What does the mutating keyword do?

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Main Source: 🔗 Ace the iOS Interview

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TL/DR

In Swift,structsare value types which means the properties contained within are immutable by default. So, if we want to be able to modify the values within astruct, we’ll need to use the mutating keyword. This keyword only applies to valuetypes as reference types are not immutable in this way.

Whenever we call a function that uses this keyword and modifies the struct's properties, Swift will generate a new struct in-place with the modifications applied and will overwrite our original struct.

struct User {
    var firstName = "Aryaman"
    var lastName = "Sharda"

    func makeLowercase() {
        // The following lines cause a compilation error:
        // Cannot assign to property: 'self' is immutable
        firstName = firstName.lowercased()
        lastName = lastName.lowercased()
    }
}

Let’s add the mutating keyword:

struct User {
    var firstName = "Aryaman"
    var lastName = "Sharda"

    mutating func makeLowercase() {
        firstName = firstName.lowercased()
        lastName = lastName.lowercased()
    }
}

When working withmutatingfunctions, we’ll need to declare the as a variable since we’re making changes to the struct's properties:

let user = User()

// Compilation Error!
// Cannot use mutating member on immutable value: 'user' is
// a 'let' constant
user.makeLowercase()

When we make it a variable, we have no such issue:

// No error
var user = User()
user.makeLowercase()