What are inout parameters?

Swift version: 5.1

Paul Hudson @twostraws October 18th 2019

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
double(&number)
print(number)

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.