# NSAssert Generates an assertion if a given condition is false. ``` #define NSAssert(condition, desc, ...) ``` ## Parameters `condition` An expression that evaluates to or . `desc` An `NSString` object that contains a `printf`-style string containing an error message describing the failure condition and placeholders for the arguments. ## Discussion Pass a comma-separated list of trailing variadic arguments to substitute into `desc`. The `NSAssert` macro evaluates the condition and serves as a front end to the assertion handler. Each thread has its own assertion handler, which is an object of class `NSAssertionHandler`. When invoked, an assertion handler prints an error message that includes the method and class names (or the function name). It then raises an `NSInternalInconsistencyException` exception. If `condition` evaluates to , the macro invokes [`handleFailureInMethod:object:file:lineNumber:description:`](/documentation/Foundation/NSAssertionHandler/handleFailureInMethod:object:file:lineNumber:description:) on the assertion handler for the current thread, passing `desc` as the description string. This macro should be used only within Objective-C methods. Assertions are disabled if the preprocessor macro `NS_BLOCK_ASSERTIONS` is defined or the `ENABLE_NS_ASSERTIONS` Xcode build setting is disabled. > Important: > Do not call functions with side effects in the `condition` parameter of this macro. The `condition` parameter is not evaluated when assertions are disabled, so if you call functions with side effects, those functions may never get called when you build the project in a non-debug configuration. > Note: > Not all release configurations disable assertions by default. ## See Also [`NSLogv(_:_:)`](/documentation/Foundation/NSLogv(_:_:)) Logs an error message to the Apple System Log facility. [`NSLog`](/documentation/Foundation/NSLog) Logs an error message to the Apple System Log facility. --- 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)