as と is の入った switch 文を if 文で記述してみる

Swiftに関するメモ その7 - 型変換で触れたとおり、switch 文の case には as や is が使えます。

Swift っぽい感じのプログラミングになって楽しいんですが、反面、慣れるまでは少し分かりづらいかもしれません。

The Swift Programming Language (iBooks Store) の「Type Casting」にある switch 文のサンプルを if で書き換えてみました。

共通部

class Movie {
  var director: String
  var name: String
  init(name: String, director: String) {
    self.director = director
    self.name = name
  }
}

var things = Any[]()

things.append(0)
things.append(0.0)
things.append(42)
things.append(3.14159)
things.append(-3.14159)
things.append("hello")
things.append((3.0, 5.0))
things.append(nil)
things.append(Movie(name: "Ghostbusters", director: "Ivan Reitman"))

という部分は共通で、The Swift Programming Language のサンプルコードは……

switch サンプルコード

for thing in things {
  switch thing {
  case 0 as Int:
    println("zero as an Int")
  case 0 as Double:
    println("zero as a Double")
  case let someInt as Int:
    println("an integer value of \(someInt)")
  case let someDouble as Double where someDouble > 0:
    println("a positive double value of \(someDouble)")
  case is Double:
    println("some other double value that I don't want to print")
  case let someString as String:
    println("a string value of \"\(someString)\"")
  case let (x, y) as (Double, Double):
    println("an (x, y) point at \(x), \(y)")
  case let movie as Movie:
    println("a movie called '\(movie.name)', dir. \(movie.director)")
  default:
    println("something else")
  }
}

それを if に書き換えると……

if サンプルコード

for thing in things {
  if thing as? Int == 0 {
    println("zero as an Int")
  } else if thing as? Double == 0 {
    println("zero as a Double")
  } else if let someInt = thing as? Int {
    println("an integer value of \(someInt)")
  } else if thing as? Double > 0 {
    let someDouble = thing as Double
    println("a positive double value of \(someDouble)")
  } else if thing is Double {
    println("some other double value that I don't want to print")
  } else if let someString = thing as? String {
    println("a string value of \"\(someString)\"")
  } else if let (x, y) = thing as? (Double, Double) {
    println("an (x, y) point at \(x), \(y)")
  } else if let movie = thing as? Movie {
    println("a movie called '\(movie.name)', dir. \(movie.director)")
  } else {
    println("something else")
  }
}

アセンブリコードレベルで同じかどうかまでの確認はしていません。あしからず。