MyDoubleDouble.swift

class MyDoubleDouble : FloatLiteralConvertible {
    var double : Double
    
    init(_ value: Double) {
        double = value * 2
    }
    
    class func convertFromFloatLiteral(value: Double) -> MyDoubleDouble {
        return MyDoubleDouble(value)
    }
}

let double = 2.0                        // double has a value of 2.0
let myDouble : MyDoubleDouble = 2.0     // myDouble.double has a value of 4.0