Storagable.swift

struct Hoge: Storagable {
    let id: Int
    let name: String
}

protocol Storagable: Codable {}

extension Storagable {
    func save(key: String = "\(Self.self)", userDefaults: UserDefaults = .standard) throws {
        let data = try JSONEncoder().encode(self)
        userDefaults.setValue(data, forKey: key)
    }

    func load(key: String = "\(Self.self)", userDefaults: UserDefaults = .standard) throws -> Self? {
        guard let data = userDefaults.data(forKey: key) else {
            return nil
        }
        let json = try JSONDecoder().decode(Self.self, from: data)
        return json
    }
}

let hoge = Hoge(id: 1, name: "fuga")
try hoge.save()
let loadedData = try hoge.load() // Hoge(id: 1, name: fuga)