Threading and Object Lifetimes with ARC

Question

As I understand ARC, without a strong reference to an object, it is fair game to be collected (since its reference count is 0).

If, in a method in class A, I do this:

ClassB* b = [[ClassB alloc] init];
[b doStuff];

And in doStuff, I do this:

NSThread* t = [[NSThread alloc] initWithTarget:self selector:@selector(theThread) object:nil];
[t start];

The reference count of b appears to be 0 since it went out of scope after the method in class A finishes. However, a thread is currently 'running' in ClassB and will need local resources.

What is the behavior here? Or perhaps, what should the behavior here be to make sure that b stays around until the thread is all finished?

Thanks!

Answer 1

The documentation for initWithTarget:selector:object: says that the thread takes ownership of (keeps a strong reference to) its target. The target will be released when the thread object is destroyed.

Be aware that this can cause a retain cycle, if the target also owns the NSThread .