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What is typecasting?

Swift version: 5.1

Paul Hudson    @twostraws   

One of the many safety features of Swift is its type safety, which means it must know what kind of data is being held by all values at all times. However, sometimes you know information that Swift doesn’t: you know that the UIViewController you have a reference to is in fact your custom PictureViewController subclass, and you want to treat it like one.

Typecasting is the process of passing on your extra knowledge to Swift – a way of saying “that thing you thought was an X is actually a Y”. Swift won’t let you typecast completely unrelated types, which means you can’t say things like “that thing you thought was a string is actually an integer”, but is instead mainly used for subclasses.

As an example, here’s how we create new view controllers from a storyboard:

let vc = storyboard?.instantiateViewController(withIdentifier: "Detail") as? DetailViewController

That as? is a typecast - we’re passing on to Swift the knowledge that the view controller we expect to be created is in fact a DetailViewController. If the typecast fails then vc will be nil, but if it succeeds then Swift will be able to use it as a DetailViewController.

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Available from iOS 8.0

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