Synthetic Codable fails with protocol typed property

Using Swift
1

Hi,

I have the following code example. In my real case the protocol P has additional properties that I want to enforce to protocols A and B, but this does not make a difference to explain the issue I have.

protocol P : Codable { }
  
protocol A : P {
   var prop : B? { get }
}
  
protocol B : P {
   var prop : String? { get }
}
  
class BB : B {
   var prop: String? = nil
}
  
class AA : A {
   var prop: B? = nil
}

Compiling it results in errors:

Error:(, ) type 'AA' does not conform to protocol 'Decodable'
Error:(, ) type 'AA' does not conform to protocol 'Encodable'

I was expecting the hierarchy of protocols and classes here to all support codable, as protocol P is Codable, and protocols A and B both inherits from protocol P. And as a result, I expected the compiler to synthesize the requirements for the Codable typealias for classes AA and BB.
If I change the protocol A definition to require a property of type String instead of B? then it works.
I know that the codable synthesis requires all the properties of the class to be codable, but I can't help but think that they all are...

Why does the compiler not synthesize in my example? Why is it unreasonable to expect it to?

2

Think about decoding an instance of AA. How would you decode prop if you only know it conforms to B? You need a concrete type to instantiate it.

(It helps me to use a concrete example, let’s say the prop conforms to some Animal protocol with concrete Cat and Dog structs conforming to it. When you want to decode it, you need a particular animal to instantiate. Is it a cat, or a dog? How would the compiler know?)

1 Like
3

Yeah, I get it, now that you point my attention to the issue of decoding.... From the information there the compiler would not have a init available for creating Bs.

I was so focused on encoding that I wouldn't see it. But of course, codable is about both encode and decode :slight_smile:

Thanks for pointing it out. Cheers!

1 Like