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

Indicates that a precondition was violated.

Use this function to stop the program when control flow can only reach the call if your API was improperly used. This function's effects vary depending on the build flag used:

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

  • In -O builds (the default for Xcode's Release configuration), stops program execution.

  • 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 preconditionFailure(_:file:line:) is called. line: The line number to print along with message. The default is the line number where preconditionFailure(_:file:line:) is called.


func preconditionFailure(_ message: @autoclosure () -> String = default, file: StaticString = #file, line: UInt = #line) -> Never