‘inout’ in Swift

Suneet Agrawal
2 min readMar 1, 2019


Recently, while working with swift I came up with a use case where I need to modify the variable passed as an argument to a function. Eventually, all the variables passed to a function are of an immutable type which cannot be changed which is similar to a let variable. If it’s a class object, you cannot create a new object but you can manipulate the properties of that class object or you can call any function with that object.

This post was originally posted at https://agrawalsuneet.github.io/blogs/inout-in-swift/ and reposted on Medium on 1st Mar 2019.
class Employee {
var name : String
var age : Int
init(name: String, age: Int){
self.name = name
self.age = age
func changeEmployeeData(emp : Employee){
employee.name = "Suneet"
employee.age = 25
let employee = Employee(name: "Random", age : 10)print(employee.name) //Random
print(employee.age) //10
changeEmployeeData(emp : employee)print(employee.name) //Suneet
print(employee.age) //25

Now the problem occurs when your argument is of primitive data type, you simply can’t change the value.

var variable: Int = 1func changeNumber (num: Int) {
num = 2
changeNumber(num : variable)

It will show the error “cannot assign to value: ‘num’ is a ‘let’ constant” which is self-explanatory. We can’t modify the parameter passed as an argument as those are of let type.

Now, what if I want to modify this value?

inout to the rescue.
You can use inout where it passes the parameter as a reference.

var variable: Int = 1func changeNumber (num:inout Int) {
num = 2
print(num) // 2
changeNumber(num : &variable)

You need to add inout before the datatype of the variable and an & while passing the parameter.

Please continue reading at https://agrawalsuneet.github.io/blogs/inout-in-swift/

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