swift - "Cannot convert value of type ' T '..."泛型

Question

无法理解如何处理泛型的类型以及如何使用带有库的未定义类型的参数:

func cellWith<T>(value: T) -> String {         // I expect Int, Double or String 
                                               // type value as argument  

    let fm = NSNumberFormatter()               // Double type argument will be
    fm.numberStyle = .CurrencyStyle            // processed like "currency"

    if value.self is Double {               // In case value have type "Double", 
                                            // like 20_000.00
        return(fm.stringFromNumber(value))  // I expect return $20,000.00
                                   ^~~~~
// ERROR: Cannot convert value of type 'T' to expected argument type 'NSNumber'

    } else {
        return("bla-bla-bla")
    }
}

Answer 1

为什么在这种情况下必须使用泛型？仅当必须保留类型信息时才使用泛型。

仅在以下情况下使用泛型:

"you are going to give you a type later and I want you to enforce that type everywhere I specify."

仅当您想告诉编译器时才使用Any:

"Don't worry about this variable no need to enforce any type here let me do whatever I want to."

话虽如此，您的用例适合第二个用例。你不想在这里搞乱泛型。你的解决方案应该是:

func cellWith(value: Any) -> String {
    let fm = NSNumberFormatter()
    fm.numberStyle = .CurrencyStyle
    if let doubleValue = value as? Double {
        return(fm.stringFromNumber(doubleValue))!
    } else {
        return("bla-bla-bla")
    }
}