GO FURTHER, FASTER: Try the Swift Career Accelerator today! >>

How to force your program to crash with assert()

Swift version: 5.10

Paul Hudson    @twostraws   

This might seem like a strange topic – after all, why would anyone want their program to crash? Well, the answer is two-fold.

First, if something has gone wrong that leaves your program in an unsafe state, continuing might mean corrupting user data.

Second, if you're debugging your app (i.e., it's still in development), having your app refuse to continue if a serious problem is found is a huge advantage and a very common way to spot problems.

Swift lets you force an app crash using the assert() function. This takes two parameters: a condition to check, and a message to print if the assertion fails. Helpfully, any calls to assert() are ignored when your app is compiled in release mode (i.e., for the App Store), which means these checks have no impact on your code's final performance.

Here are two examples of assert() being used:

assert(1 == 1, "Maths failure!")
assert(1 == 2, "Maths failure!")

The first one asserts that 1 is equal to 1, which is clearly true, so nothing will happen. The second one asserts that 1 is equal to 2, which is clearly false, so that assertion will fail: your app will halt, and the message "Maths failure!" will be printed out to help you identify the problem.

Because assertions are ignored in release builds, you don't need to worry about running expensive checks in your assertions. For example:

assert(myReallySlowMethod() == true, "The slow method returned false, which is a bad thing!")

In release builds, that code will never be run, so you won't see any performance impact.

Hacking with Swift is sponsored by try! Swift Tokyo.

SPONSORED Ready to dive into the world of Swift? try! Swift Tokyo is the premier iOS developer conference will be happened in April 9th-11th, where you can learn from industry experts, connect with fellow developers, and explore the latest in Swift and iOS development. Don’t miss out on this opportunity to level up your skills and be part of the Swift community!

Get your ticket here

Sponsor Hacking with Swift and reach the world's largest Swift community!

Available from iOS 7.0

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.

BUY OUR BOOKS
Buy Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Interview Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Hacking with watchOS Buy Hacking with tvOS Buy Hacking with macOS 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!

Average rating: 4.3/5

 
Unknown user

You are not logged in

Log in or create account
 

Link copied to your pasteboard.