What is an optional value in Swift

Question

From Apple s documentation      You can use if and let together to work with values that might be missing  These values are represented as optionals  An optional value either contains a value or contains nil to indicate that the value is missing  Write a question mark     after the type of a value to mark the value as optional    Why would you want to use an optional value

User · Answer

Lets Experiment with below code  Playground I Hope will clear idea what is optional and reason of using it   var sampleString  String     Optional  Possible to be nil  sampleString   nil     perfactly valid as its optional  sampleString    some value     Will hold the value  if let value   sampleString      the sampleString is placed into value with auto force upwraped       print value value       Sample String merged into Two    sampleString   nil    value is nil and the  if let value   sampleString       print value   value      Will Not execute and safe for nil checking         print sampleString    sampleString      this line Will crash as   operator can not add nil

User · Answer

I made a short answer  that sums up most of the above  to clean the uncertainty that was in my head as a beginner   Opposed to Objective-C  no variable can contain nil in Swift  so the Optional variable type was added  variables suffixed by            var aString   nil   error   The big difference is that the Optional variables don t directly store values  as a normal Obj-C variables would  they contain two states   has a value  or  has nil        var aString  String     Hello  World       aString   nil   correct  now it contains the state  has nil    That being  you can check those variables in different situations   if let myString   aString         println myString    else         println  It s nil      this will print in our case     By using the     suffix  you can also access the values wrapped in them  only if those exist   i e it is not nil    let aString  String     Hello  World      var anotherString  String   aString   error var anotherString  String   aString   println anotherString    it will print  Hello  World     That s why you need to use     and     and not use all of them by default   this was my biggest bewilderment   I also agree with the answer above  Optional type cannot be used as a boolean

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An optional in Swift is a type that can hold either a value or no value  Optionals are written by appending a   to any type   var name  String    You can refer to this link to get knowledge in deep  https   medium com  agoiabeladeyemi optionals-in-swift-2b141f12f870

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An optional means that Swift is not entirely sure if the value corresponds to the type  for example  Int  means that Swift is not entirely sure whether the number is an Int      To remove it  there are three methods you could employ    1  If you are absolutely sure of the type  you can use an exclamation mark to force unwrap it  like this      Here is an optional variable   var age  Int      Here is how you would force unwrap it   var unwrappedAge   age    If you do force unwrap an optional and it is equal to nil  you may encounter this crash error     This is not necessarily safe  so here s a method that might prevent crashing in case you are not certain of the type and value      Methods 2 and three safeguard against this problem    2  The Implicitly Unwrapped Optional   if let unwrappedAge   age        continue in here          Note that the unwrapped type is now Int  rather than Int     3  The guard statement   guard let unwrappedAge   age else          continue in here      From here  you can go ahead and use the unwrapped variable  Make sure only to force unwrap  with an     if you are sure of the type of the variable      Good luck with your project

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From https   developer apple com library content documentation Swift Conceptual Swift Programming Language OptionalChaining html      Optional chaining is a process for querying and calling properties  methods  and subscripts on an optional that might currently be nil  If the optional contains a value  the property  method  or subscript call succeeds  if the optional is nil  the property  method  or subscript call returns nil  Multiple queries can be chained together  and the entire chain fails gracefully if any link in the chain is nil    To understand deeper  read the link above

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In objective C variables with no value were equal to  nil  it was also possible to use  nil  values same as 0 and false   hence it was possible to use variables in conditional statements  Variables having values are same as  TRUE  and those with no values were equal to  FALSE     Swift provides type safety by providing  optional value   i e  It prevents errors formed from assigning variables of different types   So in Swift  only booleans can be provided on conditional statements   var hw    Hello World    Here  even-though  hw  is a string  it can t be used in an if statement like in objective C     This is an error  if hw          For that it needs to be created as   var nhw   String     Hello World     This is correct  if nhw

