14.4. Detecting zombies

Once an object is deallocated, the Objective-C runtime is free to reuse the associated memory for other memory allocation requests. The object doesn’t exist anymore. It’s “dead” compared to a “live” object that has one or more active owners.

When you attempt to access a dead object, you may get lucky—the object may still be sitting in memory and you won’t notice a difference (when an object is deallocated, the bookkeeping is updated, but what is in memory isn’t overwritten until that memory is reused). But the more likely scenario is that the Objective-C runtime has reassigned the associated memory for another task and attempting to access the dead object will cause an application crash.