Reference Count or Retain Count Issue (Memory Management)

Question

The below code works fine, you can run it at your system for confirmation.

My question is, as you can see, the dealloc method is only called when the retain count reaches zero, meaning the memory is freed for the RetainTracker object. However, the issue is when I log the retain count in dealloc method, it still shows a retain count of 1. Why is this?

Here is my code:

#import <Foundation/Foundation.h>
@interface RetainTracker : NSObject

@end

@implementation RetainTracker

- (id)init {

    if (self = [super init]) {
        NSLog(@"init: Retain count of %lu",(unsigned long)[self retainCount]);
    }
    return self;
}

- (void)dealloc {

    NSLog(@"Dealloc called bye bye!==>%lu",(unsigned long)self.retainCount);
    [super dealloc];
}

@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        // insert code here...
        RetainTracker *myRetainTracker = [RetainTracker new];

        [myRetainTracker retain]; // count-->2
        NSLog(@"The retain count is ==>%lu",(unsigned long)myRetainTracker.retainCount);

        [myRetainTracker release];// count -->1
        NSLog(@"The retain count is ==>%lu",(unsigned long)myRetainTracker.retainCount);

        [myRetainTracker retain];// count -->2
        NSLog(@"The retain count is ==>%lu",(unsigned long)myRetainTracker.retainCount);

        [myRetainTracker release];// count -->1
        NSLog(@"The retain count is ==>%lu",(unsigned long)myRetainTracker.retainCount);

        [myRetainTracker release];// count -->0
        NSLog(@"The retain count is ==>%lu",(unsigned long)myRetainTracker.retainCount);

    }
    return 0;
}

Here are the logs:

init: Retain count of 1
The retain count is ==>2
The retain count is ==>1
The retain count is ==>2
The retain count is ==>1
Dealloc called bye bye!==>1
The retain count is ==>1

What you want to do instead of printing out the retain count, is to run the ObjectAlloc Instrument against your code, with reference counting turned on (click on the (i) of the instrument and re-start the instrument recording your app). Then you can go to a specific instance of an object, and get the full retain/release history for it - that can help figure out if something is retaining the object that should not be. — Prashant Tukadiya, May 21 '16 at 08:06

Sulthan · Answer 1 · 2016-05-22T07:02:55.113

This is not surprising, the release code probably looks like this (in pseudocode):

- (void)release {
   if (retainCount > 1) {
      retainCount -= 1
   } else {
      // no need to set the retainCount to 0 here,
      // the object now ends its existence
      [self dealloc]
   }
}

Also, your last NSLog is actually accessing an object that no longer exists which can result in a crash.

Note that the value read from retainCount should never be relied upon. It's just an implementation detail. It is much safer to think about retain and release as transfer of ownership.

From retainCount documentation:

Do not use this method.

and

This method is of no value in debugging memory management issues. (...) it is very unlikely that you can get useful information from this method.

score -1 · Answer 2 · answered May 21 '16 at 10:31

-1

Switch your project to ARC. Seriously. I have forgotten everything I knew about retain / release three years ago. Other than that, the retain count of an object in the process of being released is meaningless. It's pointless wondering about it.

answered May 21 '16 at 10:31

gnasher729

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Reference Count or Retain Count Issue (Memory Management)

2 Answers2