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I know that how ARC and MRC works. but I am confuse while testing the below code. I don't know why this happen. Why the retain count is different in debug mode and in running mode for the same question?

    NSMutableArray *a = [NSMutableArray array];
    [a addObject:@"abc"];
    NSLog(@" 1 Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)a));
    __weak  NSMutableArray *b = a;
    NSLog(@" 2 Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)a));
    a = nil;
    NSLog(@" 3 Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)b));
    [b addObject:@"xys"];
    NSLog(@" 4 Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)b));

When I run the app in running mode the app crash on line NSLog(@" 3 Retain count is %ld", CFGetRetainCount((__bridge CFTypeRef)b)); that I understand as the b is weak reference of a. and object a get release when assign nil to it. but before that if shows the out put of first two line as the the below image. Which is also correct.

enter image description here

But When the app is debug mode (Means we have set break point and debug) then the app didn't crash and also show retain count 2 on each line. as the below image.

enter image description here

Does any one having idea why this happen? Why the same code gives two different retain value for different mode?

Crazy Developer
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    Don't rely on retainCount for anything. Especially with ARC, you can't tell where `retain`s and `release`s are inserted. In release mode, the optimizer will attempt to avoid extraneous `retain`s and `release`s, so the numbers you see will often be different. – Avi Feb 03 '16 at 08:52
  • Oh dear. Baton down the hatches! – trojanfoe Feb 03 '16 at 08:59

1 Answers1

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The very first sentence in the documentation for the retainCount method says:

Do not use this method.

And later:

it is very unlikely that you can get useful information from this method

Just don't use it.

As trojanfoe pointed out in the comment, the CFGetRetainCount that is used in the given code has a similar (maybe somewhat lesser) disclaimer posted to it.

I always understood this as "it may be useful, but the value might not be what you think it should be. Don't draw any conclusions by looking at the absolute values, because we might do every magic we want to do with it, and it's not your business".

Eiko
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  • And the documentation to `CFGetRetainCount()` (the method the OP is actually using) says "This function may useful for debugging memory leaks. You normally do not use this function, otherwise.". – trojanfoe Feb 03 '16 at 09:14
  • yes, I know that we should not use this method. but my point is that what is the issue with this method. Why it's providing wrong retainCount ? – Crazy Developer Feb 04 '16 at 07:03
  • It's probably not wrong. But there might be more (or less) retains or releases going than you might think. And they may happen at a different time as well due to optimizations the compiler makes. And then, who knows what the debug mode changes? But I agree, this semantical change seems odd! – Eiko Feb 04 '16 at 19:57