Subscript: access my dictionary values with a String enumeration

Question

I want to do something like that: access my dictionary values with a String enumeration. I am trying to overload the subscript of the dictionary but without success.

Answer 1

Try this:

extension Dictionary where Key: StringLiteralConvertible {
    subscript(index: JsonKeys) -> Value {
        get {
            return self[index.rawValue as! Key]!
        }
    }
}

Remember, with having constraint as Key: StringLiteralConvertible, the extension works for any Dictionaries with its Key conforming to StringLiteralConvertible. (You know many types other than String conform to StringLiteralConvertible.)

To call subscript self[], you need to pass a value of type Key. index.rawValue is String, which may not always be a Key in the extension.

So, the extension I have shown would work for some Dictionaries, would cause runtime crash for some other Dictionaries.

---

A little bit more type-safe way:

protocol MyJsonKeysConvertible {
    init(jsonKeys: JsonKeys)
}
extension String: MyJsonKeysConvertible {
    init(jsonKeys: JsonKeys) {self = jsonKeys.rawValue}
}
extension Dictionary where Key: MyJsonKeysConvertible {
    subscript(index: JsonKeys) -> Value {
        get {
            return self[Key(jsonKeys: index)]!
        }
    }
}