Passing property type as parameter

Question

Is there a way to pass the property to a function as a parameter ?

class Car {

    let doors : Int = 4
    let price : Int = 1000
}

Is there a way to pass the Car property as a type to a function ?

I would like to achieve the following:

func f1(car: Car, property: SomeType) {

    println(car.property)

}

let c1 = Car()

f1(c1, doors)
f1(c1, price)

Would closures help, if so how ?

score 9 · Accepted Answer · answered Mar 09 '15 at 14:35

9

I'm not sure this is what you want, but using closure:

func f1<T>(car: Car, getter: Car -> T) {
    println(getter(car))
}

let c1 = Car()

f1(c1, {$0.doors})
f1(c1, {$0.price})

answered Mar 09 '15 at 14:35

rintaro

51,423
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Awesome !! Just a little confused what $0 represents. Wouldn't $0 represent the first argument in the closure ? In this case the closure takes only one argument which is the getter. Could you pls explain – user1046037 Mar 09 '15 at 14:48
1

`getter` is a closure `Car -> T`. It receives `Car` instance and returns something `T`. In this case, `$0` is a instance of `Car`. – rintaro Mar 09 '15 at 14:52
Thanks a ton !!! pretty neat !! Just for my understanding it is the same as `func f1(car: Car, getter: (Car) -> T)` . Just writing `(Car)` within parentheses . You are spot on since `Car` is the only argument, so parentheses is not required – user1046037 Mar 09 '15 at 14:58
Yes, with or without parentheses is OK if it accept only one argument. – rintaro Mar 09 '15 at 15:01

score 4 · Answer 2 · answered Mar 09 '15 at 14:32

4

You can use key value coding:

class Car : NSObject {
    let doors : Int = 4
    let price : Int = 1000
}

Then your f1 function could be:

func f1(car: Car, property: String) {
    println(car.valueForKey(property))
}

And you can call it like so:

f1(car, property: "doors")
f1(car, property: "price")

answered Mar 09 '15 at 14:32

Rob

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Thanks Rob, I was wondering if there was a way to achieve it through closures or without inheriting from NSObject ? – user1046037 Mar 09 '15 at 14:34
Conveniently, the object I was using inherited from `NSObject`, so this solution was exactly what I needed. – Jordan Gray May 24 '17 at 10:00

Bill Chan · Answer 3 · 2022-08-05T15:58:13.043

1

Solution with key path:

class Car {

    let doors : Int = 4
    let price : Int = 1000
}

func f1<T: CustomStringConvertible>(car: Car, property: KeyPath<Car, T>) {

    print(car[keyPath: property])

}

let c1 = Car()

f1(car: c1, property: \.doors)
f1(car: c1, property: \.price)

edited Aug 05 '22 at 15:58

answered Aug 04 '22 at 10:41

Bill Chan

3,199
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1

Nice one, would be even better when you make the function generic type T which conforms to CustomStringConvertible – user1046037 Aug 04 '22 at 13:04

Passing property type as parameter

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