Create interface with generic types-CodePudding

I'm trying to create public interface for my class operating on generics.

I've created interface

public protocol StorageProtocol {
    associatedtype StorageObject: Codable
    func store(storeObject: StorageObject)
    func get() -> StorageObject?
}

and implementation

public class Storage<T: Codable>: StorageProtocol {

    public func store(storeObject: T) {}
    public func get() -> T?

Now, when I try to create instance it forces me any keyword

let myStorage: any StorageProtocol = Storage<Credentials>()

and I can't call storage.store(storeObject: Credentials()) as I'm getting error: Associated type 'StorageObject' can only be used with a concrete type or generic parameter base. What I'm missing here?

CodePudding user response：

The compiler does not know what type to require in the parameter storeObject when you constrain myStorage to be any StoreageProtocol, as the generic StorageObject could be any Codable object. A workaround could be to explicitly add a parameter for the type of the generic.

public protocol StorageProtocol<StorageObject> { // add a generic parameter
    associatedtype StorageObject: Codable
    func store(storeObject: StorageObject)
    func get() -> StorageObject?
}

and then when you define the variable

let myStorage: any StorageProtocol<Credentials> = Storage<Credentials>()

Then the compiler will know you want to store Credentials for this variable and allow you to call store as you specialized the protocol to the correct type.