Swift 中“使用未声明的类型”，即使类型是内部的，并且存在于同一模块中

Swift 是一种现代化的编程语言，它在苹果公司的开发环境中广泛使用。在 Swift 中，我们可以使用未声明的类型，即使这些类型是内部的，并且存在于同一模块中。这使得我们能够更方便地编写代码并处理复杂的数据结构。接下来，我们将详细介绍如何在 Swift 中使用未声明的类型，并通过实例代码来说明。

swift
class Playlist {
    var songs: [Song] = []
    func addSong(name: String, artist: String) {
        let song = Song(name: name, artist: artist)
        songs.append(song)
    }
}
class Song {
    var name: String
    var artist: String
    init(name: String, artist: String) {
        self.name = name
        self.artist = artist
    }
}
let playlist = Playlist()
playlist.addSong(name: "Song A", artist: "Artist A")
playlist.addSong(name: "Song B", artist: "Artist B")

swift
class Library {
    var playlists: [Playlist] = []
    func addPlaylist(name: String, songs: [Song]) {
        let playlist = Playlist(name: name, songs: songs)
        playlists.append(playlist)
    }
}
let library = Library()
library.addPlaylist(name: "Playlist A", songs: [
    Song(name: "Song A", artist: "Artist A"),
    Song(name: "Song B", artist: "Artist B")
])
library.addPlaylist(name: "Playlist B", songs: [
    Song(name: "Song C", artist: "Artist C"),
    Song(name: "Song D", artist: "Artist D")
])