NEW: Join my free 100 Days of SwiftUI challenge today! >>

What is copy on write?

Swift version: 5.1

Paul Hudson    @twostraws   

Copy on write is a common computing technique that helps boost performance when copying structures. To give you an example, imagine an array with 1000 things inside it: if you copied that array into another variable, Swift would have to copy all 1000 elements even if the two arrays ended up being the same.

This problem is solved using copy on write: when you point two variables at the same array they both point to the same underlying data. Swift promises that structs like arrays and dictionaries are copied as values, like numbers, so having two variables point to the same data might seem to contradict that. The solution is simple but clever: if you modify the second variable, Swift takes a full copy at that point so that only the second variable is modified - the first isn't changed.

So, by delaying the copy operation until it's actually needed, Swift can ensure that no wasted work is done.

Warning: copy on write is a feature specifically added to Swift arrays and dictionaries; you don't get it for free in your own data types.

LEARN SWIFTUI FOR FREE I have a massive, free SwiftUI video collection on YouTube teaching you how to build complete apps with SwiftUI – check it out!

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 OUR BOOKS
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: 3.0/5