Swift check type against a generic type

Question

I've a generic function with 1 parameter and want to check the type of the passed parameter with the generic type. Something like this:

func generic<T>(parameter: AnyObject) -> Bool {     if parameter is T {         return true     } else {         return false     } } 

But I don't know how to call this

generic<String>("Hello") 

Gives me a compiler error: "Cannot explicitly specialize a generic function generic("Hello")

Answer 1

You cannot tell a function what the types of its generic placeholders are (unlike with a generic struct). It must infer them from the context e.g. its arguments.

One way to do what you want is to add another argument related to type T. Rather than pass in a dummy value, you could use the metatype of the type you want:

func generic<T>(parameter: AnyObject, type: T.Type) -> Bool {     if parameter is T {         return true     } else {         return false     } }  let o: AnyObject = "hello" generic(o, String.self)    // true generic(o, NSString.self)  // also true generic(o, Int.self)       // false 

However, I would ask you, what is it you think you're achieving here? You've essentially done nothing more than implement is as a function:

o is String     // true o is NSString   // true o is Int        // false 

The point of generics is to operate on arguments generically, but you aren't giving the function any argument of a specific type to actually do any work on (hence the inability to infer one).

Answer 2

Checking if generic type is which class.

protocol Some {} class SomeClass: Some {} class AClass: Some {} func test(_ t: T) -> String {     if T.self == SomeClass.self {         return "Some Class"     } else {         return "Another Class"     } }  print(test(SomeClass())) // Some Class print(test(AClass())) // Another Class