NEW: Nominations are now open for the 2019 Swift Community Awards! >>

How to throw errors using strings

Swift version: 5.1

Paul Hudson    @twostraws   

Throwing functions in Swift are the main way we have of signaling that an operation failed, but sometimes it can be annoying to define a whole new error enum just to report a simple failure.

With a small extension to String you can make the whole process easier:

extension String: LocalizedError {
    public var errorDescription: String? { return self }
}

With that change you can now throw strings as errors, and they work just like regular errors. For example, you can create a throwing function like this one:

func doDangerousThing() throws {
    throw "I'm sorry, Dave, I can't do that."
}

Then attempt to run it and print any errors that are thrown, like this:

do {
    try doDangerousThing()
} catch {
    print(error.localizedDescription)
}

This only really works for errors that have one case, because you can’t match specific string errors.

SAVE 20% ON iOS CONF SG The largest iOS conference in Southeast Asia is back in Singapore for the 5th time in January 2020, now with two days of workshops plus two days of talks on SwiftUI, Combine, GraphQL, and more! Save a massive 20% on your tickets by clicking on this link.

Available from iOS 8.0

Similar solutions…

About the Swift Knowledge Base

This is part of the Swift Knowledge Base, a free, searchable collection of solutions for common iOS questions.

MASTER SWIFT NOW
Buy Testing Swift Buy Practical iOS 12 Buy Pro Swift Buy Swift Design Patterns Buy Swift Coding Challenges Buy Server-Side Swift (Vapor Edition) Buy Server-Side Swift (Kitura Edition) Buy Hacking with macOS Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with Swift Buy Dive Into SpriteKit Buy Swift in Sixty Seconds Buy Objective-C for Swift Developers Buy Beyond Code

Was this page useful? Let us know!