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

# assertionFailure(_:file:line:)

Indicates that an internal consistency check failed.

## Declaration

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

## 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.

## Discussion

Discussion This function’s effect varies depending on the build flag used: 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.

## See Also

### Testing

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