memory management - How can I demonstrate a zombie object in Swift?

Question

I've read How to demonstrate memory leak and zombie objects in Xcode Instruments? but that's for objective-c. The steps don't apply.

From reading here I've understood zombies are objects which are:

• deallocated
• but something pointer is still trying to point to them and send messages to them.

not exactly sure how that's different from accessing a deallocated object.

I mean in Swift you can do:

var person : Person? = Person(name: "John")
person = nil
print(person!.name)

Is person deallocated? Yes!

Are we trying to point to it? Yes!

So can someone share the most common mistake which leads to creating a dangling pointer?

Answer 1

Here's zombie attack in under 15 lines of code:

class Parent { }

class Child {
    unowned var parent: Parent // every child needs a parent

    init(parent: Parent) {
        self.parent = parent
    }
}

var parent: Parent? = Parent()
let child = Child(parent: parent!) // let's pretend the forced unwrap didn't happen
parent = nil // let's deallocate this bad parent
print(child.parent) // BOOM!!!, crash

What happens in this code is that Child holds an unowned reference to Parent, which becomes invalid once Parent gets deallocated. The reference holds a pointer to the no longer living parent (RIP), and accessing that causes a crash with a message similar to this:

Fatal error: Attempted to read an unowned reference but object 0x1018362d0 was already deallocated2018-10-29 20:18:39.423114+0200 MyApp[35825:611433] Fatal error: Attempted to read an unowned reference but object 0x1018362d0 was already deallocated

Note The code won't work in a Playground, you need a regular app for this.