Interview questions: what are reference types in Swift

In the Swift programming language, there are two primary categories of data types: value types and reference types. Reference types in Swift refer to data types that are passed by reference, as opposed to value types, which are passed by value. This means that when working with reference types, you are working with the same object in memory, and changes to that object will be reflected in all references to it.

Examples of reference types in Swift include:

Classes: Classes allow you to create objects with methods and properties, and their instances are passed by reference. If you modify a property of a class object through one reference, that change will be visible through all other references to that object.

class Person {
    var name: String

    init(name: String) {
        self.name = name
    }
}

let person1 = Person(name: "Alice")
let person2 = person1
person2.name = "Bob"

print(person1.name) // Outputs "Bob" because person1 and person2 both reference the same object

Closures: Closures are also passed by reference. If you assign a closure to a variable and modify it, that modification will be visible wherever that closure is used.

var incrementByTwo: () -> Int = {
    var count = 0
    return {
        count += 2
        return count
    }
}

let closure1 = incrementByTwo
let closure2 = incrementByTwo

print(closure1()) // Outputs 2
print(closure2()) // Outputs 4

These examples illustrate how reference types in Swift work by reference, and changes made in one part of the code can affect other parts of the code using the same object or closure. It’s important to be aware of this behavior when working with reference types to avoid unexpected results in your program.