---
title: "precondition(_:_:file:line:)"
framework: swift
role: symbol
role_heading: Function
path: "swift/precondition(_:_:file:line:)"
---

# precondition(_:_:file:line:)

Checks a necessary condition for making forward progress.

## Declaration

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

## Parameters

- `condition`: The condition to test. condition is not evaluated in -Ounchecked builds.
- `message`: A string to print if condition is evaluated to false in a playground or -Onone build. The default is an empty string.
- `file`: The file name to print with message if the precondition fails. The default is the file where precondition(_:_:file:line:) is called.
- `line`: The line number to print along with message if the assertion fails. The default is the line number where precondition(_:_:file:line:) is called.

## Discussion

Discussion 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.

## See Also

### Testing

- [assert(_:_:file:line:)](swift/assert(_:_:file:line:).md)
- [assertionFailure(_:file:line:)](swift/assertionfailure(_:file:line:).md)
- [preconditionFailure(_:file:line:)](swift/preconditionfailure(_:file:line:).md)