User · Answer

When i started to learn Swift it was very difficult to realize why optional  Lets think in this way  Let consider a class Person which has two property name and company  class Person  NSObject            var name   String   Person must have a value so its no marked as optional     var companyName   String     Company is optional as a person can be unemployed that is nil value is possible          init name String company String                      self name   name         self companyName   company                   Now lets create few objects of Person var tom Person   Person init name   quot Tom quot   company   quot Apple quot    posible var bob Person   Person init name   quot Bob quot   company nil     also Possible because company is marked as optional so we can give Nil  But we can not pass Nil to name var personWithNoName Person   Person init name  nil  company  nil   Now Lets talk about why we use optional   Lets consider a situation where we want to add  Inc after company name like apple will be apple Inc  We need to append Inc after company name and print  print tom companyName  quot  Inc quot      Error saying optional is not unwrapped  print tom companyName   quot  Inc quot      Error Gone  we have forcefully unwrap it which is wrong approach  Will look in Next line print bob companyName   quot  Inc quot      Crash   because bob has no company and nil can be unwrapped   Now lets study why optional takes into place  if let companyString String   bob companyName    Compiler safely unwrap company if not nil If nil no unwrap           print companyString  quot  Inc quot     Will never executed and no crash       Lets replace bob with tom if let companyString String   tom companyName    Compiler safely unwrap company if not nil If nil no unwrap           print companyString  quot  Inc quot     Will executed and no crash       And Congratulation  we have properly deal with optional  So the realization points are  We will mark a variable as optional if its possible to be nil If we want to use this variable somewhere in code compiler will remind you that we need to check if we have proper deal with that variable if it contain nil   Thank you   Happy Coding

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Optional value allows you to show absence of value  Little bit like NULL in SQL or NSNull in Objective-C  I guess this will be an improvement as you can use this even for  primitive  types      Reimplement the Swift standard library s optional type enum OptionalValue lt T gt        case None     case Some T    var possibleInteger  OptionalValue lt Int gt     None possibleInteger    Some 100       Excerpt From  Apple Inc     The Swift Programming Language     iBooks  https   itun es gb jEUH0 l

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Let s take the example of an NSError  if there isn t an error being returned you d want to make it optional to return Nil  There s no point in assigning a value to it if there isn t an error    var error  NSError    nil   This also allows you to have a default value  So you can set a method a default value if the function isn t passed anything  func doesntEnterNumber x  Int    5  - gt  Bool       if  x    5           return true       else           return false

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Here is an equivalent optional declaration in Swift   var middleName  String    This declaration creates a variable named middleName of type String  The question mark     after the String variable type indicates that the middleName variable can contain a value that can either be a String or nil  Anyone looking at this code immediately knows that middleName can be nil  It s self-documenting   If you don t specify an initial value for an optional constant or variable  as shown above  the value is automatically set to nil for you  If you prefer  you can explicitly set the initial value to nil   var middleName  String    nil   for more detail for optional read below link  http   www iphonelife com blog 31369 swift-101-working-swifts-new-optional-values

