NEW: Start my new Ultimate Portfolio App course with a free Hacking with Swift+ trial! >>

What are inout parameters?

Swift version: 5.2

Paul Hudson    @twostraws   

When you pass value types as parameters into a function, they are constants and so can’t be modified. Sometimes it would be convenient to change this so you can modify the values, and that’s what inout does for us: it lets us modify parameters inside a function, and have those changes persist outside the function.

For example, we could write a function that accepts a number and doubles it:

func double(_ number: inout Int) {
    number *= 2

That doesn’t return a value – it modifies the value that was passed in directly.

When it comes to calling functions with inout parameters, Swift has two rules: we must pass in variables, and we also need to use & before the parameter name to acknowledge that it might be changed.

So, we would call double() like this:

var number = 5

That will print 10.

inout parameters are more common than you might realize. For example, if you use += to append one string to another, it uses inout to modify the string in place.

Hacking with Swift is sponsored by ViRE

SPONSORED ViRE offers discoverable way of working with regex. It provides really readable regex experience, code complete & cheat sheet, unit tests, powerful replace system, step-by-step search & replace, regex visual scheme, regex history & playground. ViRE is available on Mac & iPad.

Download on the App Store

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

Available from iOS 8.0

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: 4.3/5

Unknown user

You are not logged in

Log in or create account

Link copied to your pasteboard.