Reference type inside value type

Question

I am exploring Swift value types particularly structs to get a better understanding of it\'s uses in different scenario. I was amazed to see how enum can be used to build Bi

Answer 1

You can test this in a playground:

class B {
    var data: Int = 0

    deinit {
        print("deallocated!")
    }
}

struct A {
    var b = B()
}

var a1: A? = A()
var a2: A? = A()
var a3: A? = a1

// Do the two instances of struct A share the same instance of class B?
a1?.b === a2?.b // false

// Do copies of instances of struct A share the same instance of class B?
a1?.b === a3?.b // true

// When will deinit be called?
a1 = nil    // Not yet, a3 still holds a strong reference to the shared instance of class B
a3 = nil    // Now! There are no longer any strong references to the shared instance of class B, so it is deallocated.

Answer 2

Every struct A copy will share the same reference to B. Every new struct A, created from scratch, will hold a completely new B object.

The B.deint will be called when there are zero strong references to it (e.g., your var b is one of these strong references). For instance, if only A values hold references to a given B object then those will need to got out of scope to zero all references to this object (or their boxed copies be deallocated as well, but this might be a topic for another question.)

Code Design. If these all sounds too confusing and is blocking your app progress (with no real practical benefit so far), you might consider refactoring B to a struct as well. For instance, even Apple recommends considering value types to design your model layer. This blog post might also help make up your mind.