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:
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?
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.
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