func assertionFailure(_: @autoclosure () -> String, file: StaticString, line: UInt)

Indicates that an internal sanity check failed.

Use this function to stop the program, without impacting the performance of shipping code, when control flow is not expected to reach the call---for example, in the default case of a switch where you have knowledge that one of the other cases must be satisfied. To protect code from invalid usage in Release builds, see preconditionFailure(_:file:line:).

  • In playgrounds and -Onone builds (the default for Xcode's Debug configuration), stop program execution in a debuggable state after printing message.

  • In -O builds, has no effect.

  • In -Ounchecked builds, the optimizer may assume that this function is never called. Failure to satisfy that assumption is a serious programming error.

Parameters: message: A string to print in a playground or -Onone build. The default is an empty string. file: The file name to print with message. The default is the file where assertionFailure(_:file:line:) is called. line: The line number to print along with message. The default is the line number where assertionFailure(_:file:line:) is called.


func assertionFailure(_ message: @autoclosure () -> String = default, file: StaticString = #file, line: UInt = #line)