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An optional in Swift is a type that can hold either a value or no value  Optionals are written by appending a   to any type  var name  String     quot Bertie quot   Optionals  along with Generics  are one of the most difficult Swift concepts to understand  Because of how they are written and used  it s easy to get a wrong idea of what they are  Compare the optional above to creating a normal String  var name  String    quot Bertie quot     No  quot   quot  after String  From the syntax it looks like an optional String is very similar to an ordinary String  It s not  An optional String is not a String with some  quot optional quot  setting turned on  It s not a special variety of String  A String and an optional String are completely different types  Here s the most important thing to know  An optional is a kind of container  An optional String is a container which might contain a String  An optional Int is a container which might contain an Int  Think of an optional as a kind of parcel  Before you open it  or  quot unwrap quot  in the language of optionals  you won t know if it contains something or nothing  You can see how optionals are implemented in the Swift Standard Library by typing  quot Optional quot  into any Swift file and  -clicking on it  Here s the important part of the definition  enum Optional lt Wrapped gt        case none     case some Wrapped     Optional is just an enum which can be one of two cases   none or  some  If it s  some  there s an associated value which  in the example above  would be the String  quot Hello quot   An optional uses Generics to give a type to the associated value  The type of an optional String isn t String  it s Optional  or more precisely Optional lt String gt   Everything Swift does with optionals is magic to make reading and writing code more fluent  Unfortunately this obscures the way it actually works  I ll go through some of the tricks later  Note  I ll be talking about optional variables a lot  but it s fine to create optional constants too  I mark all variables with their type to make it easier to understand type types being created  but you don t have to in your own code   How to create optionals To create an optional  append a   after the type you wish to wrap  Any type can be optional  even your own custom types  You can t have a space between the type and the    var name  String     quot Bob quot     Create an optional String that contains  quot Bob quot  var peter  Person    Person      An optional  quot Person quot   custom type      A class with a String and an optional String property class Car   var modelName  String    must exist var internalName  String     may or may not exist     Using optionals You can compare an optional to nil to see if it has a value  var name  String     quot Bob quot  name   nil    Set name to nil  the absence of a value if name    nil       print  quot There is a name quot     if name    nil      Could also use an  quot else quot      print  quot Name has no value quot      This is a little confusing  It implies that an optional is either one thing or another  It s either nil or it s  quot Bob quot   This is not true  the optional doesn t transform into something else  Comparing it to nil is a trick to make easier-to-read code  If an optional equals nil  this just means that the enum is currently set to  none   Only optionals can be nil If you try to set a non-optional variable to nil  you ll get an error  var red  String    quot Red quot  red   nil    error  nil cannot be assigned to type  String   Another way of looking at optionals is as a complement to normal Swift variables  They are a counterpart to a variable which is guaranteed to have a value  Swift is a careful language that hates ambiguity  Most variables are define as non-optionals  but sometimes this isn t possible  For example  imagine a view controller which loads an image either from a cache or from the network  It may or may not have that image at the time the view controller is created  There s no way to guarantee the value for the image variable  In this case you would have to make it optional  It starts as nil and when the image is retrieved  the optional gets a value  Using an optional reveals the programmers intent  Compared to Objective-C  where any object could be nil  Swift needs you to be clear about when a value can be missing and when it s guaranteed to exist   To use an optional  you  quot unwrap quot  it An optional String cannot be used in place of an actual String  To use the wrapped value inside an optional  you have to unwrap it  The simplest way to unwrap an optional is to add a   after the optional name  This is called  quot force unwrapping quot   It returns the value inside the optional  as the original type  but if the optional is nil  it causes a runtime crash  Before unwrapping you should be sure there s a value  var name  String     quot Bob quot  let unwrappedName  String   name  print  quot Unwrapped name    unwrappedName  quot    name   nil let nilName  String   name     Runtime crash  Unexpected nil    Checking and using an optional Because you should always check for nil before unwrapping and using an optional  this is a common pattern  var mealPreference  String     quot Vegetarian quot  if mealPreference    nil       let unwrappedMealPreference  String   mealPreference      print  quot Meal    unwrappedMealPreference  quot      or do something useful    In this pattern you check that a value is present  then when you are sure it is  you force unwrap it into a temporary constant to use  Because this is such a common thing to do  Swift offers a shortcut using  quot if let quot   This is called  quot optional binding quot   var mealPreference  String     quot Vegetarian quot  if let unwrappedMealPreference  String   mealPreference       print  quot Meal    unwrappedMealPreference  quot       This creates a temporary constant  or variable if you replace let with var  whose scope is only within the if s braces  Because having to use a name like  quot unwrappedMealPreference quot  or  quot realMealPreference quot  is a burden  Swift allows you to reuse the original variable name  creating a temporary one within the bracket scope var mealPreference  String     quot Vegetarian quot  if let mealPreference  String   mealPreference       print  quot Meal    mealPreference  quot      separate from the other mealPreference    Here s some code to demonstrate that a different variable is used  var mealPreference  String     quot Vegetarian quot  if var mealPreference  String   mealPreference       print  quot Meal    mealPreference  quot      mealPreference is a String  not a String      mealPreference    quot Beef quot     No effect on original      This is the original mealPreference print  quot Meal    mealPreference  quot      Prints  quot Meal  Optional  quot Vegetarian quot   quot   Optional binding works by checking to see if the optional equals nil  If it doesn t  it unwraps the optional into the provided constant and executes the block  In Xcode 8 3 and later  Swift 3 1   trying to print an optional like this will cause a useless warning  Use the optional s debugDescription to silence it  print  quot   mealPreference debugDescription  quot     What are optionals for  Optionals have two use cases   Things that can fail  I was expecting something but I got nothing  Things that are nothing now but might be something later  and vice-versa   Some concrete examples   A property which can be there or not there  like middleName or spouse in a Person class A method which can return a value or nothing  like searching for a match in an array A method which can return either a result or get an error and return nothing  like trying to read a file s contents  which normally returns the file s data  but the file doesn t exist Delegate properties  which don t always have to be set and are generally set after initialization For weak properties in classes  