Question

I created an empty iOS app on Xcode 4.4.1, and did the following:

NSNumber *n1 = @1;
NSNumber *n2 = @2;
NSNumber *n3 = @3;
NSNumber *n100 = @100;

NSString *s = @"haha";
NSArray *a = @[n1, s];
NSDictionary *d = @{ @"ha" : @1, @3 : @"hello" };

NSLog(@"retain count of @1 is %i", [n1 retainCount]);
NSLog(@"retain count of @2 is %i", [n2 retainCount]);
NSLog(@"retain count of @3 is %i", [n3 retainCount]);
NSLog(@"retain count of @100 is %i", [n100 retainCount]);

NSLog(@"retain count of @\"haha\" is %i", [s retainCount]);

NSLog(@"retain count of array literal is %i", [a retainCount]);
NSLog(@"retain count of dictionary literal is %i", [d retainCount]);

and the result is:

retain count of @1 is 10
retain count of @2 is 4
retain count of @3 is 5
retain count of @100 is 1
retain count of @"haha" is -1
retain count of array literal is 1
retain count of dictionary literal is 1

so the retain count of array literal and dictionary literal is 1, and string literal is said to exist for the whole app’s running, so that’s why it is -1 (probably meaning MAX unsigned int), but the retain count of @1 actually come out as 7, 8, and 10 at different times. Is there a rule to it? I found that I can do [n1 retain] and [n1 release] as well, and it will increase and decrease the retain count accordingly.

Answer 1

I think this points to the reason why: every time @1 is encountered, it is something like [[[NSNumber alloc] initWithInt:1] autorelease], and it appears that it returns the same object. So when the following is done:

NSMutableArray *arr = [[NSMutableArray alloc] init];
for (int i = 0; i < 300; i++) {
    [arr addObject:@1];
}

the retain count actually becomes 610. And it is the same if the following is done:

NSMutableArray *arr = [[NSMutableArray alloc] init];
for (int i = 0; i < 300; i++) {
    [arr addObject:[[[NSNumber alloc] initWithInt:1] autorelease]];
} 

(Update: it is 610, because the retain count is increased by 1 by the usage of @1, and another time when it is retained by the array, so it is 600 times total). It suggests that any usage of @1 any where will increase the retain count, and this NSNumber object is probably placed in the autorelease pool the same equal number of times.