Type Variance in Swift

http://nomothetis.svbtle.com/type-variance-in-swift

by afthonos • 12 years ago

4 2 12 years ago

nomothetis.svbtle.com

maxcan• 12 years ago

In this example:

    func countFurballs(shelter:Shelter<Animal>) {
        println("Furballs: \(shelter.inhabitants.count)")
    }
    var catShelter = Shelter(inhabitants:[Cat()])
    countFurballs(catShelter) // throws compilation error

I believe that the countFurballs function could be made more generic:

    func countFurballs(shelter:Shelter<T>) {
        println("Furballs: \(shelter.inhabitants.count)")
    }
    var catShelter = Shelter(inhabitants:[Cat()])
    countFurballs(catShelter) // works?

I don't have Xcode in front of me, so that may still be a compiler error, but I believe it works.

afthonos• 12 years ago

Yes, it could be, but that doesn't change the variance of the container itself. For instance, if instead of counting, which was admittedly a bad example, I had wanted to feed the animals, that would not have worked.

(Sorry for the delay; I assumed nobody had read it… :-) )

Type Variance in Swift

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