The thing they point to can be set to nil at any time A large resource that might have to be released to reclaim memory When you need a way to know when a value has been set  data not yet loaded  gt  the data  instead of using a separate dataLoaded Boolean  Optionals don t exist in Objective-C but there is an equivalent concept  returning nil  Methods that can return an object can return nil instead  This is taken to mean  quot the absence of a valid object quot  and is often used to say that something went wrong  It only works with Objective-C objects  not with primitives or basic C-types  enums  structs   Objective-C often had specialized types to represent the absence of these values  NSNotFound which is really NSIntegerMax  kCLLocationCoordinate2DInvalid to represent an invalid coordinate  -1 or some negative value are also used   The coder has to know about these special values so they must be documented and learned for each case  If a method can t take nil as a parameter  this has to be documented  In Objective-C  nil was a pointer just as all objects were defined as pointers  but nil pointed to a specific  zero  address  In Swift  nil is a literal which means the absence of a certain type   Comparing to nil You used to be able to use any optional as a Boolean  let leatherTrim  CarExtras    nil if leatherTrim       price   price   1000    In more recent versions of Swift you have to use leatherTrim    nil  Why is this  The problem is that a Boolean can be wrapped in an optional  If you have Boolean like this  var ambiguous  Boolean    false  it has two kinds of  quot false quot   one where there is no value and one where it has a value but the value is false  Swift hates ambiguity so now you must always check an optional against nil  You might wonder what the point of an optional Boolean is  As with other optionals the  none state could indicate that the value is as-yet unknown  There might be something on the other end of a network call which takes some time to poll  Optional Booleans are also called  quot Three-Value Booleans quot   Swift tricks Swift uses some tricks to allow optionals to work  Consider these three lines of ordinary looking optional code  var religiousAffiliation  String     quot Rastafarian quot  religiousAffiliation   nil if religiousAffiliation    nil          None of these lines should compile   The first line sets an optional String using a String literal  two different types  Even if this was a String the types are different The second line sets an optional String to nil  two different types The third line compares an optional string to nil  two different types  I ll go through some of the implementation details of optionals that allow these lines to work   Creating an optional Using   to create an optional is syntactic sugar  enabled by the Swift compiler  If you want to do it the long way  you can create an optional like this  var name  Optional lt String gt    Optional  quot Bob quot    This calls Optional s first initializer  public init   some  Wrapped   which infers the optional s associated type from the type used within the parentheses  The even longer way of creating and setting an optional  var serialNumber String    Optional none serialNumber   Optional some  quot 1234 quot   print  quot   serialNumber debugDescription  quot     Setting an optional to nil You can create an optional with no initial value  or create one with the initial value of nil  both have the same outcome   var name  String  var name  String    nil  Allowing optionals to equal nil is enabled by the protocol ExpressibleByNilLiteral  previously named NilLiteralConvertible   The optional is created with Optional s second initializer  public init nilLiteral       The docs say that you shouldn t use ExpressibleByNilLiteral for anything except optionals  since that would change the meaning of nil in your code  but it s possible to do it  class Clint  ExpressibleByNilLiteral       var name  String      required init nilLiteral                name    quot The Man with No Name quot           let clint  Clint   nil    Would normally give an error print  quot   clint name  quot    The same protocol allows you to set an already-created optional to nil  Although it s not recommended  you can use the nil literal initializer directly  var name  Optional lt String gt    Optional nilLiteral        Comparing an optional to nil Optionals define two special  quot    quot  and  quot    quot  operators  which you can see in the Optional definition  The first    allows you to check if any optional is equal to nil  Two different optionals which are set to  none will always be equal if the associated types are the same  When you compare to nil  behind the scenes Swift creates an optional of the same associated type  set to  none then uses that for the comparison     How Swift actually compares to nil var tuxedoRequired  String    nil let temp  Optional lt String gt    Optional none if tuxedoRequired    temp      equivalent to if tuxedoRequired    nil     print  quot tuxedoRequired is nil quot      The second    operator allows you to compare two optionals  Both have to be the same type and that type needs to conform to Equatable  the protocol which allows comparing things with the regular  quot    quot  operator   Swift  presumably  unwraps the two values and compares them directly  It also handles the case where one or both of the optionals are  none  Note the distinction between comparing to the nil literal  Furthermore  it allows you to compare any Equatable type to an optional wrapping that type  let numberToFind  Int   23 let numberFromString  Int    Int  quot 23 quot      Optional 23  if numberToFind    numberFromString       print  quot It s a match  quot      Prints  quot It s a match  quot     Behind the scenes  Swift wraps the non-optional as an optional before the comparison  It works with literals too  if 23    numberFromString    I said there are two    operators  but there s actually a third which allow you to put nil on the left-hand side of the comparison if nil    name           Naming Optionals There is no Swift convention for naming optional types differently from non-optional types  People avoid adding something to the name to show that it s an optional  like  quot optionalMiddleName quot   or  quot possibleNumberAsString quot   and let the declaration show that it s an optional type  This gets difficult when you want to name something to hold the value from an optional  The name  quot middleName quot  implies that it s a String type  so when you extract the String value from it  you can often end up with names like  quot actualMiddleName quot  or  quot unwrappedMiddleName quot  or  quot realMiddleName quot   Use optional binding and reuse the variable name to get around this   The official definition From  quot The Basics quot  in the Swift Programming Language   Swift also introduces optional types  which handle the absence of a value  Optionals say either    there is a value  and it equals x    or    there isn   t a value at all     Optionals are similar to using nil with pointers in Objective-C  but they work for any type  not just classes  Optionals are safer and more expressive than nil pointers in Objective-C and are at the heart of many of Swift   s most powerful features  Optionals are an example of the fact that Swift is a type safe language  Swift helps you to be clear about the types of values your code can work with  If part of your code expects a String  type safety prevents you from passing it an Int by mistake  This enables you to catch and fix errors as early as possible in the development process    To finish  here s a poem from 1899 about optionals  Yesterday upon the stair I met a man who wasn   t there He wasn   t there again today I wish  I wish he   d go away Antigonish  More resources   The Swift Programming Guide Optionals in Swift  Medium  WWDC Session 402  quot Introduction to Swift quot   starts around 14 15  More optional tips and tricks

