"static let" vs "let" for declaring class specific constants

Question

class CurrencyConverter {

    // 1
    private let conversionRate = 1.3 

    // 2
    private static let conversionRate = 1.3

    func convertToForeign(fromlocal local: Double) -> Double {
        return local * CurrencyConverter.conversionRate
    }
}

let c = CurrencyConverter()
print(c.convertToForeign(fromlocal: 5))

With reference to the code snippet above, assume that I need to use the constant conversionRate ONLY in an instance method.

What are the pros & cons of declaring conversionRate as only (1)let vs (2)static let ?

Which style would be more readable & optimised?

Additionally, let's say I will only need 1 short lived instance of CurrencyConverter and conversionRate itself is bulky (an array of 10K Doubles). Will (1)let conversionRate be more memory optimised?

Answer 1

Neither. Use an uninhabited (caseless) enum to create a Constant namespace; it reads better.

class CurrencyConverter {
    private enum Constant {
        static let conversionRate = 1.3
    }
    func convertToForeign(fromlocal local: Double) -> Double {
        return local * Constant.conversionRate
    }
}

let c = CurrencyConverter()
print(c.convertToForeign(fromlocal: 5))