How to cast generic T to conform to protocol - swift

Question

I need to cast the generic type in a class to conform to a protocol. I can not use constraints since the container class has to be serialized. So how could i cast in this case the T to ZNumeric when i allready know (i can check as you see) that it conforms to the protocol?
//: Playground - noun: a place where people can play
import UIKit
protocol ZNumeric {
}
extension Double: ZNumeric {
}
class GenericClass<T> {
}
class RestrictedGenericClass<T:ZNumeric> {
}
class Container {
required init?<T>(type: T.Type) {
let a = GenericClass<T>()
print(a)
if T.self is ZNumeric.Type {
print("is numeric")
//let b = RestrictedGenericClass<T>() // Will not work obviously
//print(b)
}
}
}
let cDouble = Container(type: Double.self) // if T.self is ZNumeric.Type is true
let cString = Container(type: String.self) // if T.self is ZNumeric.Type is false

Answer 1

Because generics have to be known at compile-time, you cannot just check at run-time for conformance (I'm pretty sure). I think this is more like what you want:
class Container {
required init?<T>(type: T.Type) {
let a = GenericClass<T>()
print(a)
}
required init?<T : ZNumeric>(type: T.Type) {
let a = GenericClass<T>()
print(a)
print("is numeric")
let b = RestrictedGenericClass<T>()
print(b)
}
}
The more specific initialiser will be chosen at compile-time.

Answer 2

I think you looking for this.
protocol ZNumeric {
}
extension Double: ZNumeric {
}
class GenericClass<T> {
}
class RestrictedGenericClass<T:ZNumeric> {
}
class Container<T> {
required init?(value: T) {
}
convenience init?(_ value: T) {
self.init(value: value)
let a = GenericClass<T>()
print(a)
}
}
extension Container where T: ZNumeric {
convenience init?(_ value: T) {
self.init(value: value)
let b = RestrictedGenericClass<T>() // Will not work obviously
print(b)
}
}
print("test Double")
let cDouble = Container(1.1) // if T.self is ZNumeric.Type is true
print("test String")
let cString = Container("") // if T.self is ZNumeric.Type is false
// test Double
// RestrictedGenericClass<Swift.Double>
// test String
// GenericClass<Swift.String>
make internal Required Initializer
make initializer for T: AnyObject
make initializer for T: ZNumeric