Struct with copy on write semantics

CopyOnWrite.swift

struct Person {
    fileprivate final class Storage {
        var name: String
        var age: UInt8
        
        init(name: String, age: UInt8) {
            self.name = name
            self.age = age
        }
    }
    
    private var storage: Storage
    
    init(name: String, age: UInt8) {
        storage = .init(name: name, age: age)
    }
}

extension Person.Storage {
    func copy() -> Person.Storage {
        .init(name: name, age: age)
    }
}

extension Person {
    private var storageForWriting: Person.Storage {
        mutating get {
            if !isKnownUniquelyReferenced(&storage) {
                storage = storage.copy()
            }
            return storage
        }
    }
    
    var name: String {
        get { storage.name }
        set { storageForWriting.name = newValue }
    }
    
    var age: UInt8 {
        get { storage.age }
        set { storageForWriting.age = newValue }
    }
}



// Protocol
protocol Storageable: AnyObject {
    func copy() -> Self
}

protocol CopyOnWriteable {
    associatedtype Storage: Storageable
    
    var storage: Storage { get set }
    var storageForWriting: Storage { mutating get }
}

extension CopyOnWriteable {
    var storageForWriting: Storage {
        mutating get {
            if !isKnownUniquelyReferenced(&storage) {
                storage = storage.copy()
            }
            return storage
        }
    }
}

struct Car: CopyOnWriteable {
    typealias Storage = CarStorage
    
    final class CarStorage: Storageable {
        var year: UInt
        var color: String
        
        func copy() -> Self {
            .init(year: year, color: color)
        }
        
        init(year: UInt, color: String) {
            self.year = year
            self.color = color
        }
    }
    
    var storage: CarStorage
    
    init(year: UInt, color: String) {
        storage = .init(year: year, color: color)
    }
    
    var year: UInt {
        get { storage.year }
        set { storageForWriting.year = newValue }
    }
    
    var color: String {
        get { storage.color }
        set { storageForWriting.color = newValue }
    }
}