User · Answer

Well           Optional  indicates your variable may contain a nil value while    unwrapper  indicates your variable must have a memory  or value  when it is used  tried to get a value from it  at runtime    The main difference is that optional chaining fails gracefully when the optional is nil  whereas forced unwrapping triggers a runtime error when the optional is nil   To reflect the fact that optional chaining can be called on a nil value  the result of an optional chaining call is always an optional value  even if the property  method  or subscript you are querying returns a nonoptional value  You can use this optional return value to check whether the optional chaining call was successful  the returned optional contains a value   or did not succeed due to a nil value in the chain  the returned optional value is nil    Specifically  the result of an optional chaining call is of the same type as the expected return value  but wrapped in an optional  A property that normally returns an Int will return an Int  when accessed through optional chaining   var defaultNil   Int      declared variable with default nil value println defaultNil   gt  gt  nil    var canBeNil   Int    4 println canBeNil   gt  gt  optional 4   canBeNil   nil println canBeNil   gt  gt  nil  println canBeNil    gt  gt     Here nil optional variable is being unwrapped using   mark  symbol   that will show runtime error  Because a nil optional is being tried to get value using unwrapper  var canNotBeNil   Int    4 print canNotBeNil   gt  gt  4  var cantBeNil   Int   4 cantBeNil   nil    can t do this as it s not optional and show a compile time error   Here is basic tutorial in detail  by Apple Developer Committee  Optional Chaining

User · Answer

You can t have a variable that points to nil in Swift     there are no pointers  and no null pointers  But in an API  you often want to be able to indicate either a specific kind of value  or a lack of value     e g  does my window have a delegate  and if so  who is it  Optionals are Swift s type-safe  memory-safe way to do this

[swift] What is an optional value in Swift?

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