When should you use inout parameters?

Paul Hudson @twostraws May 28th 2020

Updated for Xcode 14.0 beta 1

Swift’s inout parameters are very commonly used, but less commonly created. That is, they are the kind of thing you’ll rely on a lot, perhaps without even realizing, but it’s not common you’ll want to create functions with them – at least not while you’re learning.

How common are they? Well, all of Swift’s operators are actually implemented as functions behind the scenes – things like +, -, and so on. Yes, they really are just functions behind the scenes. You can imagine them a bit like this:

func +(leftNumber: Int, rightNumber: Int) -> Int {
    // code here
}

So, when writing 5 + 3 we’re actually calling a function called + takes an integer on the left (5) and an integer on the right (3), and returns a new integer containing the result.

Now think about the += operator, which both adds and assigns at the same time. That doesn’t have a return value, but actually modifies the original value directly. As a result, it looks a bit more like this:

func +=(leftNumber: inout Int, rightNumber: Int) {
    // code here
}

Broadly speaking, it’s best to avoid creating inout parameters until you feel confident they are definitely the right choice. When you’re learning (and actually when you’re very experienced too!) it’s usually better to send in some data to a function and get new data back, because it makes it easier to follow your program’s logic.

That doesn’t mean inout isn’t worth knowing, only that you want to be really sure you need it before you begin.