WWDC23 SALE: Save 50% on all my Swift books and bundles! >>

How to force your program to crash with assert()

Swift version: 5.6

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.

Save 50% in my WWDC23 sale.

SAVE 50% To celebrate WWDC23, all our books and bundles are half price, so you can take your Swift knowledge further without spending big! Get the Swift Power Pack to build your iOS career faster, get the Swift Platform Pack to builds apps for macOS, watchOS, and beyond, or get the Swift Plus Pack to learn advanced design patterns, testing skills, and more.

Save 50% on all our books and bundles!

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 Pro Swift Buy Pro SwiftUI Buy Swift Design Patterns Buy Testing Swift Buy Hacking with iOS Buy Swift Coding Challenges Buy Swift on Sundays Volume One Buy Server-Side Swift Buy Advanced iOS Volume One Buy Advanced iOS Volume Two Buy Advanced iOS Volume Three 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.