Instruments says I'm leaking a string, but I can't find it

Question

I used the leaks tool in Instruments to test the code, but the leaks tool cannot seem to find the leak.

At the end of my code, the output of NSLog(@"str count:%d",[str retainCount]); is 3. Why? I don't override the dealloc. [a.name retainCount] is there just one time and I only autorelease str for one time. So str shouldn't leak.

@interface DataMode : NSObject {

    NSString * name;
}

@property (retain) NSString * name;

- initWithName:(NSString * )name_;
@end


@implementation DataMode

@synthesize name;

- initWithName:(NSString * )name_
{
    if ([super init] != nil)
    {
        name = name_;
        return self;
    }
    return nil;
}

@end


- (void) pressed:(id)sender
{
    for( int i = 0;i<10000000;i++)
    {
        NSString * str = [NSString stringWithFormat:@"zhang"];
        DataMode * a = [[DataMode alloc] initWithName:str];
        NSLog(@"a0 count:%d",[a retainCount]);
        NSLog(@"name1 count:%d",[a.name retainCount]);
        NSLog(@"name1 count:%d",[a.name retainCount]);
        NSLog(@"a1 count:%d",[a retainCount]);
        [ a  release];
        NSLog(@"str count:%d",[str retainCount]);
        NSLog(@"str count:%d",[str retainCount]);
    }


}
@end

What troubles were you having in posting a question? Maybe we can help. — Cajunluke, Sep 23 '11 at 14:09
Also, calling `-retainCount` is discouraged, and is particularly useless on NSStrings. I'm sure someone will be along presently to elucidate. — Cajunluke, Sep 23 '11 at 14:10
As a side note, you should copy or at least retain the string object you are assigning to the `name` ivar in your initializer method. — albertamg, Sep 23 '11 at 14:29
Are you using ARC? Garbage collection? Manual memory management? — , Sep 23 '11 at 16:15
retainCount wouldn't compile in ARC and `retainCount` effectively returns `self` in GC.... gotta be manual. — bbum, Sep 23 '11 at 17:58
i think it should have leaks. because [a.name retainCount] add 1 ,But I dont't release it. thus I don't use ARC. — user961298, Sep 24 '11 at 03:40

bbum · Answer 1 · 2011-09-23T17:09:30.600

3

retainCount is useless. Don't call it.

It is not useful for finding leaks as there are much better, more accurate, and less misleading tools available.

There are several problems with your code (but leaking isn't one of them):

NSString* properties should be copy
you don't use the property to set the string value in init, thus the DataMode instances are not retaining their strings.
there is no dealloc method

~~As for the retain counts; I'm surprised it is "3". I'd expect it to be 2bazillionsomething as that is a constant string (and stringWithString: of a constant string just returns the string).~~Since you used stringWithFormat:, the constant string is turned into a non-constant string. If you had used the constant string or stringWithString:, it'd be abazillionsomething (unsigned -1... UINT_MAX...).

In any case, you have:

+1 for stringWithString:
+1 for calling a.name
+1 for calling a.name

+3 overall.

If Instruments is claiming a leak, post a screenshot.

edited Sep 23 '11 at 17:09

answered Sep 23 '11 at 16:13

bbum

162,346
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It’s not a constant string because the OP used `+stringWithFormat:`… and without a format, heh. – Sep 23 '11 at 16:23
Oh, derp. Right. `stringWithString:` does that. `stringWithFormat:` doesn't bother (though it could). – bbum Sep 23 '11 at 17:07
i think it should have leaks. tools can't find it. because [a.name retainCount] add 1 ,But I dont't release it. thus I don't use ARC. – user961298 Sep 24 '11 at 03:45
i think it should have leaks. tools can't find it. because [a.name retainCount] add 1 ,But I dont't release it.The count of str is 3. I only autorelease it a time. thus I don't use ARC. – user961298 Sep 24 '11 at 03:57

Tim · Answer 2 · 2011-09-23T14:15:00.130

I quote the NSObject protocol reference for -retainCount:

This method is typically of no value in debugging memory management issues. Because any number of framework objects may have retained an object in order to hold references to it, while at the same time autorelease pools may be holding any number of deferred releases on an object, it is very unlikely that you can get useful information from this method.

The retain count could be 3 for any number of reasons; if you can't find a leak with the leaks tool, it's likely you don't have a leak. Don't worry about the actual value of the retain count.

If you're really interested in why it's 3, recall that:

The reference from your DataMode object a will likely be held until the closest autorelease pool is drained
You're still holding a reference in the str variable
The NSString class cluster, among others, does some - unusually inexplicable - caching things internally, so you may see a few retains here and there for which nobody can account

almost right on where the retains are coming from.... The DataMode object's presence in an autorelease pool wouldn't affect the retain count of the string. — bbum, Sep 23 '11 at 16:14
The code is confusing; DataMode isn't retaining the string ever given the way the rest of the code is written. It is wrong. — bbum, Sep 25 '11 at 01:51

score 0 · Answer 3 · edited May 23 '17 at 12:19

0

Since you are using the convenience method to create str which is autoreleased you will not see determinate behavior using retain counts in this way.

Check my response to another question and add those methods to DataMode and you should see when the framework releases your objects from the autorelease pool.

Overriding release and retain in your class

edited May 23 '17 at 12:19

Community

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answered Sep 23 '11 at 14:49

EricLeaf

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Instruments says I'm leaking a string, but I can't find it

3 Answers3

retainCount is useless. Don't call it.