How do I subclass a struct in Swift?

Question

struct MyRect : CGRect {...}

Is it possible at all in swift to subclass a Structure?

I have already found out on Apple official website an example: This example extends the CGRect structure to contain a computed area property:

extension CGRect {
    var area: CGFloat {
        return width * height
    }
}
let rect = CGRect(x: 0.0, y: 0.0, width: 10.0, height: 50.0)
let area = rect.area

How can i define a subclass of a structure ?

Answer 1

It is not possible to subclass a struct in Swift, only classes can be subclassed. An extension is not a subclass, it's just adding additional functionality on to the existing struct, this is comparable to a category in Objective-C.

Please see the Apple Swift documentation here to read about the differences between struct and class.

Answer 2

You cannot subclass structures in Swift but you can, in some sense, mimic subclassing by creating a structure that simply vends out the structure that you wish to subclass. For example, if you wanted to subclass Calendar (a structure), you could create a structure that returns configured calendars.

struct WorldCalendar {
    let american1: Calendar = {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }()
    
    static let american2: Calendar = {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }()
    
    static func american3() -> Calendar {
        var c = Calendar(identifier: .gregorian)
        c.timeZone = .current
        c.locale = Locale(identifier: "en_US_POSIX")
        return c
    }
}

let worldCalendar = WorldCalendar()
let cal1 = worldCalendar.american1

let cal2 = WorldCalendar.american2

let cal3 = WorldCalendar.american3()

How you vend the configured structures (as an instance property, a static property, a static function, etc.) is just a matter of what better fits into your application and, frankly, personal preference.

Answer 3

Since Swift 5.1 it is now possible to do something similar to what you asked for: using composition, with KeyPath and dynamicMemberLookup. Have a look: https://medium.com/@acecilia/struct-composition-using-keypath-and-dynamicmemberlookup-kind-of-struct-subclassing-but-better-6ce561768612?source=friends_link&sk=da479578032c0b46c1c69111dfb6054e