Optional型の理解

optional_type.playground

// Playground - noun: a place where people can play

import UIKit

var str: String = "hoge"
// str = nil => NilLiteralConvertiable: 通常の型にnilは代入できない

var str2: Optional<String> = "hoge" // str2: String? = "hoge"と等しい
// str2 = nil => ok
// str2.upperCaseString  => error: 'Optional<String>' does not have a member named

str2!.uppercaseString // Forced Unwrapping
str2?.uppercaseString // Optional Chaining: 結果もOptional型

if let unwrapped: String = str2 {
    unwrapped.uppercaseString // Optional Binding: nilでなければString型変数に格納する
}

var str3: ImplicitlyUnwrappedOptional<String> = "hoge" // str3: String! = "hoge"のシンタックスシュガー
str3.uppercaseString // ImplicitlyUnwrappedOptional型で宣言すると使用時に自動でForced Unwrapping