Why does one of these methods cause a leak and the other not

Question

In my app i get a memory leak in

first

-(void)connectionDidFinishLoading:(NSURLConnection *)connection {  
    result = [[NSString alloc] initWithBytes:[webData mutableBytes]
                                      length:[webData length]
                                    encoding:NSUTF8StringEncoding];  
    [webData release];  
}

second

-(void)connectionDidFinishLoading:(NSURLConnection *)connection {  
    NSString * result = [[NSString alloc] initWithBytes:[webData mutableBytes]
                                                 length:[webData length]
                                               encoding:NSUTF8StringEncoding];
    [webData release];
}

in my first process I didn't get a memory leak(Declare a string object globally and I didn't release it)

In my second process i get a memory leak in string object.

1. Value stored to 'result' during its initialization is never read

2. Method returns an Objective-C object with a +1 retain count (owning reference)

3. Object allocated on line 124 and stored into 'result' is no longer referenced after this point and has a retain count of +1 (object leaked)

what is the difference?

Answer 1

In first case analyzer expects you to release your initted result string in, say, - (void)dealloc method of the class. If you don't - you'll get a leak as well (you'll see the leak if you run your app with via Instruments app with Leaks instrument added.

In second case you're creating a local variable in method scope where it should be released as well since you won't have a reference of it in any other method (i.e. if you try access result variable in other method you'll get unknown identifier error).

Answer 2

The difference is that the global String variable still points to the memory location containing the object, whereas the local variable goes out of the scope, so there is no pointer anymore and the memory has not been released.

That said, one has to understand that even situations technically not being leaks, could be seen as such, especially if one does not intend to use the global variable and just holds the reference for the lifetime of the application.