oop - A Swift protocol requirement that can only be satisfied by using a final class

Question

I'm modeling a owner/ownee scheme on Swift:

class Owner<T: Ownee> {
     // ...
}

protocol Ownee {
    var owner: Owner<Self> { get }
}

Then I have a pair of classes professor/student that adhere to the modeled types above:

class Professor: Owner<Student> {
    // ...
}

class Student: Ownee {
    let professor: Professor
    var owner: Owner<Student> {  // error here (see below)
        return professor
    }

    init(professor: Professor) {
        self.professor = professor
    }
}

However I get the following error on the definition of var owner in the Student class:

Protocol 'Ownee' requirement 'owner' cannot be satisfied by a non-final class ('Student') because it uses 'Self' in a non-parameter, non-result type position

I'm trying to understand what's the cause for this error, why making the class Student final would fix it, and if there's some workaround to be able to model this differently, without making this class final. I've googled about that error, but haven't found much so far.

Answer 1

The Error is correct. You have to make your class final, since no subclasses could conform your protocol Ownee.

Consider this subclass:

class FirstGradeStudent: Student {
   // inherited from parent
   // var owner: Owner<Student> {
   //     return professor
   //  }
}

As you can see, it would have to implement var owner: Owner<Student> because of his parent, but it should be implementing var owner: Owner<FirstGradeStudent> instead, because the protocol contains var owner: Owner<Self> { get } and in this case Self would be FirstGradeStudent.

Workaround

1: Define a superclass to Ownee, it should be used by Owner:

class Owner<T: OwneeSuper> {
    // ...
}

protocol OwneeSuper {}    
protocol Ownee: OwneeSuper {
    associatedtype T: OwneeSuper
    var owner: Owner<T> { get }
}

OwneeSuper is just a workaround to overcome this problem, otherwise we would just be using:

protocol Ownee {
    associatedtype T: Ownee
    var owner: Owner<T> { get }
}

2. In classes that conform to Ownee, you must turn the abstract type of the associatedtype into a concrete class by defining a typealias:

class Student: Ownee {
    typealias T = Student // <<-- define the property to be Owner<Student>
    let professor: Professor
    var owner: Owner<T> { 
        return professor
    }

    init(professor: Professor) {
        self.professor = professor
    }
}

3. Subclasses can now make use of the property, which will be of your defined type:

class FirstGradeStudent: Student {
    func checkOwnerType() {
        if self.owner is Owner<Student> { //warning: 'is' test is always true
            print("yeah!")
        }
    }
}