Swift基础---Optionals

let possibleNumber = "123"
let convertedNumber = possibleNumber.toInt()
// convertedNumber is inferred to be of type
// "Int?", or "optional Int"

if convertedNumber {
  println("\(possibleNumber) has an integer value of \(convertedNumber!)")
}else{
  println("\(possibleNumber) could not be converted to an integer")
}

// prints "123 has an integer value of 123"

// 模板
if let constantName = someOptional {
  statements
}

if let actualNumber = possibleNumber.toInt() {

  // 如果possibleNumber.toInt()返回可选值Int类型的值  
  println("\(possibleNumber) has an integer value of \(actualNumber)")

}else{

  // 否则
  println("\(possibleNumber) could not be converted to an integer")

// prints "123 has an integer value of 123"

var serverResponseCode: Int? = 404
// serverResponseCode 包含一个Int值404

serverResponseCode = nil
// serverResponseCode现在没有值

var surveyAnswer: String?
// surveyAnswer自动被设置为nil

let possibleString: String? = "An optional string."
println(possibleString!)
// !要求这个值可以获取到

// prints "An optional string."

let assumedString: String! = "An implicitly unwrapped optional string."
println(assumedString)
// 没有!说明这个值不是必须要获取到的

// prints "An implicitly unwrapped optional string."

if assumedString {
  println(assumedString)
}

// prints "An implicitly unwrapped optional string."

if let definiteString = assumedString {
  println(definitString)
}

// prints "An implicitly unwrapped optional string."