Question

from researching memory management on objective C, I have a question about reference count:

I have an object name obj_number. I have alloc it

    obj_number = [[NSNumber alloc] init];

and then in another method (named A)of this class, I set

    obj_number = [dataset objectAtIndex:0];

go out this method A dataset will be autorelease. I checked this after go out this method obj_number also can’t be access, maybe it also be deallocate.

I see if an object just be under one ownership, so if parent deallocate it also be deallocate, but in my case I suppose that obj_number under 2 ownership (retain count may equals to 2), so why running out method A it be released?

Answer 1

You need to provide a setter method for obj_number, which you can do by providing a @property in the .h file and a @sythesize in the .m file. The setter method will ensure that the old value is released and the new value is retained.

MyClass.h:

@interface MyClass : NSObject
{
    NSNumber *_obj_number;    // Use a different name for the ivar and property!
    ...
}

@property (retain, nonatomic, readwrite) NSNumber *obj_number;

...

@end

MyClass.m:

@implementation MyObject
@synthesize obj_number = _obj_number;

...

- (void)dealloc
{
    self.obj_number = nil;
    ...
    [super dealloc];
}

And when assigning to obj_number, you must relinquish ownership:

- (void)someMethod
{
    // We own myNumber, so release
    NSNumber *myNumber = [[NSNumber alloc] initWithUnsigned:12];
    self.obj_number = myNumber;
    [myNumber release];

    // However this is easier:
    self.obj_number = [NSNumber numberWithUnsigned:12];
}

- (void)someOtherMethod
{
    // We don't own [dataset objectAtIndex:0] so no need to release
    self.obj_number = [dataset objectAtIndex:0];
}