retainCount shows MaxInt

Question

After trying to print retainCount of object I get 2147483647. Why do I get such a result? It should be 1, shouldn't?

NSString *myStr = [NSString new];
NSUInteger retCount = [myStr retainCount];
NSLog(@"refCount = %u", retCount);

2011-09-08 17:59:18.759 Test[51972:207] refCount = 2147483647

I use XCode Version 4.1. Tried compilers GCC 4.2 and LLVM GCC 4.2 - the same result. Garbage Collection option was set to unsupported.

Answer 1

NSString is somewhat special when it comes to memory management. String literals (something like @"foo") are effectively singletons, every string with the same content is the same object because it can't be modified anyway. As [NSString new] always creates an empty string that cannot be modified, you'll always get the same instance which cannot be released (thus the high retainCount).

Try this snippet:

NSString *string1 = [NSString new];
NSString *string2 = [NSString new];
NSLog(@"Memory address of string1: %p", string1);
NSLog(@"Memory address of string2: %p", string2);

You'll see that both strings have the same memory address and are therefore the same object.

Answer 2

This doesn't directly answer your question, but retainCount is not really all that useful and should not be used for testing. See this SO post for details. When to use -retainCount?

Answer 3

While NSString's are an odd case (there are others in the framework) you might also run across this in other clases - it's one of the ways of creating a singleton object.

A singleton only exists once in the app and it's pretty important that it never gets released! Therefore, it will overwrite some methods of NSObject including (but not limited to):

- (id)release {
    // Ignore the release!
    return self;
}

- (NSUInteger)retainCount {
    // We are never going to be released
    return UINT_MAX;
}

This object can never be released and tells the framework that it's a singleton by having a ludicrously high retain count.

Checkout this link for more information about singleton objects.