Do loops and convenience methods cause memory peaks with ARC?

Question

I\'m working with ARC and seeing some strange behavior when modifying strings in a loop.

In my situation, I\'m looping using NSXMLParser delegate callbacks, but I see t

Answer 1

While you're hunting memory-related bugs, you should note that @"" and @" Hello" will be immortal objects. You can think about it as a const, but for objects. There will be one, and only one, instance of this object in memory the entire time.

As @bbum pointed out, and you verified, the @autoreleasepool is the correct way to deal with this in a loop.

In your example with the @autoreleasepool and NSMutableString, the pool doesn't really do much. The only mortal object inside the loop is your mutableCopy of @"", but that will only be used once. The other case is just an objc_msgSend to a persisting object (the NSMutableString), which only references an immortal object and a selector.

I can only assume the memory build up is inside Apple's implementation of NSMutableString, although I can wonder why you'd see it inside the @autoreleasepool and not when it's absent.

Answer 2

You are polluting the autorelease pool with tons and tons of autoreleased objects.

Surround the internal part of the loop with an autorelease pool:

for (...) {
    @autoreleasepool {
        ... your test code here ....
    }
}