSNumber is immutable, and so as an optimization NSNumber probably just gives you back an instance it has created earlier. As long as you don't look at retainCount it doesn't matter to you.

Answer 3

Do not call `retainCount`

The return value of retainCount can only be considered accurate if you are calling it on an instance of a class you wrote and you have never ever not even once passed said instance to any API provided by the system.

The absolute retain count is an implementation detail for which you might not have any control.

Assuming, for the moment, that your code was typed and, thus, the errors are not really in your original code...

NSNumber caches a subset of common values. That is, for some numeric values, there is a single instance of NSNumber that is returned for all requests to retrieve an NSNumber instance wrapping that number.