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What is mutating in swift?

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If you have ever tried using the mutating keyword in your class methods in Swift, the compiler will definitely yell at you because you are doing something wrong.
In swift, classes are reference type whereas structures and enumerations are value typesThe properties of value types cannot be modified within its instance methods by default. In order to modify the properties of a value type, you have to use the mutating keyword in the instance method. With this keyword, your method can then have the ability to mutate the values of the properties and write it back to the original structure when the method implementation ends.
Below is a simple implementation of Stack in Swift that illustrates the use of mutating functions.

struct Stack {
     var items = [Int]() // Empty items array
    mutating func push(_ item: Int) {
    mutating func pop() -> Int? {
        if !items.isEmpty {
           return items.removeLast()
        return nil

var stack = Stack()
stack.items // [4, 78]

stack.items // [4]

You can change value of items by creating instance of Stack. But without mutating keyword function can not change property.

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