How can I output to STDERR with Swift?

I'm trying to make a command line tool for OS X with Xcode 6 and the new Swift language. How can I send output to stderr? Is this done with println?

For Swift 4.x:

import Darwin
fputs("hello from libc\n", stderr)

Here is a Swift 3 snippet modified from https://gist.github.com/algal/0a9aa5a4115d86d5cc1de7ea6d06bd91.

import Foundation

var standardError = FileHandle.standardError

extension FileHandle: TextOutputStream {
  public func write(_ string: String) {
    let data = Data(string.utf8)
    self.write(data)
  }
}

print("I am printed to stderr", to: &standardError)

May be a better way to do it, but you can use NSFileHandle:

import Foundation

// Create a file handle to work with
let stderr = NSFileHandle.fileHandleWithStandardError()

// Build up a string; whatever you want
let stuff = "something"
let something = "I'm a string with \(stuff) in it\n"

// Write it
stderr.writeData(something.dataUsingEncoding(NSUTF8StringEncoding))

Not really a separate answer, but building on top of Rob Napier's answer, we can create a stderr like object so that there is not much to change when Apple comes around to providing stderr as an OutputStreamType:

import Foundation

class StandardErrorOutputStream: OutputStreamType {
  func write(string: String) {
    let stderr = NSFileHandle.fileHandleWithStandardError()
    stderr.writeData(string.dataUsingEncoding(NSUTF8StringEncoding))
  }
}

var mx_stderr = StandardErrorOutputStream()

println("on-stdout")
println("on-stderr", &mx_stderr)

EDIT: As of 8/26/2015, Xcode 7 Beta 6, you need the toStream: parameter name, like so:

println("on-stderr", toStream:&mx_stderr)

How can I output to STDERR with Swift?

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