옵셔널

소개

Swift에서 다양한 유형에 사용하는 옵셔널

var optionalVar: String?
// optionalVar is automatically set to nil]
/*
Since we define it as optional without
assigning a value, the variable 
automatically takes the value of nil.
*/

Swift의 nil과 Obj-C의 nil은 동일하지 않습니다

강제 언래핑

let convertedNumber : Int? = 123
if convertedNumber != nil {
    print("convertedNumber has an integer value of \(convertedNumber!).")
}
// Prints "convertedNumber has an integer value of 123."

옵셔널 바인딩으로 옵셔널에 접근하기

var possibleNumber : String? = "10"

if let actualNumber = possibleNumber {
    print(actualNumber)
}

예시

if let firstNumber = Int("4"), let secondNumber = Int("42"), 
  firstNumber < secondNumber && secondNumber < 100 {
    print("\(firstNumber) < \(secondNumber) < 100")
}
// Prints "4 < 42 < 100"
/*
 If at least one of them is false,
 it will not be entered in the if blog.
*/

암시적으로 언래핑되는 옵셔널

암시적 언래핑

let possibleString: String? = "An optional string."
 // requires an exclamation mark
let forcedString: String = possibleString!

let assumedString: String! = "An implicitly unwrapped optional string."
// no need for an exclamation mark
let implicitString: String = assumedString 

예시

/*
We can use it during optional binding or we can check for nil.
So we use implicitly unwrapped optional 
in the same way we use classic optional.
*/

var assumedString: String! = nil

if let definiteString = assumedString {
    print(definiteString)
}
// Prints "An implicitly unwrapped optional string."

경고

nil 병합 연산자

let defaultColorName = "red"
 // defaults to nil
var userDefinedColorName: String?  

var colorNameToUse = userDefinedColorName ?? defaultColorName
/*
userDefinedColorName is nil, 
so colorNameToUse is set to the default of "red"
*/


// a != nil ? a! : b 
/*
We can think of it as a shortcut to the above expression.
A checks if a is nil, if a is not nil, it unwrap and eturn, 
otherwise it returns the default value, ie b.
*/

축하합니다