Protocol implementation and inheritance

Question

I have two protocols and two classes implementing them as following :

protocol IMessage { }

class Message: IMessage { }

protocol IConversation {
    var messages: [IMessage] { get set }
}

class Conversation: IConversation {
    var messages: [Message] = []
}

With this code, I got the error « Type 'Conversation' does not conform to protocol IConversation »

Please do not change declarations in your post, it's cause problems with written answers. — Yury
– Yury, Commented Sep 6, 2016 at 8:40

Yury · Accepted Answer · 2016-09-06 08:56:10Z

1

Problem is in difference between IMessage and Message types. IConversation protocol expect that you are able assign to property messages variable with any type of [IMessage], not only case [Message]. Simple example with one more class:

class OtherMessage: IMessage { }

By protocol declaration you should be able to assign variable with type [OtherMessage] to messages, and class Conversation don't allow this. Fix it:

class Conversation: IConversation {
    var messages: [IMessage] = []
}

Update: if you need to work with Message type, you can use, for example, this solution:

class Conversation: IConversation {
    var messages: [IMessage] {get{return _messages}set{_messages = newValue as! [Message]}}
    var _messages: [Message] = []
}

and work with _messages inside class.

edited Sep 6, 2016 at 8:56

answered Sep 6, 2016 at 8:37

Yury

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2 Comments

Morniak Over a year ago

Thanks for the answer. So if my class Conversation is built to work only with Message, all I can do is to check the type and throw an error if it's another implementation of IMessage ?

Yury Over a year ago

@Morniak basically yes. You can use other property in your class (check update). Also this question could be related

dersvenhesse · Accepted Answer · 2016-09-06 08:35:37Z

1

Your message types don't match. Your protocol requires messages of a type [IMessage]. You're declaring it in the class with [Message].

answered Sep 6, 2016 at 8:35

dersvenhesse

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