How make SomeClass<T> Decodable?

Question

Created Aug ’23

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I am trying to encode/decode JSON data, and I have the following code:

struct SomeOrdinaryClass: Decodable {
    // ...
}

struct SomeBox<T>: Decodable {
    var data: T?
}

But Xcode gives me the following error:

myfile.swift:16:8 Type 'SomeBox' does not conform to protocol 'Decodable'

Is there anyway to overcome this?

Answered by DTS Engineer in 762458022

You're almost there…

In order to synthesize Decodable conformance for your SomeBox type, the compiler requires that each of its stored properties must conform to Decodable too. This isn't true of your generic type T — it could be anything.

Instead, you should also constrain T to Decodable:

struct SomeBox<T: Decodable>: Decodable {
    var data: T?
}

or equivalently:

struct SomeBox<T>: Decodable where T: Decodable {
    var data: T?
}

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Answer 1

DTS Engineer OP

Apple

Aug ’23

Accepted Answer

You're almost there…

In order to synthesize Decodable conformance for your SomeBox type, the compiler requires that each of its stored properties must conform to Decodable too. This isn't true of your generic type T — it could be anything.

Instead, you should also constrain T to Decodable:

struct SomeBox<T: Decodable>: Decodable {
    var data: T?
}

or equivalently:

struct SomeBox<T>: Decodable where T: Decodable {
    var data: T?
}

0