Can you explain what is meant by a 'zombie object' in programming?

In programming, the term "zombie object" usually refers to an object that has already been destroyed or freed from memory, but still has a reference to it. Such an object is no longer valid, and attempting to access it can lead to errors, such as a program crash.

In iOS (Objective-C or Swift), this can happen if an object has been deallocated, but a pointer to it remains. Attempting to access this pointer may cause the program to crash with an EXC_BAD_ACCESS error.

Example: if you have a weak reference to an object, and the object is deleted, the weak reference automatically becomes nil. But if an unsafe (unprotected) reference is used, it may point to already freed memory — this is a zombie object.

To debug such issues, Xcode provides a tool called Zombies, which helps identify accesses to already freed objects.