Swift Interview Questions and Answers

Answer 16: A defer statement is used to postpone the execution of statements till the end of scope.Defer statement is used to perform clean up which must be performed regardless of the result of current executing block. Defer keyword in other words represent statements which must be executed before control leaves the current execution scope.Few important points about defer

  • Defer statements cannot execute any statement which will transfer control outside like return, break, throw.
  • Defer statements are executed in reverse order i.e. last defer is executed first

Example from apple docs is

func processFile(filename: String) throws {
    if exists(filename) {
        let file = open(filename)
        defer {
        while let line = try file.readline() {
            // Work with the file.
        // close(file) is called here, at the end of the scope.

defer is used to call close() at the end of processFile function.

Question 17: What is difference between weak properties and unowned properties ?

Answer 17: Both weak and unowned reference types do not increase reference count of instance being referred. But there is a subtle difference between two as discussed below

  • Weak properties can be nil during the life time of the instance they are member of. Weak reference are used when the instance they are referring to can be released before there instance type thus they can be set to nil. Weak reference can only be var of optional type
  •  Unowned reference are used when it is not possible for the instance being referred ,to be released before referring instance.Unowned properties can thus be non optional.

Given below a example

class Car{
   weak var parkingReceipt:ParkingReceipt!
class ParkingReceipt{
    unowned var car : Car
        self.car = car

In the example above the ParkingReceipt will always have a shorter life time than the Car thus Car has weak ref of parking receipt  where as ParkingReceipt has unowned reference of Car

Question 18: What is @discardableResult used for in swift?

Answer 18: In swift when ever we call a function which returns a value , and we do not use the returned value of function we get a compiler warning. discardableResult attribute marks the return value of the function discardable i.e compiler will not warn you if your choose to ignore the returned value of a function/method marked with discardableResult attribute

func important()-> Bool{
    return true
func notImportant()->Bool{
    return false

func test(){
    important()  //  result of call important() is unused warning by compiler
    notImportant()  // no such warning for discardableResult


Code above shows use of discardableResult

Question 19: Remove duplicate characters from a string in Swift.

Answer 19: We can use swift Set to identify and remove duplicate characters from String

let str = "aaabbbcccdddeeeee"
var set = Set<Character>()
let nonRepeat = String(str.filter{ set.insert($0).inserted } )

print(nonRepeat)   //  "abcde"

In the example above we have used Set.inserted property to identify duplicate chars

New questions will be added regularly. You can also suggest questions in comment.

Question 20: What are non mutating setters

Answer 20: Nonmutating setters are special tye of setters which do not mutate the instance  they are part of rather they have global side effect. Detailed explaination can be found here

Question 21: Which of the following is a valid Numeric literal in swift

let paddedDouble = 000123.456
let oneMillion = 1_000_000
let justOverOneMillion = 1_000_000.000_000_1

Answer 21: All the above. Swift allows the use of leading zeros and “_” for improving the readability of numeric literals

A pat on the back !!