Default optional parameter in Swift function

Question

When I set firstThing to default nil this will work  without the default value of nil I get a error that there is a missing parameter when calling the function   By typing Int  I thought it made it optional with a default value of nil  am I right  And if so  why doesn t it work without the   nil   func test firstThing  Int    nil        if firstThing    nil           print firstThing             print  done     test

User · Answer

Optional parameter  means  type of this parameter is optional   It does not mean  This parameter is optional and  therefore  can be ignored when you call the function    The term  optional parameter  appears to be confusing  To clarify  it s more accurate to say  optional type parameter  instead of  optional parameter  as the word  optional  here is only meant to describe the type of parameter value and nothing else

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Default value doesn t mean default value of data type  Here default value mean value defined at the time of defining function  we have to declare default value of variable while defining variable in function

User · Answer

You are conflating Optional with having a default  An Optional accepts either a value or nil  Having a default permits the argument to be omitted in calling the function  An argument can have a default value with or without being of Optional type   func someFunc param1  String             param2  String    default value             param3  String     also has default value         print  param1     param1         print  param2     param2         print  param3     param3        Example calls with output   someFunc param1  nil  param2   specific value   param3   also specific value    param1   nil param2   specific value param3   Optional  also specific value    someFunc param1   has a value    param1   Optional  has a value   param2   default value param3   Optional  also has default value    someFunc param1  nil  param3  nil   param1   nil param2   default value param3   nil   To summarize    Type with    e g  String   is an Optional may be nil or may contain an instance of Type Argument with default value may be omitted from a call to function and the default value will be used  If both Optional and has default  then it may be omitted from function call OR may be included and can be provided with a nil value  e g  param1  nil

User · Answer

It is little tricky when you try to combine optional parameter and default value for that parameter  Like this   func test param  Int    nil    These two are completely opposite ideas  When you have an optional type parameter but you also provide default value to it  it is no more an optional type now since it has a default value  Even if the default is nil  swift simply removes the optional binding without checking what the default value is   So it is always better not to use nil as default value

User · Answer

Optionals and default parameters are two different things   An Optional is a variable that can be nil  that s it   Default parameters use a default value when you omit that parameter  this default value is specified like this  func test param  Int   0   If you specify a parameter that is an optional  you have to provide it  even if the value you want to pass is nil  If your function looks like this func test param  Int    you can t call it like this test    Even though the parameter is optional  it doesn t have a default value   You can also combine the two and have a parameter that takes an optional where nil is the default value  like this  func test param  Int    nil

User · Answer

in case you need to use a bool param  you need just to assign the default value   func test WithFlag flag  Bool   false           then you can use without or with the param   test     here flag automatically has the default value  false test WithFlag  true    here flag has the value  true

User · Answer

If you want to be able to call the func with or without the parameter you can create a second func of the same name which calls the other   func test firstThing  Int         if firstThing    nil           print firstThing             print  done      func test         test firstThing  nil      now you can call a function named test without or without the parameter      both work test   test firstThing  5

User · Answer

The default argument allows you to call the function without passing an argument  If you don t pass the argument  then the default argument is supplied  So using your code  this     test        is exactly the same as this   test nil    If you leave out the default argument like this     func test firstThing  Int         if firstThing    nil           print firstThing             print  done          then you can no longer do this     test     If you do  you will get the  missing argument  error that you described  You must pass an argument every time  even if that argument is just nil   test nil       this works

User · Answer

Swift is not like languages like JavaScript  where you can call a function without passing the parameters and it will still be called  So to call a function in Swift  you need to assign a value to its parameters   Default values for parameters allow you to assign a value without specifying it when calling the function  That s why test   works when you specify a default value on test s declaration   If you don t include that default value  you need to provide the value on the call  test nil    Also  and not directly related to this question  but probably worth to note  you are using the  C    way of dealing with possibly null pointers  for dealing with possible nil optionals in Swift  The following code is safer  specially in multithreading software   and it allows you to avoid the forced unwrapping of the optional   func test firstThing  Int    nil        if let firstThing   firstThing           print firstThing            print  done     test

[swift] Default optional parameter in Swift function

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