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

This is not a dangling pointer or a zombie. When you use ! you're saying "if this is nil, then crash." You should not think of person as a pointer in Swift. It's a value. That value may be .some(T) or it may be .none (also called nil). Neither of those is dangling. They're just two different explicit values. Swift's nil is nothing like null pointers in other languages. It only crashes like null pointers when you explicitly ask it to.

To create zombies, you'll need to be using something like Unmanaged. This is extremely uncommon in Swift.

Answer 2

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.