Can I specify that a generic is a value type?

Question

I know that we can essentially specify that our generics be any reference type by using AnyObject:

class Foo<T: AnyObject> {
    // ...
}

But is there a way to specify that our generics should only be value types, and not allow reference types?

Answer 1

// some code for testing    
class C { } // just a simple class as an example for a reference type
var c = C()
var d: Double = 0.9 // a value type

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Solution 1 via extension

protocol ValueType { }
extension Double : ValueType { }
extension Int : ValueType { }
// ... all value types to be added

func printT1 <T: ValueType> (input: T) {
    println("\(input) is value")
}
printT1(d) // Does work
//printT1(c) // Does not work

But as mentioned in the comments, it is working but not feasible, because user defined value types have to implement this protocol.

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Solution 2 via method signature

func printT <T: AnyObject> (input: T) {
    println("\(input) is reference")
}

func printT <T: Any> (input: T) {
    println("\(input) is value")
}

---

Solution 3 via assert

Another solution could be via assert

func printT <T: Any> (input: T) {
    print("\(input) is " + ((T.self is AnyObject) ? "reference" : "value"))
}

---

"Solution" 4 via where clauses

This would be the best solution, I think. Unfortunately, it is not possible to have

func printT <T: Any where T: ~AnyObject > (input: T) {
    println("\(input) is value")
}

or similar. Maybe it will be possible in future releases of Swift.