How to store constants at the class level that can be overriden?

Question

I want to write the following in Swift:

class A
{
    class let x = 0
    func print_class_property()
    {
        print(type(of: self).x)
    }
}

class B:A
{
    overriding class let x = 1
}

class C:A
{
    overriding class let x = 5
}

A().print_class_property() // 0
B().print_class_property() // 1
C().print_class_property() // 5

But of course, this doesn’t compile.

Instead, I can demote the class property to a variable instance property that’s overwritten in the subclass initializers, but this allocates storage for x in every instance of A, B, or C. In addition you lose the guarantee that x never changes across the lifetime of the object.

How do I store constants at the class level, that can be shared by all instances of a subclass?

Answer 1

Unfortunately Swift (as for now) cannot have class stored properties that can be overriden. But can have class computed properties, which are overridable.

You can write something like this:

class A
{
    class var x: Int {
        return 0
    }
    func print_class_property()
    {
        print(type(of: self).x)
    }
}

class B:A
{
    override class var x: Int {
        return 1
    }
}

class C:A
{
    override class var x: Int {
        return 5
    }
}

A().print_class_property() //->0
B().print_class_property() //->1
C().print_class_property() //->5

---

ADDITION

If you do not desire such re-evaluation as noted in comments, you may need to have some another static property.

For example:

class A
{
    class var x: SomeLargeObject {
        struct My {
            static let obj = SomeLargeObject("abc", 0)
        }
        return My.obj
    }
    func print_class_property()
    {
        print(type(of: self).x)
    }
}

class B:A
{
    override class var x: SomeLargeObject {
        struct My {
            static let obj = SomeLargeObject("def", 1)
        }
        return My.obj
    }
}

class C:A
{
    override class var x: SomeLargeObject {
        struct My {
            static let obj = SomeLargeObject("ghi", 5)
        }
        return My.obj
    }
}