BLACK FRIDAY: Save 50% on all our Swift books and bundles! >>

When should you use a computed property or a stored property?

Paul Hudson    @twostraws   

Updated for Xcode 13.2

Properties let us attach information to structs, and Swift gives us two variations: stored properties, where a value is stashed away in some memory to be used later, and computed properties, where a value is recomputed every time it’s called. Behind the scenes, a computed property is effectively just a function call that happens to belong to your struct.

Deciding which to use depends partly on whether your property’s value depends on other data, and partly also on performance. The performance part is easy: if you regularly read the property when its value hasn’t changed, then using a stored property will be much faster than using a computed property. On the other hand, if your property is read very rarely and perhaps not at all, then using a computed property saves you from having to calculate its value and store it somewhere.

When it comes to dependencies – whether your property’s value relies on the values of your other properties – then the tables are turned: this is a place where computed properties are useful, because you can be sure the value they return always takes into account the latest program state.

Lazy properties help mitigate the performance issues of rarely read stored properties, and property observers mitigate the dependency problems of stored properties – we’ll be looking at them soon.

Save 50% in my Black Friday sale.

SAVE 50% This Black Friday all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

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: 4.5/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.