ARC memory leaks

Question

I am experiencing memory leaks linked to NSMutableArray's in a project configured to use ARC, which I thought was supposed to handle these things for you.

The following code is triggering leaks of NSNumbers:

NSMutableArray *myArray = [[NSMutableArray alloc] init];

NSNumber  *myNumber = [NSNumber numberWithFloat:10];

[myArray addObject:myNumber];

Running the last line gives the following in the debugger:

objc[1106]: Object 0x765ffe0 of class __NSCFNumber autoreleased with no pool in place - just leaking - break on objc_autoreleaseNoPool() to debug

Aside from that, the object appears to be correctly added to the mutable array,

Am I doing something obvious wrong?

Note: There is one class in the project which I could not get to work with ARC, and so I excluded it from ARC using the compiler flag -fno-objc-arc. However, the leaks are occurring in other classes that are using ARC. Not sure if that is related.

Many thanks for your help.

Answer 1

You're probably running this code on a background thread, and don't have an autorelease pool in place. ARC will still autorelease objects for you on occasion, and if you're calling into Apple frameworks, they may still be non-ARC, so they definitely could be autoreleasing objects for you. So you still need an autorelease pool in place.

Cocoa creates an autorelease pool for you on the main thread, but doesn't do anything for you on background threads. If you're going to kick something off onto a background thread without using NSOperation or something, you'll want to wrap that thread in an @autoreleasepool, like so:

- (void)doSomething {
    [self performSelectorInBackground:@selector(backgroundSomething)];
}

- (void)backgroundSomething {
    @autoreleasepool {
        NSLog(@"Here I am in the background, doing something.");
        myArray = [[NSMutableArray alloc] init];
        // etc.
    }
}

Answer 2

Very likely you have defined the NSMutableArray as a static variable. When you do that, you fall outside the bounds of any autorelease pool, since static definitions are activated outside of any runloop. ARC is not magically, it simply automates memory management calls within the framework of the existing retain/release framework and so cannot help in those cases.

The solution is to initialize the static variable somewhere in a class so that your mutable array is built within the runloop.