Swift must always know the type of each of your variables, but sometimes you know more information than Swift does. For example, here are three classes:
class Animal { }
class Fish: Animal { }
class Dog: Animal {
func makeNoise() {
print("Woof!")
}
}
We can create a couple of fish and a couple of dogs, and put them into an array, like this:
let pets = [Fish(), Dog(), Fish(), Dog()]
Swift can see both Fish
and Dog
inherit from the Animal
class, so it uses type inference to make pets
an array of Animal
.
If we want to loop over the pets
array and ask all the dogs to bark, we need to perform a typecast: Swift will check to see whether each pet is a Dog
object, and if it is we can then call makeNoise()
.
This uses a new keyword called as?
, which returns an optional: it will be nil
if the typecast failed, or a converted type otherwise.
Here’s how we write the loop in Swift:
for pet in pets {
if let dog = pet as? Dog {
dog.makeNoise()
}
}
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