协议类型“名称”的值不能符合具有通用 function 的协议“名称”

Question

I have a function with generic parameters and when it is called I get the error Value of protocol type 'AddableObject' cannot conform to 'AddableObject'; only struct/enum/class types can conform to protocols Value of protocol type 'AddableObject' cannot conform to 'AddableObject'; only struct/enum/class types can conform to protocols

I have the following class and protocols

class DataManager{

      func updateItem<Item: AddableObject>(itemToUpdate: Item, updateValues: [String : Any], completion: @escaping ((Result<Item, Error>) -> ())){ }

}
protocol AddableObject{ }

This is the code in my VC

class vc: UIViewController{
   
   weak var task: Task?
   
   viewDidLoad(){

   super.viewDidLoad()

   }
   
  private func uploadTask(){

      guard let taskToUpdate = self.task else{return}          

      //this is where I get the error
      DataManager.shared.updateItem(itemToUpdate: taskToUpdate, updateValues: ["index": 1]){result in 

      }

  }

}

This is my task object

class Task: AddableObject{

}

When I try to call the updateItem method from my VC I get the error

Value of protocol type 'AddableObject' cannot conform to 'AddableObject'; only struct/enum/class types can conform to protocols

Why am I getting this error?

Answer 1

One obvious issue: your declaration weak var task: Task? declares an Optional. But the parameter itemToUpdate: is not an Optional. It is a generic type that conforms to AddableObject — and an Optional is not something that conforms to AddableObject.

---

If we correct for that, and for the other obvious issues in the code you have shown, the resulting code compiles perfectly:

class DataManager{

    func updateItem<Item: AddableObject>(itemToUpdate: Item, updateValues: [String : Any], completion: @escaping ((Result<Item, Error>) -> ())){ }

}
protocol AddableObject{ }

class vc: UIViewController{
   
   weak var task: Task?
   
   private func uploadTask(){

      guard let taskToUpdate = self.task else {return}

      DataManager().updateItem(itemToUpdate: taskToUpdate, updateValues: ["index": 1]){result in

      }
  }

}

class Task: AddableObject{
}

Thus I have to conclude that the trouble is in code you have not shown us, or that you have misrepresented to us.