How to use try/catch in Swift to handle exceptions

Swift version: 5.6

Paul Hudson @twostraws May 28th 2019

The try/catch syntax was added in Swift 2.0 to make exception handling clearer and safer. It's made up of three parts: do starts a block of code that might fail, catch is where execution gets transferred if any errors occur, and any function calls that might fail need to be called using try.

Here's a working example that loads an input.txt file from the app bundle into a string:

if let filename = Bundle.main.path(forResource: "input", ofType: "txt") {
    do {
        let str = try String(contentsOfFile: filename)
        print(str)
    } catch {
        print("The file could not be loaded")
    }
}

There are two other ways of using try, but neither are really recommended. The first is like this:

let filename = "somefile.txt"
let str = try! String(contentsOfFile: filename)

Note the exclamation mark: try!. This means "I realize this call might throw an exception, but trust me: it never, ever will." This is useful only if you're 100% sure the call is safe. In our example we're loading a file from the app bundle, and if that file isn't there it means our app is corrupted, so it's OK to use here. You don't need do/catch when you use try!.

The second option is try? which means "if this call throws an exception, just return nil instead." This is closer to the Objective-C way of handling errors, which was a bit scruffy. If this is your preferred way of handling errors, then go for it! You don't need do/catch when use try?, but you should check and unwrap the result carefully.