NEW: Subscribe to Hacking with Swift+ and accelerate your learning! >>

How to copy objects in Swift using copy()

Swift version: 5.2

Paul Hudson    @twostraws   

There are two main complex data types in Swift – objects and structs – and they do so many things similarly that you'd be forgiven for not being sure exactly where they differ. Well, one of the key areas is down to copying: two variables can point at the same object so that changing one changes them both, whereas if you tried that with structs you'd find that Swift creates a full copy so that changing the copy does not affect the original.

Having lots of objects point at the same data can be useful, but frequently you'll want to modify copies so that modifying one object doesn't have an effect on anything else. To make this work you need to do three things:

  • Make your class conform to NSCopying. This isn't strictly required, but it makes your intent clear.
  • Implement the method copy(with:), where the actual copying happens.
  • Call copy() on your object.

Here's an example of a Person class that conforms fully to the NSCopying protocol:

    class Person: NSObject, NSCopying {
    var firstName: String
    var lastName: String
    var age: Int

    init(firstName: String, lastName: String, age: Int) {
        self.firstName = firstName
        self.lastName = lastName
        self.age = age

    func copy(with zone: NSZone? = nil) -> Any {
        let copy = Person(firstName: firstName, lastName: lastName, age: age)
        return copy

Note that copy(with:) is implemented by creating a new Person object using the current person's information.

With that done, you can test out your copying like this:

let paul = Person(firstName: "Paul", lastName: "Hudson", age: 36)
let sophie = paul.copy() as! Person

sophie.firstName = "Sophie"
sophie.age = 6

print("\(paul.firstName) \(paul.lastName) is \(paul.age)")
print("\(sophie.firstName) \(sophie.lastName) is \(sophie.age)")
Hacking with Swift is sponsored by Paw

SPONSORED Use Paw to build, test and describe web APIs. Paw has a lightning fast native macOS interface to compose requests, collaborate in real-time on API specs, and generate client code for your applications. You can import and export API definitions.

Discover Paw for Mac

Sponsor Hacking with Swift and reach the world's largest Swift community!

Available from iOS 7.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

Buy Pro Swift Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift (Vapor Edition) Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Server-Side Swift (Kitura Edition) Buy Beyond Code

Was this page useful? Let us know!

Average rating: 3.9/5

Link copied to your pasteboard.