Inheritance of Encodable Class

Encodable and Decodable involve some code synthesis where the compiler essentially writes the code for you. When you conform BasicData to Encodable, these methods are written to the BasicData class and hence they are not aware of any additional properties defined by subclasses. You have to override the encode(to:) method in your subclass:

class AdditionalData: BasicData {
    let c: String
    let d: Int

    init(c: String, d: Int) {
        self.c = c
        self.d = d
    }

    private enum CodingKeys: String, CodingKey {
        case c, d
    }

    override func encode(to encoder: Encoder) throws {
        try super.encode(to: encoder)
        var container = encoder.container(keyedBy: CodingKeys.self)
        try container.encode(self.c, forKey: .c)
        try container.encode(self.d, forKey: .d)
    }
}

See this question for a similar problem with Decodable.