Swift optional type是用enum去實作，以下為Optional type的部分source code:
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| public enum Optional<Wrapped>: ExpressibleByNilLiteral { | |
| case none | |
| case some(Wrapped) | |
| public init(_ some: Wrapped) { self = .some(some) } | |
| public init(nilLiteral: ()) { | |
| self = .none | |
| } | |
| @inlinable | |
| public static func ==(lhs: Wrapped?, rhs: Wrapped?) -> Bool { | |
| switch (lhs, rhs) { | |
| case let (l?, r?): | |
| return l == r | |
| case (nil, nil): | |
| return true | |
| default: | |
| return false | |
| } | |
| } | |
| ….. | |
| } |
由上面我們可以知道:
- 一個非nil的值會被指定到case some(Wrapped)，而一個nil的值會被指定到case none
- 比較兩個值是否相等時，只要有一個變數是optional，在進行比較前，兩個變數都會被轉換成Optional type，再進行比較
Note: 詳細原文說明在[ref 1]的第386行
ref 1: https://github.com/apple/swift/blob/main/stdlib/public/core/Optional.swift