Checks a necessary condition for making forward progress.

func precondition(_ condition: @autoclosure () -> Bool, _ message: @autoclosure () -> String = String(), file: StaticString = #file, line: UInt = #line)



The condition to test. condition is not evaluated in -Ounchecked builds.


A string to print if condition is evaluated to false in a playground or -Onone build. The default is an empty string.


The file name to print with message if the precondition fails. The default is the file where precondition(_:_:file:line:) is called.


The line number to print along with message if the assertion fails. The default is the line number where precondition(_:_:file:line:) is called.


Use this function to detect conditions that must prevent the program from proceeding, even in shipping code.

  • 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): If condition evaluates to false, stop program execution.

  • 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.