Generic subscripts

In Swift, we can create generic subscripts, where either the subscript’s return type or its parameters may be generic. This gives our subscripts the same flexibility and type safety that we get with generic types and functions.

Let’s look at how we can create a generic subscript. In this first example, we will create a subscript that will accept one generic parameter:

Struct HashProvider {
    subscript<T: Hashable>(item: T) -> Int {
        return item.hashValue
    }
}

When we create a generic subscript, we define the placeholder type after the subscript keyword. In the previous example, we define the T placeholder type and use a type constraint to ensure that the type conforms to the Hashable protocol. This will allow us to pass in an instance of any type that conforms to the Hashable protocol.

As we mentioned at the start of this section, we can also use generics for the return type of a subscript. We define the generic...