# assert(_:_:file:line:) Performs a traditional C-style assert with an optional message. ``` func assert(_ condition: @autoclosure () -> Bool, _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line) ``` ## Parameters `condition` The condition to test. `condition` is only evaluated in playgrounds and `-Onone` builds. `message` A string to print if `condition` is evaluated to `false`. The default is an empty string. `file` The file name to print with `message` if the assertion fails. The default is the file where `assert(_:_:file:line:)` is called. `line` The line number to print along with `message` if the assertion fails. The default is the line number where `assert(_:_:file:line:)` is called. ## Discussion Use this function for internal consistency checks that are active during testing but do not impact performance of shipping code. To check for invalid usage in Release builds, see `precondition(_:_:file:line:)`. - In playgrounds and `-Onone` builds (the default for Xcode’s Debug configuration): If `condition` evaluates to `false`, stop program execution in a debuggable state after printing `message`. - In `-O` builds (the default for Xcode’s Release configuration), `condition` is not evaluated, and there are no effects. - In `-Ounchecked` builds, `condition` is not evaluated, but the optimizer may assume that it *always* evaluates to `true`. Failure to satisfy that assumption is a serious programming error. --- Copyright © 2026 Apple Inc. All rights reserved. | [Terms of Use](https://www.apple.com/legal/internet-services/terms/site.html) | [Privacy Policy](https://www.apple.com/privacy/privacy-policy)