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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() {

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 {

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