CODEWITHSUNDEEP
swiftgenerics
SteveSmith
SteveSmith

Reputation: 269

How do I turn this function to a generic type?

How would I go about turning this function into a generic type? 

func addThreeValue(value1:String,value2:String,value3:String) -> String {
    let concatenated = value1 + value2 + value3

    return concatenated
}

Upvotes: 0

Views: 39

Answers (1)

jtbandes
jtbandes

Reputation: 118731

This will not work:

func addThreeValues<T>(v1: T, _ v2: T, _ v3: T) -> T {
    return v1 + v2 + v3
}

Because the compiler doesn't know about any + operator which works with Ts.

You need to constrain the generic parameter so that it can find a suitable + operator. The IntegerType protocol provides one:

func addThreeValues<T: IntegerType>(v1: T, _ v2: T, _ v3: T) -> T {
    return v1 + v2 + v3
}

However, String doesn't conform to the IntegerType protocol:

addThreeValues("a", "b", "c")  // error: cannot invoke 'addThreeValues' with an argument list of type '(String, String, String)'

To work around this you could define your own protocol and provide conformances for whichever types you want to use:

protocol Addable {
    func +(lhs: Self, rhs: Self) -> Self
}
extension String: Addable {}
extension Int: Addable {}

func addThreeValues<T: Addable>(v1: T, _ v2: T, _ v3: T) -> T {
    return v1 + v2 + v3
}

Now this works:

addThreeValues(1, 2, 3)        // returns 6
addThreeValues("a", "b", "c")  // returns "abc"

Upvotes: 1

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